FU HUA CHEN (Eds.) Foundations on Expansive Soils 1975

Further titles in this series: 1. G. SANGLER AT THE PENETROMETER AND SOIL EXPLORATION 2. Q. ZARUBA AND V.MENCL LANDSLIDES AND THEIR CONTROL 3. E.E. WAHLSTROM TUNNELING IN ROCK 4A.R. SILVESTER COASTAL ENGINEERING, I Generation, Propagation and Influence of Waves 4B. R. SILVESTER COASTAL ENGINEERING, II Sedimentation, Estuaries, Tides, Effluents and Modelling 5. R.N. YOUNG AND B.P. WARKENTIN SOIL PROPERTIES AND BEHAVIOUR 6. E.E. WAHLSTROM DAMS, DAM FOUNDATIONS, AND RESERVOIR SITES 7. W.F. CHEN LIMIT ANALYSIS AND SOIL PLASTICITY 8. L.N. PERSEN ROCK DYNAMICS AND GEOPHYSICAL EXPLORATION Introduction to Stress Waves in Rocks 9. M.D. GIDIGASU LATERITE SOIL ENGINEERING 10. Q. ZARUBA AND V. MENCL ENGINEERING GEOLOGY 11. H.K. GUPTA AND B.K. RASTOGI DAMS AND EARTHQUAKES Developments in Geotechnical Engineering 12 FOUNDATIONS ON EXPANSIVE SOILS by FU HUA CHEN President, Chen and Associates, Consulting Soil Engineers, Denver, Colo., U.S.A. Inc., ELSEVIER SCIENTIFIC PUBLISHING COMPANY Amsterdam — Oxford — New York 1975 ELSEVIER SCIENTIFIC PUBLISHING COMPANY 3 3 5 J a n van Galenstraat P.O. Box 2 1 1 , Amsterdam, T h e Netherlands A M E R I C A N E L S E V I E R PUBLISHING COMPANY, INC. 52 Vanderbilt A v e n u e New York, New York 10017 ISBN 0-444-41393-6 C o p y r i g h t © 1 9 7 5 b y Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m All r i g h t s r e s e r v e d . N o p a r t o f t h i s p u b l i c a t i o n m a y b e r e p r o d u c e d , s t o r e d in a retrieval s y s t e m , o r t r a n s m i t t e d in a n y f o r m o r b y a n y m e a n s , e l e c t r o n i c , mechanical p h o t o c o p y i n g , recording, or otherwise, w i t h o u t the prior written permission of t h e publisher. Elsevier Scientific Publishing C o m p a n y , J a n van G a l e n s t r a a t 3 3 5 , A m s t e r d a m P r i n t e d in T h e N e t h e r l a n d s To my wife Edna with love and appreciation. vi PREFACE T h e p r o b l e m s associated w i t h expansive soils are n o t widely appreciated o u t s i d e areas of their o c c u r r e n c e . T h e a m o u n t of damage caused b y expansive soils is alarming. It has b e e n e s t i m a t e d t h a t t h e damage t o buildings, r o a d s , and o t h e r s t r u c t u r e s f o u n d e d o n expansive soils exceeds t w o billion dollars annually. In t h e past 2 0 years, considerable progress has b e e n m a d e in u n d e r s t a n d i n g t h e n a t u r e of expansive soils. This new k n o w l e d g e can be separated i n t o t w o categories. T h e first emphasizes t h e theoretical a p p r o a c h and is t h e result of studies m o s t l y b y a c a d e m i c i n s t i t u t i o n s . I n s t i t u t i o n a l research involves soil mineralogy, s t r u c t u r e , and m o d i f i c a t i o n . Academicians have also advanced n e w theories such as effective stress, soil s u c t i o n , and o s m o t i c pressure w h i c h reveal p r o p e r t i e s of swelling soils previously little k n o w n t o engineers. T h e second category is c o n c e r n e d w i t h t h e field performance of expansive soils w i t h emphasis on design criteria and construction p r e c a u t i o n s for structures f o u n d e d o n expansive soil. Practical a p p r o a c h e s of c o m b a t i n g t h e swelling soils p r o b l e m are m o s t l y u n d e r t a k e n b y soils engineers; t h e r e f o r e , t h e y m u s t offer practical and e c o n o m i c a l solutions t o their clients, so t h a t t h e s t r u c t u r e will be free damaging f o u n d a t i o n m o v e m e n t . U n f o r t u n a t e l y , p r e s e n t d a y k n o w l e d g e of expansive soils has n o t reached a stage w h e r e rational solutions can be assigned t o t h e p r o b l e m . It is difficult for t h e public t o u n d e r s t a n d w h y the soils engineer is n o t capable of offering easy solutions. When t h e first crack appears in a s t r u c t u r e , a lawsuit is t h r e a t e n e d . This b o o k provides t h e practicing engineer w i t h a s u m m a r y of t h e state-of-the-art of expansive soils and practical solutions based u p o n t h e a u t h o r ' s e x p e r i e n c e . Part I discusses t h e o r y and practice, and summarizes s o m e of t h e theoretical physical p r o p e r t i e s of expansive soils. It also discusses various t e c h n i q u e s e m p l o y e d t o found s t r u c t u r e s on expansive soils such as drilled pier f o u n d a t i o n , m a t f o u n d a t i o n , m o i s t u r e c o n t r o l , soil r e p l a c e m e n t , and chemical stabilization. Part II presents typical case studies. T h e a u t h o r has found t h a t few records are available on t h e cause of structural distress, their remedial measures, and m o r e i m p o r t a n t , t h e degree of success after t h o s e measures have b e e n c o m p l e t e d . I n . t h e last 15 years, t h e a u t h o r has investigated m a n y t h o u s a n d s of building sites*in expansive soil areas in t h e R o c k y M o u n t a i n region. He has also investigated over 1,000 cracked buildings and has suggested remedial measures. It is t h e a u t h o r ' s h o p e t h a t b y sharing his k n o w l e d g e and t h e k n o w l e d g e of o t h e r practicing engineers, a b e t t e r u n d e r s t a n d i n g of expansive soil p r o b l e m s can b e achieved. T h e a u t h o r wishes t o t h a n k t h e entire staff of C h e n and Associates for sharing t h e w o r k load while t h e a u t h o r was devoting his t i m e t o writing this b o o k and also t h e assistance given b y t h e m in t h e p r e p a r a t i o n of t h e m a n u s c r i p t . Many t h a n k s t o t h e various consulting firms, especially Woodward-Clyde and Associates, Jorgensen and H e n d r i c k s o n , Ketchum-Konkel-Barrett-NickelAustin, and Ε. H. Tippets C o m p a n y for allowing t h e publication of their valuable findings. Mr. B y r o n Eskesen has c o n d u c t e d m o s t of t h e field investigation and l a b o r a t o r y testing presented in this b o o k . Denver, C o l o r a d o August, 1975 from T h e U. In Israel. T h e first significant n a t i o n a l conference on expansive clay p r o b a b l y was o n e held at t h e C o l o r a d o School of Mines in G o l d e n . brick veneer residences b e c a m e widely used.S. the shale of t h e Ecca Series. Israel. t h e dolerite sills and d y k e s in the central region of S o u t h Africa and the g a b b r o s and norities west of Pretoria N o r t h . b e d r o c k shale found in the Pierre F o r m a t i o n and the m o r e r e c e n t Laramie and Denver F o r m a t i o n s are examples of this t y p e of rock. T h e damages were a t t r i b u t e d t o s h o d d y c o n s t r u c t i o n and s e t t l e m e n t of the f o u n d a t i o n at o n e corner. It was t h e n t h a t t h e o w n e r found cracks developing in t h e brick course. there are t h e marls and limestones and in S o u t h Africa. . 1 9 4 0 .Chapter 1 NATURE OF EXPANSIVE SOILS INTRODUCTION T h e p r o b l e m of expansive soils was n o t recognized b y soil engineers until t h e latter p a r t of 1930. Engineers from C a n a d a . in 1 9 7 3 . T o d a y . *Numbers in brackets refer to items in the references at the end of each chapter. t h e feldspar and p y r o x e n e minerals of the p a r e n t rocks have d e c o m p o s e d to form m o n t m o r i l l o n i t e and o t h e r secondary minerals. has The increasingly extensive use of c o n c r e t e slab-on-ground c o n s t r u c t i o n . T h e first g r o u p comprises t h e basic igneous r o c k s . By 1 9 3 0 . W. further ORIGIN O F EXPANSIVE SOILS G. w i t h o u t recognition of t h e role of expansive soils. C o l o r a d o in 1 9 5 9 . Bureau of R e c l a m a t i o n [ 1 ] * first recognized t h e swelling soil p r o b l e m in 1938 in c o n n e c t i o n w i t h a f o u n d a t i o n for a steel siphon at their O w y h e e Project in Oregon. T h e second g r o u p comprises t h e s e d i m e n t a r y r o c k s t h a t c o n t a i n m o n t m o r i l l o n i t e as a c o n s t i t u e n t w h i c h breaks d o w n physically to form expansive soils. m o s t of t h e lightly loaded buildings in t h e United States consisted of frame dwellings. such as t h e basalts of t h e Deccan Plateau in India. t h e r e is a world-wide interest in expansive clays and shales. engineers realized t h e cause of damage was s o m e t i m e s o t h e r t h a n s e t t l e m e n t . S o u t h Africa. Transvaal. In these soils. Such structures could w i t h s t a n d considerable m o v e m e n t w i t h o u t exhibiting noticeable cracks. In N o r t h America. and their third conference in Haifa. and the United States have c o n t r i b u t e d i m m e n s e l y t o the knowledge and t h e p r o p e r design for s t r u c t u r e s on expansive soils. after increased t h e damage to s t r u c t u r e s caused b y expansive soils. Israel. Since t h a t t i m e . Prior t o 1 9 2 0 . Australia. D o n a l d s o n [ 2 ] classified the p a r e n t materials t h a t can be associated w i t h expansive soil i n t o t w o groups. T h e I n t e r n a t i o n a l Research and Engineering Conference on Expansive Soils held their first and second conferences at T e x a s A & M University in 1965 and 1 9 6 9 . (After Tourtelot. materials I V or Figure 1. and t o t h e east t h e Great Plains regions were once ocean basins w h e r e the Pierre and Bearpaw shales and their equivalent were deposited. Inland. . sediments S volcan*: . T h e source of the sediments consists of volcanic r o c k in t h e n o r t h e r n part ( M o n t a n a ) and a range of rock t y p e s in t h e s o u t h e r n part.2 FOUNDATIONS ON EXPANSIVE SOILS / i Γ I Ά Λ /V ) m — —ι— . Highland source of. r i Marine mud j and clay in I ocean basin . the shale consists almost entirely of clay-size material with high swell p o t e n t i a l . Geographic setting of deposition of Pierre and Bearpaw Shales and related rocks in Late Cretaceous time in Rocky Mountain and Great Plains region. In t h e Late Cretaceous t i m e . T h e shale is sandier and siltier adjacent t o the coast and possesses a lower swelling p o t e n t i a l . Separating t h e coastal plain from t h e ocean basin is a belt of sandy deposit. 1973) T o u r t e l o t [ 3 ] r e c o n s t r u c t e d the paleogeographic c o n d i t i o n in t h e R o c k y M o u n t a i n and Great Plains regions as s h o w n on figure 1.. t o t h e west of t h e R o c k y M o u n t a i n s were high-to-moderate u p l a n d s . T h e p r o d u c t s of weathering and erosion of the rocks in t h e highlands were carried b y streams t o the coastal plains. Potentially expansive soils can be found almost anywhere in the world. in a city of . m u c h of the expansive soil p r o b l e m s m a y n o t have been recognized. M o n t m o r i l l o n i t e is regionally a b u n d a n t in c o n t i n u o u s geologic formations t h r o u g h o u t the R o c k y M o u n t a i n s . a n d t h e Mississippi E m b a y m e n t as well as California and t h e Pacific N o r t h w e s t . D o n a l d s o n [21 s u m m a r i z e d t h e d i s t r i b u t i o n of r e p o r t e d instances of expansive soils a r o u n d the world (fig. This follows t h e t h e o r y t h a t in semi-arid zones. Even t h o u g h t h e damage caused b y expansive soils is m o d e r a t e . 1 9 6 5 . volcanic e r u p t i o n s . the a b u n d a n c e of m o n t m o r i l l o n i t e in b e d r o c k geologic formations in t h e United States.NATURE OF EXPANSIVE SOILS 3 T h e m o n t m o r i l l o n i t e was p r o b a b l y f o r m e d from t w o separate origins. t h e lack of leaching has aided t h e f o r m a t i o n of m o n t m o r i l l o n i t e . in a general w a y . W.A. It is t o be e x p e c t e d t h a t m o r e expansive soil regions will be discovered each y e a r as the a m o u n t of c o n s t r u c t i o n increases r World problem of expansive soils T h e status of t h e art of dealing with world p r o b l e m s on expansive clay soils was s u m m a r i z e d in t h e I n t e r n a t i o n a l Panel Review during t h e first conference o n expansive clay soils at T e x a s A & M. large parts of t h e Gulf Coastal Plains.S. These ashes were altered t o m o n t m o r i l l o n i t e . sending u p clouds of ash. 3 ) . DISTRIBUTION O F EXPANSIVE SOILS G. In the u n d e r d e v e l o p e d n a t i o n s . Expansive soils are in a b u n d a n c e where t h e a n n u a l é v a p o t r a n s p i r a t i o n e x c e e d s t h e p r e c i p i t a t i o n . m o s t of t h e Great Plains. Venezuela Figure 3 indicates t h a t t h e p o t e n t i a l l y expansive soils are confined t o t h e semi-arid regions of the tropical and t e m p e r a t e climate z o n e s . T h e following are t h e typical findings in each c o u n t r y : Australia — T h e major city t h a t experienced expansive soil p r o b l e m s is Adelaide in S o u t h Australia. T h e c o u n t r i e s in w h i c h expansive soils have been r e p o r t e d are as follows: Argentina Australia Burma Canada Cuba Ethiopia Ghana India Israel Iran Mexico Morocco Rhodesia S o u t h Africa Spain Turkey U. fell on t h e plains and t h e seas. Figure 2 illustrates. Meanwhile. T h e fine grained soils eventually b e c a m e shale a c c u m u l a t i n g in t h e ocean basin. General abundance of montmorillonite in near-outcrop bedrock formations.4 FOUNDATIONS ON EXPANSIVE SOILS Figure 2. 1973) . (Modified from Tourtelot. r e n o w n e d r e p u t a t i o n for s e t t l e m e n t p r o b l e m s . T h e soils in this region are generally desiccated. W. Distribution of reported instances of heaving. India . In t h e clay soil area. 0 0 0 i n h a b i t a n t s . Israel has a rainy w i n t e r season and a h o t . So .T h e so-called black c o t t o n soils cover a large area of a p p r o x i m a t e l y 2 0 0 . shallow b a s e m e n t s placed on shallow footings are c o m m o n l y used. in t h e heart of India. (After G. Israel — Expansive soil p r o b l e m s exist t h r o u g h o u t Israel. T h e p r o b l e m of expansive clays in Mexico is n o t considered t o be very serious t o d a t e .NATURE OF EXPANSIVE SOILS 5 Figure 3. In Western Canada. 0 0 0 square miles. t h e aggregate damage associated w i t h f o u n d a t i o n cracks is a substantial a m o u n t . This soil is characterized b y its e x t r e m e hardness w h e n d r y and with high swelling p o t e n t i a l during the process of wetting. Donaldson) s o m e 6 0 0 . Basement floors have been k n o w n t o heave as m u c h as 6 inches in 18 m o n t h s . m o n t m o r i l l o n i t e m a y be p r e s e n t in q u a n t i t i e s ranging from 4 0 t o 8 0 p e r c e n t of the soil. including Saskatchewan and Alberta. T h e soils are primarily alluvium o r r e w o r k e d t r a n s p o r t e d alluvium which originates from t h e weathering of either basalt or l i m e s t o n e . dry s u m m e r . in this area of C a n a d a . Canada . T h e r e have been very m a n y cases w h e r e pressures of t h e expansive clays have caused lateral deflections of b a s e m e n t walls. Also.T h e wide range of climate and geology in Canada p r o d u c e s a great variety of foundation problems. Mexico — Mexico City has a w o r l d . expansive clay p r o b l e m s are strongly evident. the a b u n d a n c e of M o n t m o r i l l o n i t e is c o m m o n in b o t h clays and claystone shales. t h o u g h generally t h e y d o n o t go over 8 0 . In t h e province of Madrid. covering a large p a r t of S o u t h Africa.6 FOUNDATIONS ON EXPANSIVE SOILS far. Figure 4 indicates t h e states where t h e State Highway D e p a r t m e n t s have sponsored research c o n c e r n i n g expansive soils [ 4 ] . shales with expansive properties are found. Andalucia and Madrid. T h e S o u t h African I n s t i t u t i o n of Civil Engineers published the first symposium on expansive clays in 1 9 5 7 . A m o n g the various regions w h e r e such p h e n o m e n a have been observed. In m o s t p a r t s of t h e c o u n t r y . is responsible for the f o u n d a t i o n p r o b l e m s at Odendaalsrus in t h e Orange F r e e States Goldfields. In one instance near t h e city. Severe foundation m o v e m e n t p r o b l e m s were recorded at Leeuhof. and Pretoria in Transvaal. T h e states t h a t experience various degrees of expansive soil p r o b l e m s are listed as follows: Severe: Colorado Texas Wyoming Moderate: California Utah Nebraska South Dakota . where t h e fluvio-lacustrine deposits are t h e source of swelling soils. Some of these soils have swelling pressures of 13 t o n s per sq. present n o swelling p r o b l e m . In a great p a r t of the m e t r o p o l i t a n areas. Research has been carried o u t o n expansive soils in m a n y states t h r o u g h o u t t h e United States. therefore. It is interesting t o n o t e t h e similarity b e t w e e n figures 2 and 4 . t h e y have been e n c o u n t e r e d in o n l y a b o u t five t o w n s of r a t h e r m e d i u m size. t h e r e are t w o provinces which m a y be regarded as t y p i c a l . and occasionally u p t o 2 8 t o n s per sq. ft. b u t t h e p r o b l e m is p o t e n t i a l l y m o r e serious because n e w t o w n s are being c o n s t r u c t e d and small t o w n s are being e x p a n d e d . T h e Ecca shale. t h e climate is arid and t h e é v a p o t r a n s p i r a t i o n is several times greater t h a n t h e precipitation resulting in swelling p h e n o m e n a . t h e p r o b l e m of expansive soils was b r o u g h t t o t h e a t t e n t i o n of the engineers as early as 1 9 5 0 . Spain — In Spain. T h e r e p o r t e d p r o b l e m areas are m o s t l y located in t h e regionally a b u n d a n t m o n t m o r i l l o n i t e areas indicated in figure 2. m a n y clay f o r m a t i o n s of s e d i m e n t a r y origin w i t h high plasticity can be found. ft. t h e soils for the m o s t p a r t consist of m o n t m o r i H o n i t i c clays. Venezuela — T h e first r e p o r t of swelling clays in Venezuela c a m e from t h e vicinity of t h e City of C o r o where m a n y buildings are badly cracked. t h e highly plasticity clays are covered w i t h a sufficient d e p t h of sandy clay s e d i m e n t s . Vereeniging. movement Distribution of expansive soils in the United States In t h e United States. S o u t h Africa — In S o u t h Africa. These soils r e a c h a liquid limit of 2 5 0 . from t h e Gulf of Mexico t o t h e Canadian Border and from Nebraska t o the Pacific Coast. NATURE OF EXPANSIVE SOILS Figure 4 . (After Sallbert and Smith) 7 . State Highway Departments that are sponsoring or have recently sponsored research concerning expansive clay soils. parking areas Highway and streets U n d e r g r o u n d utilities and service Airports U r b a n landslides Others According t o t h e above e s t i m a t e . and t h e chance of lightly loaded structures cracking due t o s e t t l e m e n t is r a t h e r r e m o t e . there are a large n u m b e r of instances where heavy cracks have appeared in the b a s e m e n t walls t h a t were n o t caused b y f o u n d a t i o n heaving b u t by e a r t h pressure exerted o n the wall. in a d d i t i o n to t h e lack of expansion j o i n t s . . A great deal of s t r u c t u r a l m o v e m e n t has been u n d u l y b l a m e d on expansive soils.140 100 40 25 100 Total $ 2.8 FOUNDATIONS ON EXPANSIVE SOILS Mild: Oregon Montana Arizona Oklahoma Kansas Alabama Mississippi D A M A G E C A U S E D BY E X P A N S I V E S O I L S J o n e s and Holtz r e p o r t e d in A S C E in 1 9 7 3 [ 5 ] expansive soil m o v e m e n t as follows: E s t i m a t e d average annual loss. the soils are generally stiff. In expansive soil areas. In m o s t cases where vertical or h o r i z o n t a l cracks developed in the b a s e m e n t wall. This is especially true for large warehouse floors w h e r e p r o p e r curing and design is essential. Curling of c o n c r e t e slabs has a strong resemblance t o heaving floors caused b y swelling soils. drives. It is a well k n o w n fact t h a t i m p r o p e r curing of c o n c r e t e . expansive soil damages n o w exceed t h e c o m b i n e d average annual damages from floods. and t o r n a d o s . e a r t h q u a k e s . generally c o m p o u n d e d b y seepage pressure. will cause cracking. millions of dollars $ 300 360 80 110 1. M a n y floor slabs c o n s t r u c t e d in an expansive soil area crack and s o m e t i m e s heave d u e t o i m p r o p e r l y designed concrete. Diagonal cracks t h a t develop b e l o w w i n d o w s and above d o o r s are a strong indication of swelling m o v e m e n t . e a r t h pressure p r o b l e m s are suspect. At t h e same t i m e . hurricanes.255 the estimated damage a t t r i b u t e d to C o n s t r u c t i o n category Single-family h o m e s C o m m e r c i a l buildings Multi-story buildings Walks. While it is t r u e t h a t swelling soils are p r o b a b l y responsible for m o s t of the cracking and m o v e m e n t of lightly loaded s t r u c t u r e s . b a s e m e n t wall cracks are caused b y careless c o n s t r u c t i o n crews. 0 0 2 m m ) or less. Backfill should n o t be placed against t h e wall until t h e wall has been p r o p e r l y restrained at t o p and b o t t o m . can result in cracks and m o v e m e n t . o t h e r aspects of f o u n d a t i o n m o v e m e n t c a n n o t a n d should n o t be ignored. Failure t o d o so m a y result in an arched c o n d i t i o n . micas. e x c e p t vermiculite. o x i d a t i o n . T h e three m o s t i m p o r t a n t groups of clay minerals are m o n t m o r i l l o n i t e . which are crystalline h y d r o u s aluminosilicates. F r o m t h e mineralogical s t a n d p o i n t . Expansive soils are o f t e n t i m e s blamed for arching of a wall w h e n actually improper r e i n f o r c e m e n t and restraint is t h e real p r o b l e m . Split level houses are generally c o n s t r u c t e d w i t h grade b e a m s placed at different levels. Such grade b e a m s . T h e colloidal particle consists primarily of clay minerals t h a t were derived from p a r e n t r o c k b y weathering.NATURE OF EXPANSIVE SOILS 9 S o m e t i m e s . Structural defects are s o m e t i m e s m i s t a k e n for distress caused b y swelling soils. Formation of clay minerals T h e clay minerals are formed t h r o u g h a c o m p l i c a t e d process from an a s s o r t m e n t of p a r e n t materials. T h e alteration process includes disintegration. their exchangeable ions. These particles are said t o be in t h e colloidal state. if n o t p r o p e r l y tied t o g e t h e r w i t h r e i n f o r c e m e n t . B a c k h o e or o t h e r e a r t h moving e q u i p m e n t b u m p i n g against the wall can cause vertical or h o r i z o n t a l cracks. and l i m e s t o n e . T h e p a r e n t materials include feldspars. T h e a l t e r a t i o n process t h a t takes place on land is referred t o as w e a t h e r i n g and t h a t on t h e sea floor or lake b o t t o m as halmyrolysis. and also as a specific mineral n a m e [ 7 ] . generally backfill is so loosely c o m p a c t e d t h a t distress caused b y lateral e x p a n s i o n of backfill is very u n c o m m o n . and kaolinite. t h e electrical forces acting on t h e surface of t h e particle are m u c h greater t h a n the gravitational force. h y d r a t i o n . illite. A b s o r p t i o n of w a t e r b y clays leads t o e x p a n s i o n . and leaching. Such p h e n o m e n o n s o m e t i m e s is e r r o n e o u s l y i n t e r p r e t e d as h o r i z o n t a l swelling pressure being e x e r t e d against t h e wall. electrolyte c o n t e n t of a q u e o u s phase. . F o r small size particles. While it is possible t h a t a large a m o u n t of swelling pressure can be e x e r t e d h o r i z o n t a l l y against a wall. CLAY MINERALS Most soil classification systems arbitrarily define clay particles as having an effective d i a m e t e r of t w o m i c r o n s ( 0 . and t h e internal s t r u c t u r e . Probably t h e m o s t i m p o r t a n t grain p r o p e r t y of fine-grained soils is t h e minealogical c o m p o s i t i o n [ 6 1 . T h e n a m e ' ' m o n t m o r i l l o n i t e " is used c u r r e n t l y b o t h as a g r o u p n a m e for all clay minerals w i t h an e x p a n d i n g lattice. Particle size alone does n o t d e t e r m i n e clay mineral. M o n t m o r i l l o n i t e is t h e clay mineral t h a t presents m o s t of t h e expansive soil p r o b l e m s . t h e m a g n i t u d e of e x p a n s i o n d e p e n d s u p o n t h e kind and a m o u n t of clay minerals p r e s e n t . and t h e exchange reaction does n o t affect t h e s t r u c t u r e of t h e silica-alumina p o c k e t . F r o m table 1. T h e exchangeable ions are held a r o u n d the outside of the silica-alumina clay-mineral structural u n i t . U n d e r these c o n d i t i o n s . t h e m o s t c o m m o n exchangeable cations are Ca^. Such c o n d i t i o n s are favorable in semi-arid regions w i t h relatively low rainfall or highly seasonal m o d e r a t e rainfall. T h e p a r e n t minerals for t h e f o r m a t i o n of m o n t m o r i l l o n i t e often consist of ferromagnesium minerals. Swelling clays are c o m m o n l y referred t o as b e n t o n i t i c soils b y l a y m e n . Certain relationships exist b e t w e e n soil p r o p e r t i e s such as A t t e r b e r g limits. Actually. B e n t o n i t e is a clay c o m p o s e d primarily of m o n t m o r i l l o n i t e which has b e e n formed b y the chemical weathering of volcanic ash. Cations (positive ions) are m o r e readily absorbed t h a n anions (negative ions). particularly w h e r e e v a p o r a t i o n exceeds p r e c i p i t a t i o n . Mg**". so t h a t m a g n e s i u m . it is seen t h a t the cation exchange . This is caused b y t h e large net negative charge carried by t h e m o n t m o r i l l o n i t e particle and its greater specific surface as c o m p a r e d w i t h kaolinite and illite. Na +. T h e existence of such charges is indicated b y t h e ability of clay t o absorb ions from t h e solution. T h e situations in w h i c h m o n t m o r i l l o n i t e can form require t h a t leaching be restricted. is readily a t t r a c t e d from a salt solution and a t t a c h e d t o a clay surface. t h e t y p e of clay mineral. strong h y d r a t i o n . h e n c e . Since c o m m e r c i a l for b e n t o n i t e is w h i t e . and m a n y volcanic r o c k s . F r o m tables 1 and 2. In clay minerals. N H 4 +. A cation. and iron cations m a y a c c u m u l a t e in t h e s y s t e m . negative charges m u s t be p e r d o m i n a n t on t h e clay surface. such as Na +. s o d i u m . e n o u g h w a t e r is available for the alteration process. Cation exchange Clay minerals have t h e p r o p e r t y of sorbing certain anions and cations and retaining t h e m in an exchangeable s t a t e . volcanic glass. Table 2 indicates t h e liquid limit and t h e plasticity index of each g r o u p of clay minerals. the w h i t e calcium streaks present in stiff clays are often m i s t a k e n b e n t o n i t e . calcic feldspars. However. frequently in a b o u t t h a t order of general relative a b u n d a n c e . FT. t h e absorbed Na + ion is n o t p e r m a n e n t l y a t t a c h e d .10 FOUNDATIONS ON EXPANSIVE SOILS T o u r t e l o t [31 p o i n t e d o u t t h a t t h e setting for t h e f o r m a t i o n of m o n t m o r i l l o n i t e is e x t r e m e disintegration. clays w i t h an a b u n d a n c e of calcium seldom exhibit swelling characteristics. Typical ranges of cation exchange capacities of various clay minerals are s h o w n in table 1. it can be replaced by K + ions if the clay is placed in a s o l u t i o n of potassium chloride KCL. and restricted leaching. T h e cation exchange capacity is t h e charge or electrical a t t r a c t i o n for cation per unit mass as measured in millequivalent p e r 100 grams of soil. calcium. t h e f o r m a t i o n of m o n t m o r i l l o n i t i c minerals is aided b y an alkaline e n v i r o n m e n t . T h e process of r e p l a c e m e n t by excess cations is called cation exchange [8]. T h u s . b u t t h e a c c u m u l a t e d cations will n o t be r e m o v e d b y flush rain. K +. and t h e n a t u r e of t h e adsorbed i o n . it is seen t h a t m o n t m o r i l l o n i t e s are 10 times as active in absorbing cations as kaolinites. and a lack of leaching. presence of magnesium ions. T h e cation exchange capacity of different t y p e s of clay minerals m a y be m e a s u r e d b y washing a sample of each w i t h a solution of a salt such as a m m o n i u m chloride N H 4 C L and the a m o u n t of adsorbed N H ^ b y measuring t h e difference b e t w e e n t h e original and the final c o n c e n t r a t i o n of t h e washing solution. NATURE OF EXPANSIVE SOILS 11 Table 1 . T h e silica sheets and t h e alumina sheets c o m b i n e t o form the basic s t r u c t u r a l u n i t s of t h e clay particle. T h e basic principle involved in the chemical stabilization of expansive soil is the increase in t h e ionic c o n c e n t r a t i o n in t h e free w a t e r and base e x c h a n g e p h e n o m e n o n . T h e basic building blocks of t h e clay minerals are t h e silica t e t r a h e d r o n and t h e alumina o c t a h e d r o n . Ranges of cation exchange capacities of various clay minerals Kaolinite Particle thickness Particle diameter Specific surface (sq. a plate-shaped layer is f o r m e d .5 .10 microns 50 . T h e greater t h e cation exchange capacity of clay.840 3-15 10-40 70-80 (After Woodward-Clyde & Associates. Kaolinite is a typical two-layer mineral having a single t e t r a h e d r a l sheet j o i n e d b y a single octahedral sheet t o form w h a t is called a 2 t o 1 lattice s t r u c t u r e ..10 microns 65 .5-4 microns 10-20 Illite 0. T h e results of studies using t h e e l e c t r o n m i c r o s c o p e and X-ray diffraction t e c h n i q u e s s h o w t h a t t h e clay minerals have a lattice s t r u c t u r e in w h i c h t h e a t o m s are arranged in several sheets. .003 .0 . Clay structure Philip L o w [9] p o i n t e d o u t t h e t w o f u n d a m e n t a l m o l e c u l a r s t r u c t u r e s as the basic u n i t s of the lattice s t r u c t u r e .180 Montmorillonite Less than 9. 1967) capacity of a clay has definite relation with t h e A t t e r b e r g limits. Cation exchange p h e n o m e n o n takes place in everyday life. when each a l u m i n u m a t o m is shared b y t w o o c t a h e d r o n .5 A 0. a sheet is f o r m e d . T h e alumina o c t a h e d r o n consists of an a l u m i n u m a t o m s u r r o u n d e d octahedrally b y six o x y g e n ions as s h o w n on figure 5 b .05 . 1 microns 0. meter/gram) Cation exchange capacity (milliequivalents per 100g) 0. T h e a r r a n g e m e n t and t h e chemical c o m p o s i t i o n of these sheets d e t e r m i n e t h e t y p e of clay mineral. A simple and well k n o w n e x a m p l e of t h e i o n exchange reaction is t h e softening of water b y t h e use of p e r m u t i t e s or c a r b o n exchangers. similar t o t h e pages of a b o o k . Various clay minerals differ in t h e stacking configuration. t h e greater the effect of changing t h e adsorbed c a t i o n . Similarly. T h e silica t e t r a h e d r o n consists of a silicon a t o m s u r r o u n d e d tetrahedrally b y four o x y g e n ions as s h o w n on figure 5a. When each o x y g e n a t o m is shared b y t w o t e t r a h e d r a . These are t h e silica t e t r a h e d r o n and t h e alumina o c t a h e d r o n .5-2 microns 0. T h e b l o c k s c o m b i n e i n t o t e t r a h e d r a l and o c t a h e d r a l sheets to p r o d u c e t h e various types of clays. Cation Clay mineral Kaolinte Illite Montmorillonite 29 61 344 1 27 251 35 81 161 7 38 104 34 90 166 8 50 101 39 83 158 11 44 99 percent Plasticity index.. percent Liquid limit.A.12 FOUNDATIONS ON EXPANSIVE SOILS Table 2 . Atterberg-limit values of clay minerals with various adsorbed cations Na+ Liquid limit. 1958) Figure 5. percent Ca" Plasticity index. percent Liquid limit. percent (After W. Polyhedra composing the structure of montmorillonite: (b) the alumina octahedron. White. . percent Mg" Plasticity index. percent Liquid limit. (After Philip Low. 1973) (a) the silica tetrahedron. percent Plasticity index. (After Philip Low. sandwiched Illite has similar s t r u c t u r e w i t h t h a t of m o n t m o r i l l o n i t e . 2. the attractive and the repulsive forces. Electrostatic force . in a d d i t i o n . 1. the w a t e r within t h e clay is called adsorbed water. the resulting e q u a t i o n is the Poisson-Boltzmann e q u a t i o n and is t h e Figure 6. therefore. T h e resulting repulsive pressure is. 1973) . t h e o s m o t i c pressure. t w o attractive forces p r e d o m i n a t e . T h e closer t h e dipolar w a t e r molecules and cations are t o t h e flat plate surface. Van der Waals' force — d e p e n d s on t h e distance b e t w e e n t h e layers. p o t a s s i u m ions are present b e t w e e n t h e t e t r a h e d r a l sheet and adjacent crystals. t h e w a t e r and ions w i t h t h e clay lattice c o n s t i t u t e t h e diffuse d o u b l e layer. charged c o n d e n s e r plate and t h e ions are assumed t o be non-interacting p o i n t charges. Model of a layer of montmorillonite. b u t s o m e of t h e silican a t o m s are replaced b y a l u m i n u m .NATURE OF EXPANSIVE SOILS 13 M o n t m o r i l l o n i t e is a three-layer mineral having a single o c t a h e d r a l sheet b e t w e e n t w o t e t r a h e d r a l sheets t o give a 2 t o 1 lattice s t r u c t u r e as s h o w n on figure 6. In t h e clay-water-air s y s t e m .d e p e n d s o n t h e c o m p o s i t i o n of the mineral. T w o forces exist in t h e system. it is possible t o use Poisson's e q u a t i o n from t h e t h e o r y of electrostatics. T h e double-layer t h e o r y assumes t h a t t h e clay particle is a flat. T h e high c o n c e n t r a t i o n of cations near the surface of t h e clay particle creates a repulsive force b e t w e e n t h e diffuse double-layer system. t h e m o r e strongly t h e y are a t t r a c t e d . T h e interlayer solution has a higher c o n c e n t r a t i o n of dissolved e l e c t r o l y t e t h a n the e x t e r n a l solution and the s u b s e q u e n t e n t r y of water b y osmosis. H e n c e . a n d . A t small interlayer distances. By c o m b i n i n g Poisson's e q u a t i o n w i t h B o l t z m a n n ' s e q u a t i o n of o s m o t i c pressure. As a result. t h e ion c o n c e n t r a t i o n b e t w e e n the particles will increase and t h e o s m o t i c pressure in t u r n increases. if t h e soil is subjected t o e x t e r n a l pressure. It is therefore. A n equilibrium is finally reached w h e n t h e o s m o t i c pressure equals the e x t e r n a l pressure. T h e reverse process involves the decrease of . the distance b e t w e e n particles will decrease and w a t e r will be squeezed o u t . Based on t h e t h e o r y t h a t o s m o t i c pressure is t h e only internal pressure acting b e t w e e n particles. [ 1 1 ] c o n c l u d e d t h a t t h e swelling of b o t h illitic clays and m o n t m o r i l l o n i t e clays is caused b y the excess o s m o t i c pressure in t h e adsorbed layer of ions. T h e typical result of t h e i n t e g r a t i o n of t h e Poisson-Boltzmann equation is given in figure 7. H. Warkentin and Bolt [101 observed t h a t e x p e r i m e n al curves of swelling pressure versus interlayer half-distance for N a . a n d . Osmotic pressure Osmosis is t h e passage of solvent t h r o u g h a semi-permeable m e m b r a n e from a s o l u t i o n of lesser c o n c e n t r a t i o n t o one of higher c o n c e n t r a t i o n . G. as early as 1 9 5 6 .C 2) O s m o t i c pressure Gas c o n s t a n t ( B o l t z m a n n c o n s t a n t ) Absolute temperature C o n c e n t r a t i o n of a n y ionic species (in ions p e r c m 3 ) Ionic c o n c e n t r a t i o n of the ionic species in the e x t e r n a l s o l u t i o n (in ions p e r c m 3 ) It is well recognized t h a t o s m o t i c pressure can be e x p e c t e d t o take place in t h e soil-water s y s t e m . It is seen from figure 7 t h a t t h e calculated repulsive pressures increase rapidly as the half-distance b e t w e e n the particles decreases.14 FOUNDATIONS ON EXPANSIVE SOILS basic differential e q u a t i o n of t h e double-layer t h e o r y . in w h i c h t h e surface charge density is d e t e r m i n e d b y dividing the cation ion exchange capacity b y the surface area. Assuming t h a t the d o u b l e layer system exists in t h e soil lattice. Research m a d e in t h e last decade strongly suggests t h a t o s m o t i c pressure indeed develops in t h e soil-water system and is responsible for the swelling m e c h a n i s m . and o s m o t i c pressure is t h e pressure w h i c h m u s t be applied t o t h e solution t o prevent t h e flow of solvent which tries t o dilute t h e solution. Bolt.m o n t m o r i l l o n i t e has t h e same shape as figure 7. n o t surprising t h a t t h e swelling pressure of expansive clays s o m e t i m e s reaches m o r e t h a n 25 t o n s per square foot. However. the c o n c e n t r a t i o n of ions being held b y t h e attractive force prevents t h e ions from moving away from the d o u b l e layer. water is able to move in and dilute t h e c o n c e n t r a t i o n . c o n s e q u e n t l y . O s m o t i c pressure can be evaluated b y V a n ' t Hoff e q u a t i o n as follows: P0 In w h i c h : PQ R Τ Cj C2 = = = = = = R T ( C i . a semi-permeable m e m b r a n e effect is achieved. Bolt claimed t h a t t h e o s m o t i c pressure of the s y s t e m might r e a c h a value of 50 t o 100 t o n s per square foot. T h e basic relation b e t w e e n d r y density and swelling pressure developed by the a u t h o r in figure 2 8 also assumes t h e same p a t t e r n . (After Philip Low) .NATURE OF EXPANSIVE SOILS 15 Figure 7. Calculated repulsive pressures at different half-distances between adjacent montmorillonite particles (layers) for two values of the surface charge density (cr). b u t are impractical and u n e c o n o m i c a l for practicing engineers. such as t h e i n d e x p r o p e r t y . PVC m e t h o d . The various methods listed above should generally be used in combination. offers the m o s t useful d a t a for a practicing engineer. t h e test results require e x p e r t i n t e r p r e t a t i o n and the . mineralogical identification. T h e first. can be useful in t h e evaluation of t h e material b u t is n o t sufficient in itself w h e n dealing w i t h n a t u r a l soils. A n o t h e r g r o u p includes t h e indirect m e t h o d s . it is claimed b y t h e clay mineralogist t h a t t h e swelling p o t e n t i a l of a n y clay can b e evaluated b y identification of t h e c o n s t i t u e n t mineral of this clay. Dye adsorption. H e n c e . T h e distance b e t w e e n the particles w o u l d increase. t h o u g h a great deal of research has been d o n e in t h e various fields of mineralogical s t u d y . " T h e negative electric charges o n the surface of t h e clay minerals. RECOGNITION O F EXPANSIVE SOILS There are three different m e t h o d s of classifying p o t e n t i a l l y expansive soils. Testing should be p e r f o r m e d on a n u m b e r of samples r a t h e r t h a n of a few t o avoid e r r o n e o u s conclusions. and t h e cation exchange capacity all c o n t r i b u t e to the swelling p o t e n t i a l of t h e clay. and activity m e t h o d w h i c h are valuable tools in evaluating t h e swelling p r o p e r t y . T h e third m e t h o d . T h e various m e t h o d s of mineralogical identification are i m p o r t a n t in a research l a b o r a t o r y in exploring t h e basic p r o p e r t i e s of clays. direct m e a s u r e m e n t . N o n e of the indirect m e t h o d s should be used i n d e p e n d e n t l y . T h e i m b i b a t i o n of w a t e r is the m o s t i m p o r t a n t cause of swelling. T h e five t e c h n i q u e s w h i c h m a y be used are as follows: X-ray diffraction. E r r o n e o u s conclusions can be drawn w i t h o u t the benefit of direct tests.16 FOUNDATIONS ON EXPANSIVE SOILS e x t e r n a l pressure and t h e s u c t i o n of liquid b y o s m o t i c pressure b e t w e e n t h e particles t o dilute t h e c o n c e n t r a t i o n of ions. T h e tests are simple t o perform and d o n o t require a n y costly and e x o t i c l a b o r a t o r y e q u i p m e n t . Chemical analysis. U n f o r t u n a t e l y . Mineralogical identificatioη T h e mineralogical c o m p o s i t i o n of expansive soils has an i m p o r t a n t bearing on t h e swelling p o t e n t i a l as explained u n d e r "Clay S t r u c t u r e . This process c o n t i n u e s until a n e w equilibrium is established. resulting in volume increase and a r e d u c t i o n of o s m o t i c pressure. and E l e c t r o n m i c r o s c o p e resolution. t h e different t y p e s of clay minerals present in a given soil can be evaluated q u a n t i t a t i v e l y . Using c o m b i n a t i o n s of t h e m e t h o d s . Soil s u c t i o n m a y prove to be very useful w i t h m o r e general application and improved testing t e c h n i q u e s . t h e strength of t h e interlayer b o n d i n g . A w o r d of c a u t i o n should be i n t r o d u c e d h e r e . Differential t h e r m a l analysis. minéralogie . Chemical analysis can be a valuable s u p p l e m e n t t o o t h e r m e t h o d s such as X-ray analysis in identifying clays. Microscopic e x a m i n a t i o n of clay minerals offers a direct observation of t h e material. Brindley [12] claimed t h a t the use of self-recording counter for s p e c t r o m e t e r s in lieu of p h o t o g r a p h i c t e c h n i q u e s increases considerably b o t h the a c c u r a c y and t h e convenience of t h e X-ray m e t h o d . chemical analysis can be used t o d e t e r m i n e t h e n a t u r e of i s o m o r p h i s m and t o show t h e origin a n d l o c a t i o n of the charge o n t h e lattice. It is well established as a t e c h n i q u e for t h e c o n t r o l of materials w h i c h u n d e r g o characteristic changes on heating. t h e s u b s t i t u t i o n of F e for Al in t h e o c t a h e d r a l c o o r d i n a t i o n . t h e color assumed b y t h e adsorbed d y e d e p e n d s o n t h e base exchange capacity of t h e various clay minerals present. and i n t e r n a l s t r u c t u r e . Chemical Analysis. T h e X-ray diffraction m e t h o d used in d e t e r m i n i n g t h e p r o p o r t i o n of the various minerals present in a colloidal clay consists essentially of c o m p a r i n g the ratios of t h e intensities of diffraction lines from t h e different minerals w i t h t h e intensities of lines from t h e standard substance. t h e i s o m o r p h o u s c h a r a c t e r of t h e m o n t m o r i l l o n i t e g r o u p can p r o b a b l y be s h o w n in n o o t h e r way. Differential t h e r m a l analysis w h e n used in c o n j u n c t i o n w i t h X-ray diffraction a n d chemical analysis enables t h e identification of o t h e r w i s e difficult materials.NATURE OF EXPANSIVE SOILS 17 specialized a p p a r a t u s r e q u i r e d are costly a n d n o t e c o n o m i c a l l y available in m o s t soil testing laboratories. and t h e s u b s t i t u t i o n of Mg for Al in t h e o c t r a h e d r a l positions. When a clay sample has been p r e t r e a t e d w i t h acid. A brief description of the various t e c h n i q u e s is as follows: X-Ray Diffraction M e t h o d . t e x t u r e . Brindley also believes t h a t t h e X-ray m e t h o d q u a n t i t a t i v e d e t e r m i n a t i o n s should be applied w i t h considerable c i r c u m s p e c t i o n . a n d t h a t in favorable cases t h e possibility of 1 identifying species b y X-ray analysis can be regarded w i t h restrained o p t i m i s m . T h e i s o m o r p h i s m involves t h r e e basic variations in t h e s u b s t i t u t i o n : t h e s u b s t i t u t i o n for Al for Si in t e t r a h e d r a l p o s i t i o n s in t h e lattice. T h e relatively simple testing p r o c e d u r e and speed of d y e staining tests c o m p a r e d w i t h X-ray diffraction and differential t h e r m a l analysis justify wider a p p l i c a t i o n of t h e color m e t h o d . In t h e m o n t m o r i l l o n i t e g r o u p of clay minerals. Electron Microscope R e s o l u t i o n . T h e main p u r p o s e of t h e microscopic e x a m i n a t i o n is t o d e t e r m i n e c o m p o s i t i o n . A c c o r d i n g t o Kelley [ 1 4 ] . T h e presence of m o n t m o r i l l o n i t e can be d e t e c t e d if its a m o u n t is greater t h a n a b o u t 5 t o 10 p e r c e n t . T w o clays m a y give t h e same X-ray p a t t e r n and t h e same differential t h e r m a l curve b u t will s h o w u p distinct m o r p h o l o g i c a l characteristics u n d e r e l e c t r o n m i c r o s c o p e resolution. W. Dye A d s o r p t i o n . T h e use of differential t h e r m a l analysis t e c h n i q u e in identifying expansive soil is n o t always accurate [ 1 3 ] . Differential T h e r m a l Analysis. Dyestuffs and o t h e r reagents which e x h i b i t characteristic colors w h e n adsorbed b y clay have been used t o identify clay. G. Holtz and Gibbs [ 1 6 ] d e m o n s t r a t e d in 1956 t h a t plasticity index and liquid limit are useful indices for d e t e r m i n i n g the swelling characteristics of m o s t clays. ridged. T h e above e q u a t i o n applies o n l y t o soils w i t h clay c o n t e n t s b e t w e e n 8 and 6 5 p e r c e n t a n d the computed value is p r o b a b l y accurate t o within a b o u t 3 3 p e r c e n t of t h e laboratory d e t e r m i n e d swell p o t e n t i a l . The swell p o t e n t i a l is defined as t h e percentage swell of a laterally confined sample w h i c h has soaked u n d e r a surcharge of 1 p o u n d per square inch after being c o m p a c t e d t o m a x i m u m density at o p t i m u m m o i s t u r e c o n t e n t according to t h e A A S H O c o m p a c t i o n test. Linear shrinkage tests.l i k e t e x t u r e .15 10 35 20-55 35 and Above M 2 . A t t e r b e r g Limits. It might be possible t o evaluate some p r o p e r t i e s of t h e expansive soil b y observing t h e degree of crinkling and interparticle b o n d i n g from scanning an electron m i c r o s c o p e . Since liquid limit and swelling of clays b o t h d e p e n d o n the a m o u n t of water a clay tries t o i m b i b e . W o o d w a r d . and Lundgren [ 1 7 ] have d e m o n s t r a t e d t h a t t h e plasticity index alone can be used as a preliminary indication of swelling characteristics of m o s t clays. Seed. h o n e y c o m b . Single index method Simple soil p r o p e r t y tests can be used for the evaluation of the swelling p o t e n t i a l of expansive soils. Such tests are easy t o p e r f o r m and should be included as r o u t i n e tests in t h e investigation of building sites in t h o s e areas having expansive soil. it is n o t surprising t h a t t h e y are related. relatively thick plates while m o n t m o r i l l o n i t e s have a crinkly. F r o m this. Relation b e t w e e n swelling p o t e n t i a l of clays and plasticity i n d e x can be established as follows: Swelling p o t e n t i a l Low Medium High Very high Plasticity i n d e x 0 . and Colloid c o n t e n t tests. and L u n d g r e n established t h e following simplified relationship: S = 60K(PI) in w h i c h : and S = Swell p o t e n t i a l Κ = 3. W o o d w a r d .18 FOUNDATIONS ON EXPANSIVE SOILS Ravina [ 1 5 ] m a d e extensive s t u d y of t h e mineralogical c o m p o s i t i o n of expansive clays b y the use of the scanning electron m i c r o s c o p e .6 X 1 0 " 5 and is a c o n s t a n t . Such tests m a y i n c l u d e : A t t e r b e r g limits tests. Seed. Free swell tests. It s h o w e d t h a t the nonswelling clays a p p e a r as flat. F r e e swell tests consist of placing a k n o w n v o l u m e of d r y soil in w a t e r and n o t i n g t h e swelled v o l u m e after t h e material settles. and soils having free swell value below 5 0 p e r c e n t seldom exhibit appreciable v o l u m e change even u n d e r very light loadings. failed t o show conclusive evidence of t h e correlation b e t w e e n swelling p o t e n t i a l and shrinkage limit.8 0 . w i t h o u t a n y surcharge. Experiments conducted b y Holtz [16] indicated t h a t a good grade of high swelling commercial b e n t o n i t e will have a free swell value of from 1 2 0 0 t o 2 0 0 0 p e r c e n t . Colloid C o n t e n t . Holtz suggested t h a t soils having free swell value as low as 100 p e r c e n t can cause considerable damage t o lightly loaded s t r u c t u r e s . T h e difference b e t w e e n t h e final and initial v o l u m e . 1-psi surcharge for a sample c o m p a c t e d at o p t i m u m m o i s t u r e c o n t e n t t o . expressed as percentage of swell u n d e r m a x i m u m density in s t a n d a r d A A S H O c o m p a c t i o n test. is t h e free swell value. T h e swell test is very crude and was used in t h e early d a y s w h e n refined testing m e t h o d s were n o t available. W o o d w a r d . however. Linear Shrinkage. t h e a m o u n t of swell will increase w i t h t h e a m o u n t of clay present in t h e soil as s h o w n on figure 8. t h e converse is n o t true. However. F o r a n y given clay t y p e . expressed as a p e r c e n t a g e of initial v o l u m e . It was suggested b y A l t m e y e r in 1955 [ 1 8 ] as a guide t o t h e d e t e r m i n a t i o n of p o t e n t i a l expansiveness for various values of shrinkage limits and linear shrinkage as follows: Shrinkage limit as a percentage Less t h a n 10 10-12 G r e a t e r t h a n 12 Linear shrinkage as a percentage Greater than 8 5 . Seed. In t h e o r y it appears t h a t t h e shrinkage characteristics of the clay should be a consistent and reliable i n d e x t o t h e swelling p o t e n t i a l . particularly t h e colloid c o n t e n t . t o t h e b o t t o m of a g r a d u a t e d cylinder. and L u n d g r e n [ 1 7 ] believed t h a t there is n o correlation* b e t w e e n swelling p o t e n t i a l and percentage of clay sizes.NATURE OF EXPANSIVE SOILS 19 While it m a y be t r u e t h a t high swelling soil will manifest high index p r o p e r t y . T h e grain size characteristics of a clay appear t o have a bearing on its swelling p o t e n t i a l .5 Degree of expansion Critical Marginal Non-critical R e c e n t research. T h e swell p o t e n t i a l is p r e s u m e d t o be related t o t h e o p p o s i t e p r o p e r t y of linear shrinkage measured in a very simple test. for a given clay t y p e . F r e e Swell. t h e relationship b e t w e e n t h e swelling p o t e n t i a l and p e r c e n t a g e of clay size can be expressed by t h e e q u a t i o n : S where: S = = KCX Swelling p o t e n t i a l . H o l t z [ 1 9 ] p r o p o s e d t h e identification criteria of expansive clay as follows: . 0 0 2 mm) Figure 8. By utilizing r o u t i n e l a b o r a t o r y tests such as A t t e r b e r g limits. c o n t r o l s t h e a m o u n t of swell. Some of these m e t h o d s are as follows: U S B R M e t h o d — Developed b y H o l t z and G i b b s [ 1 6 ] is based o n t h e s i m u l t a n e o u s consideration of several soil p r o p e r t i e s . Relationship between percentage of swell and percentage of clay sizes for experimental soils. and others. Woodward & Lundgren) Percentage of clay sizes finer t h a n 0 . colloid c o n t e n t s . A n e x p o n e n t d e p e n d i n g on t h e t y p e of clay. Based o n t h e curves presented in figure 9 . w h i c h is reflected b y X and Κ in t h e above e q u a t i o n . a n d Coefficient d e p e n d i n g o n t h e t y p e of clay. shrinkage limits. t h e quality o r kind of colloid. Where t h e q u a n t i t y o f t h e clay size particles is d e t e r m i n e d b y a h y d r o m e t e r test. (After Seed. t h e swelling potential can b e evaluated without resorting to direct m e a s u r e m e n t . 0 0 2 m m . .20 70 FOUNDATIONS ON EXPANSIVE SOILS Clay co mponent: C >mmercial Εîentonite J ι l 1 NOTE : Percent swell measured under 1 psi surcharge for sample compacted at optimum water content to maximum density IP «tnnHnrri Δ Δ ^ΠΗ t * * t 50 % 40 \y I h i Comme -cial Illite/ Bentonite / A 20 C X Κ = = = Classification method . T h e typical relationships of these p r o p e r t i e s w i t h swelling p o t e n t i a l are s h o w n o n figure 9. Colloid c o n t e n t as well as A t t e r b e r g limits should be i n c l u d e d in t h e r o u t i n e l a b o r a t o r y investigation o n expansive soils.6\ Com rcercial Kao unite/Senti >nite ^3-1 Com nercial III) e/Bentonit 1 •mmercial JL-C< I 1 lite ^<_|:| Commercial Illite/Kaol nite 1 Commercial Kaolinite 100 40 50 60 70 30 PERCENT CLAY SIZES (finer than 0 . 0 8 3 8 . kaolinite. T h e relationship b e t w e e n swell p o t e n t i a l and plasticity i n d e x can b e expressed as follows: S = in w h i c h i B e A( p ) A = 0 . and fine sand. T h e activity m e t h o d a p p e a r s t o be an i m p r o v e m e n t over t h e U S B R m e t h o d in t h a t t h e shrinkage limit did n o t e n t e r in t h e evaluation of swell p o t e n t i a l . Activity M e t h o d . consideration should be given in t h e differentiation of soil behavior b e t w e e n u n d i s t u r b e d and r e m o l d e d samples. Also.2558 F r o m figure 10 it is seen t h a t with increase of plasticity i n d e x . t h e increase of swelling p o t e n t i a l is m u c h less t h a n predicted b y H o l t z and Gibbs or from Seed.NATURE OF EXPANSIVE SOILS Table 3 .D a t a for making estimates of probable volume changes for expansive soils Data from index tests* Colloid content. and L u n d g r e n ( 1 6 ) was based on r e m o l d e d . percent total vol. T h e expansion was measured as p e r c e n t swell on soaking from u n d e r a surcharge of 1 psi. Especially. 0 0 2 m m . t h e r e f o r e . C d e n o t e s t h e percentage clay size finer t h a n 0 . and Β = 0. T h e activity for t h e artificially prepared sample was defined as: 100 p e r c e n t m a x i m u m d e n s i t y and o p t i m u m m o i s t u r e c o n t e n t in standard A A S H O c o m p a c t i o n test Activity = J2_ In t h e above. . percent minus 0. W o o d w a r d .0 psi.T h e activity m e t h o d p r o p o s e d b y Seed. change >30 20-30 10-30 <10 21 Degree of expansion Very high High Medium Low •Based on vertical loading of 1. an a t t e m p t has b e e n m a d e t o differentiate b e t w e e n u n d i s t u r b e d and r e m o l d e d samples. T h e a u t h o r has over t h e past 15 years p e r f o r m e d m a n y t h o u s a n d s of tests o n p o t e n t i a l swell and index p r o p e r t i e s . All tests refer to a surcharge pressure of 1 psi w i t h m o i s t u r e c o n t e n t b e t w e e n 15 and 2 0 p e r c e n t and dry density b e t w e e n 100 and 110 pcf.001 mm >28 20-13 13-23 >15 Plasticity index >35 25-41 15-28 <18 Shrinkage limit <11 7-12 10-16 >15 Probable expansion. T h e p r o p o s e d classification chart is s h o w n on figure 1 1 . t h e d a t a a c c u m u l a t e d is n o t sufficient t o form accurate empirical relationship b e t w e e n m e a s u r e d e x p a n s i o n and t h r e e i n d i c a t o r tests. artificially prepared soils c o m p o s e d of 2 3 m i x t u r e s of b e n t o n i t e . (After Holtz & Gibb) It should be p o i n t e d o u t t h a t figure 9 is based o n actual e x p a n s i o n tests for only 4 5 u n d i s t u r b e d and r e m o l d e d samples and. W o o d w a r d and L u n d g r e n . F r o m t h e test results of 3 2 1 u n d i s t u r b e d samples. illite. . a regression curve can b e fitted as s h o w n o n figure 10. per cu. in.1· \ \ // 40 COLLOID CONTENT (% less than 0. T h e Ladd and L a m b e m e t h o d aided b y a PVC m e t e r is p r o b a b l y t h e simplest and quickest m e t h o d . PVC Meter. F r o m figure 12. T h e sample was first c o m p a c t e d in a fixed ring c o n s o l i d o m e t e r w i t h c o m p a c t i o n effort of 5 5 . and w a t e r added t o t h e sample w h i c h is partially restrained from vertical expansion b y a proving ring. L a m b e established t h e following categories of PVC rating: PVC Rating Less t h a n 2 2 —4 4 . W./ / —/ 1 • 1.22 FOUNDATIONS ON EXPANSIVE SOILS f .•/ • r 1 / · • //' •l -/i • -/ hAr /. plasticity index. T h e reading is converted t o pressure and is designated as Swell I n d e x . while t h e soil suction m e t h o d is considered t o b e a n e w a p p r o a c h t o w a r d t h e m e a s u r e m e n t of swelling p o t e n t i a l and swelling pressure.6 Greater than 6 Category Non-critical Marginal Critical V e r y critical .T h e d e t e r m i n a t i o n of t h e p o t e n t i a l v o l u m e change (PVC) of soil was developed b y T. Relation of volume change to colloid content. T h e proving ring reading is t a k e n at t h e end of 2 h o u r s .0 1 τ 7 !•/ '.-lbs. 0 0 0 ft.001 mm) % • 24 SHRINKAGE LIMIT (%) λ* A 20 40 PLASTICITY INDEX Figure 9. / . and shrinkage limit (air-dry to saturated condition under a load of 1 lb. R e m o l d e d samples were specified. per sq.) (After Holtz and Gibbs) Indirect measurement Indirect m e a s u r e m e n t of swelling p o t e n t i a l of expansive soils has b e e n a p p r o a c h e d b y m a n y investigations. t h e swell index can b e converted t o p o t e n t i a l v o l u m e change. V \ 1 • • \ V . ft. L a m b e u n d e r t h e auspices of t h e Federal Housing A d m i n i s t r a t i o n [ 2 0 ] . T h e n an initial pressure of 2 0 0 psi was applied.. . NATURE OF EXPANSIVE SOILS 23 Figure 10. and Chen. Seed. . Relationship of volume change to plasticity index as predicted by Holtz. Woodward & Lundgren) T h e PVC m e t e r m e t h o d has b e e n widely utilized b y t h e Federal Housing A d m i n i s t r a t i o n as well as t h e C o l o r a d o State Highway D e p a r t m e n t . Soil Suction. Classification chart for swelling potential (After Seed. In engineering practice. It should b e p o i n t e d o u t t h a t t h e PVC m e t e r test in itself d o e s n o t m e a s u r e t h e swell p o t e n t i a l . T h e m e t h o d has n o t received wide a t t e n t i o n . however. . T h e capillary p o t e n t i a l can b e considered as being equivalent t o t h e negative p o r e pressure at low level of m a t r i x s u c t i o n . T h e capillary potential of an u n s a t u r a t e d soil is often identified in t e r m s of its soil s u c t i o n . it is considered satisfactory t o c o n d u c t l a b o r a t o r y analysis b y simulating t h e actual capillary p o t e n t i a l in t h e soil. — In theoretical analysis. Ladd and L a m b e [ 2 1 ] p r o p o s e d a classification system in 1961 w h e r e b y soils are classified w i t h respect t o p o t e n t i a l v o l u m e change d u e t o b o t h swelling and shrinkage. T h e PVC m e t e r test should be used only as a comparison b e t w e e n various swelling soils. t h e t o t a l suction can be considered t o consist of t h e o s m o t i c (or solute) p o t e n t i a l . T h e t r u e swell p o t e n t i a l of clay measured can be m u c h greater t h a n t h e indicated value.002mm) Figure 11. gravitational p o t e n t i a l .5% I l _ I Ο 10 20 30 Percent 40 50 60 70 80 90 100 Clay Sizes (finer than 0. and m a t r i x or capillary p o t e n t i a l .24 FOUNDATIONS ON EXPANSIVE SOILS *> Swelling Potential = 2 5 % Swelling Potential = 5% Swelling Potential = 1. " Federal Housing Administration Publication No. (From "FHA Soil PVC Meter Publication. Swell index versus potential volume change.NATURE OF EXPANSIVE SOILS 25 2 Non Critical 3 Marginal 4 5 Critical 6 7 Very 8 9 ΙΟ II 12 _ Critical IPVC) POTENTIAL VOLUME CHANGE Figure 12. 701) . has been a c c u m u l a t e d governmental organizations o n expansive tests using a c o n s o l i d o m e t e r . Since t h e m e m b r a n e is pervious t o ions of dissolved salts in t h e soil-water. O s m o t i c cell-consolidometer a p p a r a t u s has recently been developed t o m e a s u r e t h e swelling properties of soils under variable suction conditions. academic or evaluated easily w i t h o u t resorting t o devices t o hold t h e soil v o l u m e c o n s t a n t . it is difficult t o evaluate and c o m p a r e t h e test d a t a [ 2 5 ] . T h e c o n s o l i d o m e t e r can b e platform t y p e .000 c e n t i m e t e r s ( 2 0 . Direct Measurem en t T h e m o s t satisfactory and convenient m e t h o d of d e t e r m i n i n g t h e swelling p o t e n t i a l and swelling pressure of an expansive clay is b y direct m e a s u r e m e n t . It consists of t w o units separated b y a semi-permeable m e m b r a n e . T h e load can b e applied with air as in t h e case of C o n b e l c o n s o l i d o m e t e r or b y direct weight as in t h e case of cantilever consolidometer. w a t e r can flow i n t o t h e soil b e n e a t h t h e excavated area and cause swelling. After t h e soil has reached its m a x i m u m v o l u m e increase. simple in c o n s t r u c t i o n . Obermeier developed an osmotic c o n s o l i d o m e t e r which is similar t o t h a t used b y Kassiff and Ben-Shalom [ 2 4 ] b u t is inexpensive. T h u s . Vertical expansion m e a s u r e m e n t is r e p o r t e d as percentage of t h e initial height of the sample and is frequently referred t o as t h e p e r c e n t of swell. At oven d r y c o n d i t i o n . T h e a m o u n t of soil suction of a sample at equilibrium w i t h free w a t e r is z e r o . T h e soil sample is enclosed b e t w e e n t w o p o r o u s plates and confined in a m e t a l ring. the a m o u n t of soil suction rises rapidly. T h u s . and so forth [ 2 2 ] . dissimilar test p r o c e d u r e s have been used. t h e sample can b e reloaded A great and swelling pressure d e t e r m i n e d deal of d a t a ( c h a p t e r 2 ) . swelling pressure can be in files of soil engineers. T n e soil sample can b e flooded b o t h from t h e b o t t o m and from t h e t o p . t h e value m a y b e several thousand atmospheres. T h e disadvantage of such tests is t h a t there is a long t i m e required t o reach equilibrium. the stress release during excavation can result in significantly m o r e negative p o r e pressure in t h e u n d e r l y i n g soils. T h e long-term heave p o t e n t i a l t h a t results from stress release during t h e excavation of clay-shales m a y s o m e d a y be predicted b y the use of a suction test. t h e system controls m a t r i x suction. Such a device enables an easy and accurate m e a s u r e m e n t of t h e swelling p o t e n t i a l of a clay u n d e r various c o n d i t i o n s . O b e r m e i e r [ 2 3 ] claimed t h a t for a saturated clay mass. p F of 4 represents 10. U p o n drying. T h u s . T h e u l t i m a t e goal of t h e m e a s u r e m e n t of soil suction is t h e p r e d i c t i o n of m o i s t u r e m o v e m e n t and m o i s t u r e equilibria r a t h e r t h a n t h e direct m e a s u r e m e n t of t h e swell p o t e n t i a l . and easily a d a p t e d t o existing l a b o r a t o r y e q u i p m e n t . Better l a b o r a t o r y t e c h n i q u e s for measuring heaving potential of swelling soils subjected t o stress release are n e e d e d . A . or o t h e r a r r a n g e m e n t . scale t y p e . T h e thickness of t h e s a m p l e ranges from one-half t o 1 inch. T h u s . a p F of 2 represents 100 c e n t i m e t e r s of h y d r o s t a t i c heads ( 2 0 5 psf). Direct m e a s u r e m e n t of expansive soils can be achieved b y t h e use of t h e conventional one-dimensional c o n s o l i d o m e t e r . O b e r m e i e r further believed that b o t h shear and tensile stresses m a y have been an i m p o r t a n t c o n t r i b u t i o n t o t h e heaving of clay shales. U n f o r t u n a t e l y .26 FOUNDATIONS ON EXPANSIVE SOILS Soil suction is expressed in a term designated as p F which is t h e log of t h e equivalent capillary rise in c e n t i m e t e r s of water. 5 0 0 psf). T h e d i a m e t e r of t h e ring ranges from 2 t o 4 inches d e p e n d i n g u p o n t h e t y p e of sampling device. A solution of p o l y e t h y l e n e glycol is placed in unit I and t h e soil sample in u n i t II. When t h e m o i s t u r e c o n t e n t of the clay is changed. and stress h i s t o r y of t h e s a m p l e . can b e outlined as follows: Moisture content of high swelling p o t e n t i a l . 3 . T h e initial m o i s t u r e c o n t e n t is affected b y : (a) T h e t i m e allowed for t h e sample to remain in t h e ring before w e t t i n g . Increasing t h e applied load will r e d u c e t h e m a g n i t u d e of swell. as in t h e case of a rising w a t e r table. F o r an u n d i s t u r b e d sample. t h e m o l d i n g w a t e r c o n t e n t . the floor will heave b u t the e x t e n t of cracking will n o t be severe. T h e t y p e s of soils. t h e r e will be n o v o l u m e change. if t h e c o n c e p t of swelling pressure as discussed in c h a p t e r 2 is fully u n d e r s t o o d . a reliable and r e p r o d u c i b l e test can b e o b t a i n e d . t h e m o r e i m p o r t a n t variables involved are as follows: 1. Slight changes of m o i s t u r e c o n t e n t . if t h e m o i s t u r e c o n t e n t of t h e clay remains Irrespective u n c h a n g e d . Moisture c o n t e n t . and (c) T h e t e m p e r a t u r e and h u m i d i t y of t h e l a b o r a t o r y . sampling m e t h o d . in the m a g n i t u d e of o n l y 1 t o 2 p e r c e n t . will take place. F o r r e m o l d e d samples. T i m e allowed. and s t r u c t u r e s founded on clays w i t h c o n s t a n t m o i s t u r e c o n t e n t will n o t be subject to m o v e m e n t caused b y heaving. b o t h in the vertical and h o r i z o n t a l d i r e c t i o n . while swelling is absent o r limited in illite and kaolinite. t h e dry d e n s i t y . and t h e c o n d i t i o n s u n d e r w h i c h t h e m o s t critical situation exists. It is k n o w n t h a t floor slabs f o u n d e d on expansive soils cracked m o s t severely w h e n t h e m o i s t u r e c o n t e n t increased slightly d u e to local w e t t i n g . T h e lower t h e initial m o i s t u r e c o n t e n t t h e higher t h e swell. and thickness of t h e sample. If t h e floor slab is flooded. In the p e r f o r m a n c e of a typical swell test. (a) T h e e x t e n t of evaporation allowed while t h e sample is in t h e ring. Surcharge load for m o s t l a b o r a t o r y practice ranges from 1 t o 10 psi. a t t e m p t s were m a d e to duplicate t h e surcharge load w i t h t h e actual footing dead load. are sufficient t o cause d e t r i m e n t a l swelling. Also. The time required to fully complete t h e swell process m a y vary considerably and d e p e n d s on t h e p e r m e a b i l i t y of t h e clay. this would include t h e m e t h o d of c o m p a c t i o n . it m a y require as m u c h as several d a y s t o c o m p l e t e t h e total available swell. Surcharge load. In the l a b o r a t o r y . curing t i m e before and after c o m p a c t i o n . and compaction density. S o m e t i m e s . State of sample. m a n y of the variables m e n t i o n e d above can be simplified. PHYSICAL PROPERTIES O F EXPANSIVE SOILS It is well k n o w n t o soil engineers t h a t m o n t m o r i l l o n i t e clays swell w h e n t h e m o i s t u r e c o n t e n t is increased. this w o u l d include t h e c o n d i t i o n of t h e sample. 4. 2. . C o m p l e t e s a t u r a t i o n is n o t necessary t o accomplish swelling. F o r an u n d i s t u r b e d sample having a thickness of 1 inch. t h e direct m e a s u r e m e n t m e t h o d is t h e m o s t i m p o r t a n t and reliable test on expansive soils. By standardizing the above variables. U n d o u b t e d l y .NATURE OF EXPANSIVE SOILS 27 s t a n d a r d i z a t i o n of test p r o c e d u r e of a one-dimensional swell test d o e s n o t a p p e a r difficult and will salvage m u c h of t h e valuable d a t a a c c u m u l a t e d in t h e h a n d s of t h e private c o n s u l t a n t s . v o l u m e e x p a n s i o n . clay samples swell in the c o n s o l i d o m e t e r w i t h slight increase of h u m i d i t y . percent total volume change >10 3-10 1-5 < 1 Swelling pressure. it is realistic to use a vertical load of 1. Soils w i t h d r y densities in excess of 110 pcf generally exhibit high swelling p o t e n t i a l . Conversely.000 psf t o gauge the swelling p o t e n t i a l . Table 4 is a guide for estimating t h e p r o b a b l e v o l u m e changes of expansive soils. liquid limit. In the highly expansive clay areas of Denver. T h e simplified classification of the expansive p r o p e r t i e s can be conveniently used b y engineers as a guide for t h e choice of t y p e of f o u n d a t i o n on expansive soils. V e r y d r y clays w i t h n a t u r a l m o i s t u r e c o n t e n t below 15 p e r c e n t usually indicate danger. Such clays will easily absorb m o i s t u r e t o as high as 35 p e r c e n t w i t h resultant damaging e x p a n s i o n t o s t r u c t u r e s . p e n e t r a t i o n resistances as high as 3 0 are n o t uncommon. clays w i t h m o i s t u r e c o n t e n t s above 3 0 p e r c e n t indicate t h a t m o s t of t h e expansion has already t a k e n place and further e x p a n s i o n will be small. This is true both for soils in undisturbed and in r e m o l d e d states. the dry density of the clay is a n o t h e r i n d e x of expansion. 200 sieve >35 60-95 30-60 <30 Liquid limit. R e m a r k s m a d e b y excavators complaining t h a t the soils are as hard as a r o c k is an indication t h a t soils inevitably will p r e s e n t e x p a n s i o n p r o b l e m s . Dry density ' Directly related t o initial m o i s t u r e c o n t e n t . m o i s t clays m a y desiccate d u e to lowering of w a t e r table or o t h e r changes in physical c o n d i t i o n s and u p o n s u b s e q u e n t w e t t i n g will again exhibit swelling p o t e n t i a l .28 FOUNDATIONS ON EXPANSIVE SOILS T h e initial m o i s t u r e c o n t e n t of t h e expansive soils controls the a m o u n t of swelling. Index properties T h e a u t h o r has a c c u m u l a t e d years of test d a t a o n expansive soils in t h e R o c k y M o u n t a i n area and found t h a t it is m o r e c o n v e n i e n t t o correlate t h e expansive properties w i t h t h e percentage of silt and clay ( . ksf >20 5-20 3-5 1 Degree of expansion Very high High Medium low . the relationship b e t w e e n t h e initial m o i s t u r e c o n t e n t and t h e capability of swelling has b e e n studied b y Holtz [ 1 6 ] . Clays w i t h p e n e t r a t i o n resistance in excess of 15 usually possess some swelling p o t e n t i a l .2 0 0 ) . F o r e x a m p l e .000 psf on t h e footings. for Table 4 .. Seed [ 1 7 1 . Since m o s t lightly loaded s t r u c t u r e s will exert a m a x i m u m dead load pressure of a b o u t 1. As previously discussed. percent >60 40-60 3040 <30 Standard penetration resistance. blows/ft >30 20-30 10-20 <30 Probable expansion. T h e d r y density of t h e clays is also reflected b y the standard p e n e t r a t i o n resistance test results. Data for making estimates of probable volume changes for expansive soils Laboratory and field data Percentage passing No. However. and m a n y o t h e r s . and field p e n e t r a t i o n resistance. then dessicated to initial moisture content ( 1 1 . if sufficient r e i n f o r c e m e n t is provided in t h e f o u n d a t i o n walls t o c o m p e n s a t e for slight m o v e m e n t s . 1965) . Figure 13 shows a typical l a b o r a t o r y fatigue curve of swelling. then allow full expansion a g a i n . 0 0 0 t o 5 . 5 ι 1 1 1 1 1 1 1 ζ ο </) ρ ζ < £ UJ I Claystone Soil Sample. t h e n is saturated again. 5 % ) . individual footings or p a d s can b e used w h e r e t h e dead load of t h e s t r u c t u r e can be c o n c e n t r a t e d to an i n t e n s i t y of 3 . It was observed t h a t t h e soil showed signs of fatigue after each cycle of d r y i n g and w e t t i n g [ 2 6 ] . Chu believed t h a t if d r y i n g and w e t t i n g cycles are r e p e a t e d . Sample saturated to allow full expansion. F o r soils of high-to-very-high degree of e x p a n s i o n . Piers w i t h sufficient d e a d load pressure and e n o u g h anchorage as described in c h a p t e r 4 should be used. Fatigue of swelling A clay sample is subjected t o full swelling in t h e c o n s o l i d o m e t e r . F o r soils of m e d i u m degree of e x p a n s i o n . special c o n s i d e r a t i o n should be given as t o t h e f o u n d a t i o n t y p e .NATURE OF EXPANSIVE SOILS 29 soils w i t h a low degree of e x p a n s i o n . Ο 1 2 3 4 OF WETTING 8 5 6 7 NUMBER OF CYCLES DRYING Figure 13. T h e fatigue of swelling p r o b a b l y can furnish t h e answer. Fatigue of swelling (After Chen. This is repeated for a n u m b e r of cycles. spread footing t y p e f o u n d a t i o n s can usually be used. This p h e n o m e n o n has n o t b e e n u n d e r full investigation. t h e swelling d u r i n g t h e first cycle w o u l d be appreciably higher t h a n t h a t in s u b s e q u e n t cycles. 0 0 0 psf. Fatigue of swelling was also observed b y C h u [ 2 7 ] in his research o n controlled suction test. It has b e e n n o t e d t h a t p a v e m e n t s f o u n d e d on expansive clays w h i c h have u n d e r g o n e seasonal m o v e m e n t d u e t o w e t t i n g and drying have a t e n d e n c y t o reach a p o i n t of stabilization after a n u m b e r of years. allowed t o desiccate t o its initial m o i s t u r e c o n t e n t . 658. W. Vol. [15] Ravina.C. E. T. 1956. and Miller. Bulletin 169.. [ I I ] Bolt. D.. Mielenz..The Hidden Disaster. "Engineering Properties of Expansive Clays." Proceedings. J. W.30 FOUNDATIONS ON EXPANSIVE SOILS REFERENCES [I] [2] [3] [4] Holtz. 8. G. Peck. "Foundation Engineering.." Civil Engineering. W. G. R. [18] Altmeyer. Bulletin 169. J. Vol. I." ASCE Transactions Paper No. "Engineering Properties of Expansive Clays. "Prediction of Swelling Potential for Compacted Clays. ASCE. Bolt. W.. T.. R. J. W. J. E. [14] Kelley." Geotechnique. 1973. 1967. Grim. Jones. No... R. I. Texas A & M Press. [20] "The Character and Identification of Expansive Soils. P. Texas A & M Press. "Fundamental Mechanisms Involved in Expansion of Clays as Particularly Related to Clay Mineralogy." Clays and Clay Technology. Jr. 1973. of Mines. 81. 80. H. Div. B. "Physical-Chemical Properties and Engineering Performance of Clays. . M. H." ASTM Journal of Materials. "Expansive Clay .. 88." Second International Research and Engineering Conference on Expansive Clay Soils.. S. R. March. Haifa.. W. of Mines. 1974. 5th International Conference on Soil Mechanics and Foundation Engineering. 1954." Engineering Effects of Moisture Change in Soils. March. I." John Wiley & Sons. [24] Kassiff. and Smith.Properties and Problems." Proceedings. Vol. G." Proceedings of the Third International Conference on Expansive Soils. [13] Dodd. 2. 1969. Η. "Geologic Origin and Distribution of Swelling Clays. 1955. G.. and Ben-Shalom.." Clays and Clay Technology. G. G." Colorado School of Mines Quarterly. Aug. "Swelling of Clays. Soil Mechanics and Foundations Div. "Expansive Soils . Donaldson. Vol. E. W. and Lundgren. Vol. C. [16] Holtz. H. Separate No." Proceedings of Workshop on Expansive Clay and Shale in Highway Design and Construction. Vol. [22] "A Review Paper on Expansive Clay Soils.F. [5] [6] [71 [81 [9] [101 Warkentine. "Swelling Pressure of Montmorillonite. G. D." by Woodward-Clyde & Assoc. 1960. Proceeding 2 1 . Div. and Holtz.. 1956. R. [17] Seed." Soil Science Society of America." A Report Completed for the Technical Studies Program of the Federal Housing Administration.. International Research and Engineering Conference on Expansive Clay Soils. May. "Clay Mineralogy. I. I.. Nov. 1955. "Dye Adsorption as Method of Identification of Clays. B. Α. [12] Brindley. and Gibbs. 43." Clays and Clay Technology. Israel.P.. "Identification of Clay Minerals by X-Ray Diffraction Analysis.G. Vol. [23] Obermeier. 54... and Thornburn. Div. T. 1973.." Proceedings ASCE. "The Identification and Behavior of Expansive Clays. 1957.. 1962." Journal ASCE. [21] Ladd. "Pavement Design over Expansive Clay: Current Practices and Research in the United States. 1961. Mineralogical Composition and Microstructure. J. Vol.. 1955. P. 1955. Tourtelot. R. 6. "Physical-Chemical Analysis of the Compressibility of Pure Clays." Clay and Clay Technology. Α. Hanson. Paris. W. and Lambe. C.. and King. Woodward. No. G. "Discussion of Engineering Properties of Expansive Clays. C. Dept. Bulletin 169. of Mines Bulletin 169. Low. Salbert." Proceedings of Workshop on Expansive Clay and Shale in Highway Design and Construction.. "The Occurrence of Problems of Heave and the Factors Affecting its Nature. 1973. 1968. 2814.. [19] Holtz. 121. H. "Evaluation of Laboratory Techniques for Measurement of Swell Potential of Clays. 1965. "Interpretation of Chemical Analysis of Clays. G. W. 1973. 4. "Apparatus for Measuring Swell Potential Under Controlled Moisture Intake.. I. W. C. 1955. of Natural Resources.. I. Vol..." McGraw-Hill Book Co. R. Vol. 1971. Vol. P. and Gibbs. State of California. 1959. Vol." Proceedings of Workshop on Expansive Clays and Shales in Highway Design and Construction. 1973. "The Need for Uniformity in Testing of Expansive Soils. I. H. L. . [26] Chen. Concluding Proceedings International Research and Engineering Conference on Expansive Clay Soils.NATURE OF EXPANSIVE SOILS 31 [25] Kraynski. "Volume Change Characteristics of Expansive Soils Determined by Controlled Suction Tests. and Mou. Haifa. C. Texas A & M Press. F." Proceedings of Workshop on Expansive Clays and Shales in Highway Design and Construction. T." Engineering Effects of Moisture Changes in Soils. [27] Chu. 1965. Vol... Israel. 1973. H. M.." Proceedings of the Third International Conference on Expansive Soils. "The Use of Piers to Prevent the Uplifting of Lightly Loaded Structures Founded on Expansive Soils. swelling p o t e n t i a l . In fine grained soils. swelling will n o t t a k e place. and t h e United States in an a t t e m p t t o stabilize p a v e m e n t s c o n s t r u c t e d in expansive soil areas. Shrinkage cracks w h i c h develop due t o surface desiccation provide easy access of w a t e r i n t o t h e d e e p soils. mineralogical c o m p o s i t i o n . t o p o g r a p h i c features. Moisture transfer T h e m o s t c o m m o n m e t h o d of m o i s t u r e transfer is b y gravity. climatic c o n d i t i o n s . and g r o u n d . if t h e e n v i r o n m e n t of t h e expansive soil has n o t b e e n changed. By far t h e m o s t i m p o r t a n t e l e m e n t and of m o s t c o n c e r n t o t h e practicing engineer is t h e effect of w a t e r on expansive soils. and v o l u m e increase because of t h e i n t r o d u c t i o n of m o i s t u r e . In clean. p r e c i p i t a t i o n . S o u t h Africa. MOISTURE MIGRATION The pattern of moisture migration depends on the geological formations. E n v i r o n m e n t a l change can consist of pressure release d u e t o excavation. " With t h e i n t r o d u c t i o n of water. t h e capillary rise is insignificant. U n d e r artesian c o n d i t i o n s . Much research has b e e n c o n d u c t e d in r e c e n t years b y highway organizations in Australia. t h e flow can be u p w a r d .Chapter 2 MECHANICS OF SWELLING INTRODUCTION In c h a p t e r 1. T h e seepage of surface w a t e r . t h e rise is a few inches. This c h a p t e r p r e s e n t s a discussion o n t h e m i g r a t i o n of w a t e r . T h e m o i s t u r e migration can o c c u r in all directions. and swelling pressure. coarse gravel. desiccation caused b y t e m p e r a t u r e increase. t h e flow generally occurs in t h e b e d d i n g planes or follows c o n t i n u o u s fractures and fissures. in fine . v o l u m e t r i c e x p a n s i o n takes place. and snow m e l t i n g i n t o t h e soil are c o m m o n e x a m p l e s . In stiff clays and in shale b e d r o c k . and t h e basic s t r u c t u r e of expansive soil were o u t l i n e d . I m p o r t a n t differences in t h e m o i s t u r e migration p a t t e r n b e t w e e n covered and n a t u r a l areas have b e e n studied extensively b y t h e C o m m o n w e a l t h Scientific and Industrial Research Organization in Australia. In clean sands.w a t e r level. capillary force is a significant m e a n s of w a t e r transfer. Kraynski [ 2 2 ] stated in his review p a p e r o n expansive soils. " T h e r e m u s t be a p o t e n t i a l gradient which can cause w a t e r migration and a c o n t i n u o u s passage t h r o u g h w h i c h w a t e r transfer can t a k e p l a c e . If pressure is applied t o p r e v e n t e x p a n s i o n . t h e pressure required t o m a i n t a i n t h e initial v o l u m e is t h e swelling pressure. T h e height of w a t e r rise i n t o t h e capillary fringe varies inversely w i t h t h e radius of t h e capillary t u b e . t h e origin. Obviously. soil t y p e s . a rise of m o r e t h a n 1000 feet is theoretically possible. t h e rise is 1 or 2 feet. When t h e area is covered b y a building o r p a v e m e n t ." Soil suction as briefly described in c h a p t e r 1 is considered a basic p a r a m e t e r t o soil-water equilibria and m o v e m e n t of w a t e r in soil. E x p e r i m e n t s c o n d u c t e d at P r i n c e t o n University show t h a t a t e m p e r a t u r e differential of Γ C was at least equivalent t o a h y d r o s t a t i c head of 3 feet in its ability t o cause m o i s t u r e m i g r a t i o n . the value of soil suction is reduced b y the o v e r b u r d e n pressure a n d / o r t h e e x t e r n a l stress. . T h e r m a l gradients can also cause m o i s t u r e migration t h r o u g h t h e liquid p h a s e of t h e soils. in silt. and t h e capillary forces. T h e difference b e t w e e n t h e p o r e pressure and a t m o s p h e r i c pressure is t h e value of soil suction [ 2 9 ] . T h e r m a l gradient reaches m a x i m u m efficiency w h e n t h e m o i s t u r e c o n t e n t in t h e soil is near plastic limit. Moisture equilibria of t h e partially saturated soil is in general In n a t u r a l g r o u n d . and t h e p o r e w a t e r pressure. T h e new equilibrium c o n d i t i o n . T h e m a g n i t u d e of p o r e pressure in t h e soil can be obtained from t h e soil suction as follows: u where: a Ρ s u = = = = = aP-s fraction of t h e n o r m a l pressure effective in changing t h e value of soil s u c t i o n . t h e m e a s u r e d soil s u c t i o n .34 FOUNDATIONS ON EXPANSIVE SOILS sands. It is well recognized b y observant soil engineers t h a t t h e heaving of expansive soils m a y t a k e place w i t h o u t t h e presence of free w a t e r . V a p o r and liquid m o i s t u r e transfer u n d e r t h e r m a l gradient can be an i m p o r t a n t cause of t h e swelling of moisture-deficient soils. generally the covered area b e n e a t h a s t r u c t u r e . t h e evaporation and t r a n s p i r a t i o n forces are eliminated and a new set of equilibrium m u s t be requires t h e flow of m o i s t u r e c o m p a t i b l e w i t h t h e new established. t h e t o t a l n o r m a l stress o n t h e soil e l e m e n t . u p t o 10 t o 12 feet. t h e forces d u e t o evaporation and transpiration at ground surface. T h e force causing t h e m o i s t u r e change or flow is t e r m e d soil suction. T h e review panel of engineering c o n c e p t s of Moisture Equilibria and Moisture Changes in Soil B e n e a t h Covered Areas [ 2 8 ] states. c o n d e n s a t i o n can t a k e place and provide sufficient m o i s t u r e t o initiate swelling. T h e p o r e pressure of an u n d i s t u r b e d sample o b t a i n e d at t h e g r o u n d surface (above t h e w a t e r table w i t h n o externally applied stress) is less t h a n a t m o s p h e r i c . Water v a p o r at a t e m p e r a t u r e higher t h a n its surroundings will migrate t o w a r d t h e cooler area t o equalize the t h e r m a l engery of t h e t w o areas. " T h e major change induced in soil b y a surface cover is caused b y an alteration in t h e rates and q u a n t i t i e s of w a t e r able t o e n t e r and leave t h e soil at t h e surface. When w a t e r reaches t h e cooler area. t h e m o i s t u r e c o n t e n t equilibrium w i t h t h e applied stress. V a p o r transfer plays an i m p o r t a n t role in providing the m e a n s for t h e v o l u m n e increase of expansive soils. Below t h e g r o u n d surface or u n d e r structural load. Covering t h e ground surface a r o u n d t h e building w i t h plastic m e m b r a n e s creates a thermal gradient which may encourage moisture from lawn watering t o transfer t o the f o u n d a t i o n soils. and in clay. K r a y n s k i referred t o this d e p t h as D e p t h of Desiccation. S t u d y showed t h a t t h e soils u n d e r a covered area are isolated from rapid changes and tend t o w a r d a stable equilibrium m o i s t u r e d i s t r i b u t i o n . indicated in figure 14 d e p e n d s on t h e variation of surface m o i s t u r e . and climatic c o n d i t i o n s . t h e value of a varies b e t w e e n zero and u n i t y . T h e t o t a l suction b e n e a t h t h e covered area t e n d s t o change w i t h climatic c o n d i t i o n s . t h e H s d e p t h m a y be only a few feet. This d e p t h represents t h e t o t a l thickness of material w h i c h has a p o t e n t i a l t o e x p a n d because of w a t e r deficiency. permeability of t h e soils. Aichison presented empirical m e t h o d s predicting equilibrium soil s u c t i o n u n d e r s t r u c t u r e s b y utilizing t h e results of a broad-scale field investigation b e n e a t h sealed p a v e m e n t t h r o u g h o u t Australia.i n n o n c o n s o l i d a t a b l e soil a is z e r o . and t h e m i n i m u m d e p t h is equal t o t h e d e p t h of t h e seasonal m o i s t u r e c o n t e n t fluctuation described b e l o w .m a d e e n v i r o n m e n t . t h e H s d e p t h can reach 10 feet or more. T h e soil suction c o n c e p t provides a m e a n s of d e t e r m i n i n g . T h e r e is n o gain or loss of m o i s t u r e t o t h e a t m o s p h e r e . discharge of r o o f drains.MECHANICS OF SWELLING 35 In s a t u r a t e d clay a is u n i t y . Aichison and R i c h a r d s [ 3 1 ] claim t h a t in u n s a t u r a t e d soils. T h e u p p e r p o r t i o n of curve 3 can e x t e n d b e y o n d curve 1 in very w e t seasons and b e h i n d curve 1 in dry seasons. t h e influence of e v a p o r a t i o n decreases w i t h d e p t h and at s o m e d e p t h . C o n s e q u e n t l y . t h e m o i s t u r e c o n t e n t equilibrium remains t h e same as t h e covered c o n d i t i o n . n o c o n s i d e r a t i o n has been given t o t h e m a n . planting of trees and s h r u b s . t h e t y p e of soil. and t h e location of t h e w a t e r table. f o r m a t i o n of drainage channels and swales. . H ^ . However. H s. During w e t m o n t h s w i t h heavier p r e c i p i t a t i o n and higher h u m i d i t y . t h e greater t h e d e p t h of desiccation. and t h e possibility of utility line leakage will all increase t h e value of H s. In areas w h e r e p r e c i p i t a t i o n and evaporation are fairly c o n s t a n t . T h e value of H ^ d e p e n d s c n t h e climate c o n d i t i o n . moisture Depth of moisture fluctuation K r a y n s k i [ 2 2 ] explained t h e m o i s t u r e c o n t e n t variation w i t h d e p t h in a h o m o g e n e o u s soil b y figure 14. t h e classic D a r c y ' s Law should be modified t o c o n f o r m with flow of w a t e r in u n s a t u r a t e d soils. T h e calculated m o i s t u r e will c o r r e s p o n d t o m e a s u r e d field c o n t e n t if field p o r e pressure can b e accurately d e t e r m i n e d [ 3 0 ] . T h e m a x i m u m d e p t h of H(j is equal t o t h e d e p t h t o t h e w a t e r table. It should b e n o t i c e d t h a t in t h e above evaluation of t h e d e p t h H s. t h e p o r e pressures are negative and t h a t t h e p e r m e a b i l i t y Κ is n o longer assumed t o b e a c o n s t a n t . T h e h o t t e r and drier t h e climate. It is n o t u n c o m m o n t h a t H s d e p t h can reach as m u c h as 25 feet (see c h a p t e r 7). When a long d r o u g h t is followed b y an intense rainfall. In u n s a t u r a t e d clay. The* d e p t h of seasonal m o i s t u r e c o n t e n t f l u c t u a t i o n . t h e m o i s t u r e c o n t e n t of near-surface soil increases and t h e m o i s t u r e profile represented b y curve 2 alters its shape t o curve 3. w i t h o u t direct m e a s u r e m e n t . It is impossible t o d e t e r m i n e t h e value of H(j. T h e watering of lawns. E v a p o r a t i o n causes loss of m o i s t u r e in t h e soil n e a r t h e ground surface. In a covered area. t h e m o i s t u r e profile is s h o w n b y curve 1. Curve 2 indicates t h e m o i s t u r e c o n t e n t variation w i t h d e p t h in t h e same area in uncovered natural c o n d i t i o n s . t h e soil m o i s t u r e c o n t e n t . T h e m o i s t u r e c o n t e n t of t h e soil decreases w i t h d e p t h . (After Kraynski.Dessicated Moisture Content H $. Moisture content variation with depth below ground surface.Dessicated Moisture Content 3 .Wet Season Moisture Content 2 .Depth of Seasonal Moisture Content Fluctuation H d-Depth of Dessication Figure 14. 1967) .36 FOUNDATIONS ON EXPANSIVE SOILS INCREASING MOISTURE CONTENT OF SOIL 2 . such m o v e m e n t w o u l d n o t have been n o t e d had n o t a precise leveling b e e n c o n d u c t e d . An u n u s u a l o p p o r t u n i t y afforded t h e m a k i n g of a precise m e a s u r e m e n t o f a school building founded o n piers. Seasonal fluctuation of m o i s t u r e c o n t e n t along t h e edges of highway p a v e m e n t o r parking areas can be e x p e c t e d . In t h e course of investigating a cracked building. Since m o i s t u r e transfer is a slow process. T h e m o v e m e n t of t h e piers was m e a s u r e d each m o n t h for a period of 11 m o n t h s . b u t at t h e central p o r t i o n of a covered area. t h e p h e n o m e n o n of swelling and shrinkage can be explained at t h e same t i m e . t h e m o v e m e n t graph lags b e h i n d the p r e c i p i t a t i o n graph. The shifting of t h e m o i s t u r e profile of a swelling soil from t h e natural conditions represented b y curves 1 and 2 t o curve 3 in figure 14 for covered c o n d i t i o n s is t h e cause of significant d a m a g e . it is n o t surprising t h a t distress of a building often takes place several years after o c c u p a n c y . Theoretically. T h e m o v e m e n t of b o t h t h e interior and e x t e r i o r piers was r e c o r d e d and p l o t t e d along w i t h t h e average m o n t h l y p r e c i p i t a t i o n . It has b e e n claimed t h a t shrinkage is t h e m i r r o r reflection of e x p a n s i o n . t h e d e p t h of seasonal m o i s t u r e c o n t e n t fluctuation H s can a p p r o a c h t o d e p t h of desiccation H ^ . shrinkage can result in s e t t l e m e n t . It is e x p e c t e d b y measuring soil s u c t i o n w h i c h evaluates t h e t o t a l negative pressures in t h e soil-water s y s t e m . t h e difference b e t w e e n t h e m a x i m u m and m i n i m u m pier m o v e m e n t seldom exceeded one-half inch. It is the c o n t i n u o u s increase in m o i s t u r e c o n t e n t b e n e a t h t h e covered area t h a t i n t r o d u c e s the d a m a g e in an expansive soil area. In fact. sidewalks o r a p r o n s . m u c h a t t e n t i o n has b e e n focused b y various investigators o n t h e m e c h a n i c s of shrinkage. In t h e school building previously discussed. b u t lags b e h i n d rainfall [ 3 2 ] . evaporation is blocked or partially r e t a r d e d . it is n o t u n u s u a l t o find t h a t t h e m o i s t u r e c o n t e n t of t h e soils b e n e a t h t h e covered area or in the vicinity of t h e covered area had substantially increased. capillary action. Such m o v e m e n t was insufficient t o manifest noticeable d a m a g e t o t h e building.MECHANICS OF SWELLING 37 When areas are covered b y structures such as buildings. T h e m o i s t u r e c o n t e n t b e n e a t h t h e covered area increases d u e t o gravitational migration. shrinkage seldom takes place even u n d e r a prolonged arid climate. in the course of several years. p a v e m e n t s . b u t t h e r e is very little evidence that t h e r e is appreciable d o w n w a r d m o v e m e n t u n d e r covered areas in a building. T h e cyclic u p and d o w n m o v e m e n t is believed to o c c u r in phases. for t h e interior piers. v a p o r and liquid t h e r m a l transfer a n d . as s h o w n in figure 15. Shrinkage In reviewing literature o n expansive soil. M o v e m e n t m e a s u r e m e n t s of buildings in good c o n d i t i o n have seldom been performed. T h e end result of shrinkage a r o u n d or b e n e a t h a covered area seldom causes any s t r u c t u r a l d a m a g e . is n o t an i m p o r t a n t item t o be considered b y soil engineers. This is e x p e c t e d since it takes a d d i t i o n a l t i m e for m o i s t u r e or v a p o r t o migrate from t h e e x t e r i o r of t h e building t o t h e interior piers. h e n c e . . it should b e n o t e d t h a t over a period of 11 m o n t h s . It is believed t h a t seasonal m o i s t u r e c o n t e n t fluctuation can result in heaving and settling of t h e s t r u c t u r e . Monitoring cracked building m o v e m e n t c a n n o t reflect t h e effect of seasonal m o i s t u r e variation. However. T h e d r y and w e t p e r i o d s coincide fairly well w i t h t h e u p w a r d and d o w n w a r d m o v e m e n t of the e x t e r i o r piers. T h e r e is an u r g e n t need for u n i f o r m i t y in testing expansive soils. T h e swell index is defined as t h e slope of t h e e-log ρ curve.1 kg per c m 2 w a s used. SWELLING POTENTIAL A l t h o u g h t h e swelling p h e n o m e n o n has b e e n fully recognized for m a n y years. C o n s e q u e n t l y . T e x a s is discussed. T h e p r o p o s e d m e t h o d utilizes t h e c o n v e n t i o n a l fixed-ring consolidometer for c o n d u c t i n g t h e tests. L a m b e [ 3 3 ] used t h e swell index t o m e a s u r e t h e expansion characteristics of clay. T h e m e t h o d also involves shrinkage tests and permeability tests. T h e m e t h o d has b e e n s u b m i t t e d for consideration as a standard t o ASTM C o m m i t t e e D-18 and is given in A p p e n d i x A. Movement of exterior and interior piers in a school building with respect to precipitation. G. T h e standard p r o c e d u r e involves t h e testing of t w o similar samples.38 FOUNDATIONS ON EXPANSIVE SOILS „ MOVEMENT OF PIER MONTHS (SHIFTED) ι N O 1 J 1 1 F 1 M 1 A 1 M 1 J MONTHS 1 1 J 1 A 1 S 1 O 1 N 1 J D Figure 15. after being c o m p a c t e d t o m a x i m u m density at o p t i m u m w a t e r c o n t e n t in t h e standard A A S H O c o m p a c t i o n test. it is n o t t h e same standard as t h e swelling of t h e clay shale in t h e R o c k y M o u n t a i n area. In 1 9 6 2 . T h e difficulty in providing a suitable yardstick for measuring swelling characteristics is t h e presence of t h e n u m e r o u s variables involved. Seed's definition is confined t o r e m o l d e d soil while t h e slope of t h e e-log ρ curve d e p e n d s o n initial-moisture c o n t e n t as well as surcharge pressure.0 t o 0. A s t a n d a r d m e t h o d for general use was p r o p o s e d b y W.e x p a n d e d test and t h e o t h e r for e x p a n d e d . o n e for l o a d e d . T h e pressure i n c r e m e n t from 1. Procedures are given for testing both u n d i s t u r b e d and r e m o l d e d specimens. Seed [ 1 7 ] defined swelling p o t e n t i a l as t h e p e r c e n t a g e of swell of a laterally confined sample on soaking u n d e r a 1-psi surcharge. . H o l t z [ 3 4 ] .a n d .a n d . a definite m e t h o d of measuring t h e swelling p o t e n t i a l of clay has n o t been established. w h e n high swelling soil in San A n t o n i o .l o a d e d test. t o this d a t e . It is clear t h a t b o t h definitions have their limitations. Initial d r y d e n s i t y . Sample thickness affects t h e t i m e required for t o t a l s a t u r a t i o n . In general. T h e t i m e required for t h e soil t o reach its m a x i m u m swell p o t e n t i a l m a y vary considerably d e p e n d i n g essentially o n t h e initial d e n s i t y . Initial m o i s t u r e c o n t e n t . t h e initial c o m p a c t i o n c o n d i t i o n is critical. sample thickness of less t h a n 1 inch should be used. This requires a m i n i m u m curing time of 6 h o u r s for r e p r o d u c i b l e results. t h e d i a m e t e r should n o t b e less t h a n 2 inches. care should b e t a k e n in selecting the sample w i t h t h e m o s t critical m o i s t u r e c o n t e n t . Size and thickness. 2. T o e x p e d i t e testing t i m e . Swell tests m a y d e c i d e t h e degree of c o m p a c t i o n required in t h e p l a c e m e n t of fill. for u n d i s t u r b e d high d e n s i t y clay shale. These and other e n v i r o n m e n t a l c o n d i t i o n s are e x t r e m e l y i m p o r t a n t in d e t e r m i n i n g t h e a m o u n t of swell. Field c o n d i t i o n and c o n s t r u c t i o n specifications d i c t a t e t h e m o i s t u r e requirement. A small surcharge load in t h e range of 0. For remolded samples. 4 . and seams in t h e soil. T h e d i a m e t e r of t h e sample is also significant. t h e initial a d d e d w a t e r m u s t b e evenly d i s t r i b u t e d . a l t h o u g h o n l y vertical rise is m e a s u r e d in t h e c o n s o l i d o m e t e r . t h e larger t h e effect of side friction. Usually. and w h e n t h e w e t t i n g takes place or w h e n t h e load is applied. a great deal of a t t e n t i o n should b e directed t o t h e time e l e m e n t . T i m e allowed for swell. . In a d d i t i o n . In testing u n d i s t u r b e d samples. " t h e i m p o r t a n c e of initial moisture content and initial d r y d e n s i t y has b e e n discussed. 3 . T h e smaller t h e d i a m e t e r . fissures. T h e following factors influence t h e results o b t a i n e d in loaded swell tests o n soils of any mineralogical composition: 1. it m a y require several d a y s or even a w e e k before c o m p l e t e s a t u r a t i o n can be achieved.35 t o 1 psi has b e e n suggested for a seating load in t h e swell test.MECHANICS OF SWELLING Factors affecting volume change 39 In t h e preceding c h a p t e r u n d e r t h e "Physical P r o p e r t i e s of Expansive S o i l . and t h e thickness of t h e sample. G r e a t e r thickness m a y i n t r o d u c e excessive side friction. t h e use of low surcharge pressure m a y lead t o erratic and e r r o n e o u s test results. A t t h e same t i m e . Sample d i a m e t e r is c o n t r o l l e d b y a sampling device. F o r r e m o l d e d samples. t h e y should b e e x a m i n e d c o n c u r r e n t l y . it is r e c o m m e n d e d t h a t this value b e used for a surcharge load. tests should b e p e r f o r m e d o n t h e driest sample. Surcharge pressure. A t t h e s a m e t i m e . generally 2 4 h o u r s is sufficient t o o b t a i n 9 5 p e r c e n t of t h e total available swell.000 psf o n the soil. S t a n d a r d i z a t i o n o n b o t h size and thickness o f sample is a p p a r e n t l y necessary for c o m p a r a b l e results. Detailed discussions are given later u n d e r "Swelling P r e s s u r e . F r e q u e n c y of testing is i m p o r t a n t so as t o cover all possible c o n d i t i o n s . 5. Since swell is very sensitive t o changes in pressure in t h e lower ranges of pressure (less t h a n a b o u t 1 psi). Since m o i s t u r e c o n t e n t and d r y d e n s i t y are closely related. p e r m e a b i l i t y . Since m o s t footing f o u n d a t i o n s can exert a pressure of a b o u t 1.t h e t i m e elapsed b e t w e e n sampling and testing and t h e t i m e elapsed w h e n t h e sample is placed in t h e c o n s o l i d o m e t e r . T h e single m o s t i m p o r t a n t factor affecting swelling characteristics of swelling soils is d e n s i t y . in a c u t t i n g from an u n d i s t u r b e d s a m p l e . " In r e m o l d e d tests. F o r r e m o l d e d samples. it is o b v i o u s t h a t t h e initial m o i s t u r e c o n t e n t will c o n t r o l t h e v o l u m e change. small thickness m a y i n t r o d u c e surface d i s t u r b a n c e and e x c l u d e t h e possible effect of granular particles. little d a m a g e t o t h e f o u n d a t i o n can t a k e place. " T h e d e p t h and degree of desiccation affects the amount of swell in a given soil h o r i z o n . t h e swelling characteristics will vary greatly w i t h t h e variation of one or m o r e of t h e above environmental or placement conditions. S o m e of t h e variables are described as follows: 1. as outlined above. Soil profile. therefore. Climate c o n d i t i o n partially affects t h e desiccation. it is obvious t h a t for a certain clay w i t h k n o w n p r o p e r t i e s . G r o u n d w a t e r . With the i n t r o d u c t i o n of t h e c o n c e p t of swelling pressure. leads t o t h e prediction of t o t a l heave o r t h e m a x i m u m p o t e n t i a l m a g n i t u d e of heaving of a s t r u c t u r e . T h e in situ variables involved are always m o r e c o m p l i c a t e d t h a n those used in l a b o r a t o r y tests. o n e set of s t a n d a r d s can hardly cover t h e very c o m p l i c a t e d variables involved.40 FOUNDATIONS ON EXPANSIVE SOILS F r o m t h e preceding analysis. Climate. T h e thickness of t h e swelling soil s t r a t u m is. the p o t e n t i a l total heave will be small. 2. and receding g r o u n d w a t e r s o m e t i m e s c o n t r i b u t e s to a d d i t i o n a l swelling. However. A reliable and r e p r o d u c i b l e test w h i c h is t o be considered as a basis for t h e classification of p o t e n t i a l expansive soil m u s t b e s t a n d a r d i z e d at least for t h e following e n v i r o n m e n t a l c o n d i t i o n s : F o r u n d i s t u r b e d sample: Surcharge pressure Size and thickness T i m e required for t h e test Initial m o i s t u r e c o n t e n t and density m u s t b e qualified F o r r e m o l d e d sample: Initial m o i s t u r e c o n t e n t Initial d r y d e n s i t y M e t h o d of c o m p a c t i o n Surcharge pressure Size and thickness T i m e required for t h e test Curing t i m e allowed Clearly. As long as t h e thickness of t h e swelling soil is less t h a n 2 4 inches. e v a p o r a t i o n and transpiration affect t h e m o i s t u r e in t h e soil as previously discussed u n d e r " M o i s t u r e T r a n s f e r . It w o u l d be e r r o n e o u s to c o m p a r e the swelling characteristics of different soils w i t h o u t first clearly defining t h e p l a c e m e n t c o n d i t i o n . swelling soil located b e l o w g r o u n d w a t e r will n o t pose a p r o b l e m t o t h e s t r u c t u r e . " Total heave T h e d e t e r m i n a t i o n of swelling p o t e n t i a l in t h e l a b o r a t o r y . T h e thickness of expansive soil u n d o u b t e d l y affects t h e m a g n i t u d e of t o t a l heave. Damaging swelling soil p r o b l e m s are . At t h e same t i m e . If t h e thickness of high swell p o t e n t i a l soils is thin. Climate c o n d i t i o n s involving p r e c i p i t a t i o n . Evidently. t h e above e n v i r o n m e n t a l c o n d i t i o n can be simplified and test results can be c o m p a r e d as later explained u n d e r "Swelling P r e s s u r e . g r o u n d w a t e r fluctuates. for a thick s t r a t u m of low swelling soil. t h e t o t a l heave can be considerable over a long period of t i m e . 3 . limited b y the d e p t h t o g r o u n d water. especially w h e r e only a single sample is used t o represent an entire f o u n d a t i o n . and t h e p a t t e r n of m o i s t u r e m i g r a t i o n . S e t t l e m e n t d u e t o load i n c r e m e n t Δ Ρ can be calculated from t h e change of void-ratio e 0 . T h e general test p r o c e d u r e is given as follows: T w o c o n s o l i d o m e t e r rings are filled w i t h u n d i s t u r b e d samples from adjacent l o c a t i o n s . T h e second sample is flooded w i t h w a t e r at low pressure of 2 0 psf. T h e d o u b l e o e d o m e t e r m e t h o d developed b y Jennings and Knight is based on t h e c o n c e p t of effective stress and has received wide a t t e n t i o n . All suggested m e t h o d s have limitations. S e t t l e m e n t of clay u n d e r load will t a k e place w i t h o u t the aid of w e t t i n g . T h e t o t a l a m o u n t of heave d e p e n d s o n t h e e n v i r o n m e n t a l c o n d i t i o n . T h e void-ratio at t h e o v e r b u r d e n pressure is e G. a c o n s o l i d a t i o n test is perfor«med in t h e c o n v e n t i o n a l m a n n e r . T h e search b y various investigators for a reliable m e t h o d for predicting t h e t o t a l heave is p r o b a b l y affected b y t h e c o n c e p t of u l t i m a t e s e t t l e m e n t in t h e t h e o r y of c o n s o l i d a t i o n . T h e d o u b l e o e d o m e t e r m e t h o d is based o n t h e a s s u m p t i o n t h a t t h e r e is a p o i n t d u r i n g c o m p r e s s i o n at w h i c h t h e initially u n s a t u r a t e d soils pass from an applied pressure t o an effective p h e n o m e n o n and t h e c o m p r e s s i o n curve joins w i t h t h e virgin consolidation curve. t h e d u r a t i o n of w e t t i n g . resulting in e 0 having a n e w effective pressure P 0 + U L represented in t h e u p p e r saturated curve b y P 0 + U L and e 2. If n o load is applied.MECHANICS OF SWELLING 41 seldom e n c o u n t e r e d for a soil profile w i t h g r o u n d w a t e r located s h o r t distances b e n e a t h t h e footings. t h e L a m b e and W h i t m a n m e t h o d [ 3 3 ] . F o r m a n y years engineers have b e e n familiar w i t h t h e calculation of u l t i m a t e s e t t l e m e n t and differential s e t t l e m e n t of a s t r u c t u r e f o u n d e d on clay. S o m e of t h e m are as follows: 1. 2. If D is t h e d e p t h t o t h e w a t e r table and Y w is t h e u n i t weight of water. A soil sample t a k e n at d e p t h ζ has an o v e r b u r d e n pressure P 0 = Yz w h e r e Y is t h e d e n s i t y of t h e soil. while e x p a n s i o n of clay will n o t be realized w i t h o u t m o i s t u r e increase. U L = Y w (D . T h e t w o c o m p r e s s i o n curves are p l o t t e d o n t h e same diagram and o n e of t h e curves is selected for vertical a d j u s t m e n t so as t o coincide w i t h t h e virgin sections of t h e curves as s h o w n o n figure 16. These are t h e d o u b l e o e d o m e t e r m e t h o d [ 3 5 1 . V a r i o u s m e t h o d s have b e e n p r o p o s e d t o predict t h e a m o u n t of total heave u n d e r a given structural load. 4 . good surface drainage a r o u n d t h e s t r u c t u r e r e d u c e s t h e swelling p r o b l e m as discussed in detail in c h a p t e r 8.z). T h e r e are s o m e f u n d a m e n t a l differences b e t w e e n t h e behavior of settling and heaving soil. . T h e effect of t h e load i n c r e m e n t is t h e n again t a k e n i n t o c o n s i d e r a t i o n and t h e final values are ( P 0 + U L + Δ Ρ and e 3 ). and the F H A m e t h o d [ 3 7 ] . Typical heave calculations are s h o w n o n table 5. t h e soil u n d e r a covered area will gain m o i s t u r e a n d swelling will t a k e place. T h e final c o n d i t i o n s of m o v e m e n t m a y t h e n be p r e d i c t e d b y adding t h e voidration changes over t h e w h o l e profile. After t h e sample is fully w e t t e d . As e x p e c t e d . and it is assumed t h a t t h e total heave can also be p r e d i c t e d . T h e first sample is k e p t at its n a t u r a l m o i s t u r e c o n t e n t and a confined c o m p r e s s i o n test is p e r f o r m e d . t h e McDowell m e t h o d [ 3 6 ] . and c o n s e q u e n t l y .t l . a n y t o t a l heave p r e d i c t i o n c a n b e entirely erroneous. T h e c o n d i t i o n will alter P 0 . Capillary m o v e m e n t and v a p o r transfer will b e of such m a g n i t u d e t h a t it takes only a s h o r t period of t i m e before t h e thin layer of swelling soil below t h e s t r u c t u r e becomes completely saturated. such as t h e e x t e n t of w e t t i n g . Such variables c a n n o t b e ascertained. Drainage. 42 FOUNDATIONS ON EXPANSIVE SOILS Figure 16. Log P curves showing adjustment to bring straight line portions coincident. (After Jennings & Knight, 1958) 3. Differential settlement is usually described as a percent of the ultimate settlement. However, in the case of expansive soils, one corner of the building may be subjected to maximum heave due to excessive wetting while another corner may have no movement. Therefore, in the case of swelling soils, differential heaving can equal the total heave. No correlation between differential and total heave can be established. Effective stress Terzaghi developed the principle of effective stress in the early 1920's. It is believed that the principle can be applied to all soil behaviors. In gernal terms, the principle of the effective stress states that the behavior of an element of soil mass depends not on the total stress applied to the element but rather on the difference between this total stress and the stress present in the pore fluid. For saturated soil, the effective stress is defined as: = G-yL where: - effective stress - total normal stress M = stress in pore fluid or pore water pressure MECHANICS OF SWELLING Table 5-. Results of heave calculations 1 2 3 4 5 6 7 Depths at top and bottom of layer, feet Depth of double Oedom. test, feet VOID RATIO 8 9 10 <u 43 11 ο ο 12 Β S) Β ra Χ) 13 <5 C O Β Ό g ^ ΔΡ at mean depth, ton/sq. ft. P0 at mean depth, ton/sq. ft. UL at mean depth, ton/sq. ft. • o e ο .s ^ ο C Layer No. o ei e2 e3 Ο Ο) > ι ο D U I-H 00 Β & ë ι ο s κ * — Iπ c Ό =• 1 ζ r »° 1+ 1.342 0.525 0.474 0.343 0.524 0 3.208 1 ζ « 5 •j A f 5 6 4.006.56 6.568.87 8.8710.80 10.8013.35 13.3518.81 18.81- 5.25 7.87 9.87 11.74 14.85 18.87 0.30 0.45 0.59 0.73 0.97 1.21 0.10 0.07 0.06 0.04 0.04 0.04 0.97 0.89 0.84 0.78 0.70 0.59 0.419 0.428 0.418 0.544 0.626 0.523 0.418 0.427 0.417 0.543 0.625 0.522 0.492 0.460 0.449 0.563 0.640 0.524 TOTALS: 0.481 0.455 0.447 0.562 0.639 0.523 1.585 0.623 0.506 0.362 0.565 0.017 3.658 0.021 0.019 0.016 0.020 0.040 0.017 0.133 (After Jennings & Knight, 1958) T h e preceding e q u a t i o n is based on saturated soils. F o r partly saturated soils, c o n s i d e r a t i o n should b e given t o t h e fact t h a t p o r e pressure acts o n l y o n p a r t of t h e area of a n y plane t h r o u g h t h e soil. In a d d i t i o n , an electro-chemical force of a t t r a c t i o n and repulsion force m a y act t h r o u g h t h e spaces n o t filled w i t h w a t e r . C o n s e q u e n t l y , t h e effective e q u a t i o n should b e modified b y adding t h e v e c t o r sum of attractive forces and s u b t r a c t i n g t h e v e c t o r sum of repulsive forces. Aichison and Richards [ 3 1 ] s t a t e d , " I t is n o t o n l y possible b u t essential for t h e effective stress principle t o be used in t h e quantification of expansive soil b e h a v i o r . " L a m b e [ 3 3 ] stated in 1959 after m a k i n g in-depth research on t h e application of t h e c o n c e p t of effective stress t o explain t h e swelling soil behavior, " O n e w o n d e r s w h e t h e r , for e x a m p l e , we are hindering o u r u n d e r s t a n d i n g of the n a t u r e of t h e e x t r e m e l y plastic swelling soils b y forcing t h e m t o fit t h e effective stress c o n c e p t . " T h e a u t h o r shares t h e t h i n k i n g of L a m b e and believes t h a t t h e m e c h a n i c s of expansive soil are totally different from t h e t h e o r y of c o n s o l i d a t i o n and t h e shearing s t r e n g t h of soils. It should be considered as a n e w phase of soil m e c h a n i c s . SWELLING PRESSURE In the course of t h e last 15 years, t h o u s a n d s of swell tests were c o n d u c t e d b y t h e a u t h o r o n various k i n d s of expansive soils found in t h e R o c k y M o u n t a i n area. T h e s t a n d a r d p r o c e d u r e used t o c o n d u c t these tests is t o place t h e u n d i s t u r b e d sample in a c o n s o l i d o m e t e r u n d e r a surcharge 44 FOUNDATIONS ON EXPANSIVE SOILS load of 1,000 psf ( a b o u t 7 psi) for 2 4 h o u r s , s a t u r a t e t h e sample, m e a s u r e and record t h e a m o u n t of v o l u m e change. T h e a m o u n t of v o l u m e change exhibited b y various soils u n d e r various p l a c e m e n t c o n d i t i o n s varies greatly. F u r t h e r m o r e , it was found t h a t soil o b t a i n e d from b e n e a t h a s t r u c t u r e t h a t has u n d e r g o n e severe f o u n d a t i o n m o v e m e n t m a y n o t possess high swell p o t e n t i a l . It was suspected t h a t t h e r e m u s t b e a single soil p r o p e r t y t h a t governs t h e swelling characteristics. After t h e sample had swelled t o its m a x i m u m e x t e n t , the specimen was loaded until it r e t u r n e d to its initial v o l u m e and t h e pressure r e q u i r e d t o d o this was designated as swelling pressure. It is suspected t h a t t h e swelling pressure is t h e built-in p r o p e r t y of expansive soil and will n o t be affected b y p l a c e m e n t c o n d i t i o n or e n v i r o n m e n t a l c o n d i t i o n . Test procedure T h e clay selected for m a k i n g this s t u d y is a c l a y s t o n e shale typical of t h o s e found in s o u t h e a s t Denver. Such soil has caused a great deal of d a m a g e t o lightly loaded s t r u c t u r e s such as residential houses. Most of t h e h o u s e s in this area are f o u n d e d w i t h piers b o t t o m e d in a z o n e where a change of m o i s t u r e c o n t e n t is unlikely t o t a k e p l a c e ; however, slabs placed on such soil have experienced generally severe m o v e m e n t . Uplift m o v e m e n t in excess of 6 inches is n o t uncommon. T h e physical p r o p e r t i e s of such clay are as follows: Liquid limit Plasticity i n d e x Shrinkage limit Sand Silt Clay ( p e r c e n t smaller t h a n 0 . 0 0 5 m m ) Optimum moisture content M a x i m u m d r y d e n s i t y (standard P r o c t o r test) F r e e swell ( U S B R m e t h o d ) Specific gravity 108.4 pcf 75.0% 2.67 44.4% 24.4% 14.5% 0% 63.0% 37.0% 19.5% In a d d i t i o n t o t h e above, t h e mineral c o n t e n t , in p e r c e n t , of such clay is as follows: Montmorillonite Calcite Quartz Feldspar Kaolinite 25.0 5.0 25.0 10 t o 2 5 . 0 5.0 Because of t h e erratic f o r m a t i o n of n a t u r a l soil strata, for research p u r p o s e s , it will be necessary t o use only r e m o l d e d samples so t h a t the p l a c e m e n t c o n d i t i o n can b e d u p l i c a t e d . T h e air d r y sample is prepared b y passing it t h r o u g h a N o . 4 0 sieve. Moisture c o n t e n t is added t o t h e air d r y clay and t h e n allowed t o age in a sealed c o n t a i n e r for a period of 4 8 h o u r s . MECHANICS OF SWELLING 45 The sample is compacted in a 2-inch-diameter, 1-inch-thick consolidometer ring in three layers with predetermined compaction effort. The required density is obtained as near as possible by an experienced technician. It should be noted that in the test results presented, some deviation of density has taken place which results in some erratic test results. Surcharge pressure It is a well recognized fact that if sufficient load is applied on an expansive clay, the detrimental volume increase can be controlled. The surcharge pressure applied to the soil sample in the consolidometer simulates the dead load pressure exerted on the footings or pier foundation. Figure 17 and table 6 indicate that with a surcharge pressure of 1,000 psf, upon wetting the clay swelled 5.9 percent with a swelling pressure of 12,000 psf. By increasing the surcharge pressure to 5,000 psf, the amount of volume increase was limited to 1.6 percent, but the swelling pressure remains unchanged. Figure 18 indicates the relationship between volume change with surcharge pressure. These curves have a hyperbolic shape and the intersection of the curves with the abscissa indicates the pressure required for zero volume change. This pressure by definition is the swelling pressure. Figure 17. Relationship between surcharge pressure and volume increase for constant density and moisture content samples. 46 FOUNDATIONS ON EXPANSIVE SOILS Table 6—. Effect of varying pressure on volume change and swelling pressure for constant density and moisture content samples. Applied pressure (psf) 1,000 2,000 3,000 5,000 7,000 Average Moisture content, percent Initial 11.90 11.90 11.90 11.90 11.90 11.90 Final 24.58 25.08 24.94 25.02 25.49 25.02 Initial density, pcf 105.58 106.08 105.96 105.90 105.96 105.95 Volume increase, percent 5.90 3.90 2.80 1.60 1.00 Swelling pressure, psf 12,000 13,000 12,000 12,500 12,500 12,400 SURCHARGE PRESSURE ( p s f ) Figure 18. Effect of varying pressure on volume change for constant density and moisture content sample. Surcharge load is essential to control foundation m o v e m e n t . With swelling pressure d e t e r m i n e d , a reasonable f o u n d a t i o n design can be a p p r o a c h e d . If t h e swelling pressure is n o t excessive, say, on t h e o r d e r of 5 , 0 0 0 psf, a spread footing f o u n d a t i o n can be used. T h e r e q u i r e m e n t will b e t o assign a m i n i m u m dead load pressure of 5,000 psf so t h a t the v o l u m e change of t h e soil will n o t be allowed even in excessive w e t t i n g c o n d i t i o n . In t h e design of t h e A differential uplift of three-fourths of an inch generally is considered t o b e tolerable. F r o m t h e pressure versus v o l u m e change curve and t h e tolerable uplift. With 3/4-inch allowable differential uplift.25 Swelling pressure. Sufficient t i m e was allowed for all a d d e d w a t e r t o soak i n t o t h e sample.0 105.6 106.500 12.53 6. Moisture content. a t t e n t i o n m u s t be given t o t h e additional swelling effect o n t h e shaft of t h e pier e m b e d d e d in swelling soil.100 . a w o r k i n g swelling pressure can be established. F r o m figure 19 it is seen t h a t t h e a m o u n t of v o l u m e change Table 7 .0 82. t h e required dead load pressure can b e drastically r e d u c e d . t o achieve t h e same effect in t h e l a b o r a t o r y t h e degree of s a t u r a t i o n on the sample was varied.9 106.MECHANICS OF SWELLING 47 footing f o u n d a t i o n .0 67. F o r highly swelling soil w i t h swelling pressure in excess of 5. it is advisable t o actually d e t e r m i n e t h e swelling pressure.66 9.3 93. Since it is difficult in a short sample height t o c o n t r o l t h e d u r a t i o n of w e t t i n g . percent 1. F o r a rational design.35 4. and a m e a s u r e d a m o u n t of w a t e r was t h e n i n t r o d u c e d i n t o the sample. Degree of saturation This series of tests was p e r f o r m e d t o s t u d y t h e effect of d u r a t i o n of w e t t i n g o n swelling characteristics.000 15. Since t h e v o l u m e change for surcharge pressure of 1 psi and t h e v o l u m e change u n d e r a surcharge pressure of 10 psi will vary considerably.66 9. pcf 106. A n c h o r a g e of t h e pier in a z o n e n o t affected b y m o i s t u r e change should b e used t o assist the dead load pressure r e q u i r e m e n t .000 15. T h e swelling pressure e x e r t e d o n t h e shaft of t h e pier can b e m a n y times greater t h a n t h e pressure e x e r t e d at t h e b o t t o m of the pier.000 17.66 rage 9. However.000 psf. It is a well established p h e n o m e n o n t h a t p r o l o n g e d w e t t i n g will result in m o r e damage t o a s t r u c t u r e t h a n s h o r t d u r a t i o n w e t t i n g .83 3. 0 0 0 psf in a small d i a m e t e r pier.93 Degree of saturation. Dead load pressure alone generally is n o t sufficient t o prevent t h e uplifting of t h e pier.000 15.7 105. percent 61.0 86.66 Final 13. T h e samples were c o m p a c t e d in t h e c o n s o l i d o m e t e r w i t h u n i f o r m d e n s i t y and m o i s t u r e c o n t e n t .07 14.66 9.6 106.50 19.35 5.58 18. Effect of varying degree of saturation on volume change and swelling pressure for constant density and moisture content samples. a pier f o u n d a t i o n will be required t o c o n c e n t r a t e t h e dead load pressure o n a small area. it m a y be possible t o allow certain a m o u n t s of uplift m o v e m e n t so as t o minimize t h e required dead load pressure. percent Initial 9.2 Volume increase.53 17.0 Initial density. It is n o t difficult t o assign a dead load pressure in excess of 2 0 .66 9. t h e m e r i t of using swelling pressure as a direct m e a s u r e m e n t of t h e swelling characteristics can be seen at o n c e .. psf 16. Uplift m o v e m e n t can be tolerated in certain s t r u c t u r e s in t h e same m a n n e r as s o m e s e t t l e m e n t can be t o l e r a t e d in m o s t s t r u c t u r e s . T h e a m o u n t of v o l u m e change and t h e swelling pressure recorded are s h o w n o n table 7. . Effect of varying degree of saturation on volume change for constant density and moisture content sample.48 FOUNDATIONS ON EXPANSIVE SOILS Figure 19. It has been a c o m m o n practice t o install drain tiles a r o u n d the building in an a t t e m p t t o remove free w a t e r and to stop foundation movement. . This is t h e reason w h y it is so difficult t o c o n t r o l slab m o v e m e n t for slab-on-ground construction. a short d u r a t i o n w e t t i n g can cause equally heavy damage t o lightly loaded s t r u c t u r e s as long d u r a t i o n wetting. W i t h o u t s u p e r i m p o s e d load. an a t t e m p t is m a d e t o Figure 20. Since the swelling pressure remains c o n s t a n t . Initial moisture content Expansive soils will n o t be subject t o v o l u m e change unless t h e r e is an increase in m o i s t u r e c o n t e n t . A subdrain will n o t arrest the migration of moisture. t h e swelling of t h e soil c a n n o t be controlled even w i t h a m i n i m u m a m o u n t of m o i s t u r e change. C o n s e q u e n t l y . Relationship between degree of saturation and volume increase for constant density and moisture content samples. In this series of tests. C o m p l e t e s a t u r a t i o n is n o t required t o result in a large v o l u m e change. Figure 20 indicates t h a t t h e swelling pressure is c o n s t a n t . A drier soil will swell m o r e t h a n a wet soil. This reflects directly in t h e m i s c o n c e p t i o n t h a t b y removing free w a t e r t h e swell can be controlled. swelling can be substantial. or the pressure required t o m a i n t a i n c o n s t a n t v o l u m e is i n d e p e n d e n t of t h e d u r a t i o n of w e t t i n g or the degree of s a t u r a t i o n .MECHANICS OF SWELLING 49 increases in direct p r o p o r t i o n t o t h e degree of s a t u r a t i o n at t h e end of test. Figure 22 indicates t h e variation of m o i s t u r e c o n t e n t versus v o l u m e change. t h e soils w i t h low initial m o i s t u r e c o n t e n t swell m o s t . Figure 21 s h o w s the results of a series of tests indicating the a m o u n t of v o l u m e change of. footings f o u n d e d o n swelling soil will experience the same swelling pressure Figure 21. Kassiff & Baker in 1971 [ 3 8 ] stated t h a t if clay is given e n o u g h time for aging. and s o m e variation in swelling pressure occurs. T h e result s h o w n in table 8 confirms this s t a t e m e n t . F o r all practical p u r p o s e s . D u e to variation in l a b o r a t o r y controlled c o n d i t i o n s . Relationship between initial moisture content and volume increase for constant density samples. t h e v o l u m e change should be negligible. t h e swelling pressure required for zero v o l u m e change r e m a i n e d practically c o n s t a n t . T h u s . t h e swelling pressure is a c o n s t a n t value. soil samples c o m p a c t e d at c o n s t a n t density b u t varying m o i s t u r e c o n t e n t s . Table 8 indicates t h a t the swelling pressure for t h e various m o i s t u r e c o n t e n t s ranges from 7 .50 FOUNDATIONS ON EXPANSIVE SOILS d e t e r m i n e t h e effect of increasing t h e initial m o i s t u r e c o n t e n t o n t h e v o l u m e change as well as swelling pressure. T h e results of t h e above l a b o r a t o r y tests indicate t h a t the increase of m o i s t u r e c o n t e n t of an expansive soil is n o t a positive m e t h o d of controlling t h e e x p a n s i o n of the soil. 0 0 0 t o 12. As e x p e c t e d . that for the same dry d e n s i t y . However. t h e initial d e n s i t y is n o t entirely c o n s t a n t . These tests indicate t h a t w i t h m o i s t u r e c o n t e n t slightly higher t h a n o p t i m u m m o i s t u r e c o n t e n t . the slope of t h e e-log ρ curve decreases as t h e initial m o i s t u r e c o n t e n t increases. . however. Even w i t h high m o i s t u r e c o n t e n t . t h e swell pressure is n o t affected b y m o i s t u r e c o n t e n t .500 psf. MECHANICS OF SWELLING 51 Ο» 0 1 1 5 10 Moisture I 15 Content (%) I 20 \ 25 Figure 22. Effect of varying moisture content on volume changes from constant density samples. 52 FOUNDATIONS ON EXPANSIVE SOILS Table 8 - . Effect of varying moisture content on volume change and swelling pressure for constant density samples. Initial density, pcf 106.97 105.93 106.27 105.60 106.47 106.37 105.46 105.73 106.35 îrage 106.13 Moisture content, percent Initial 5.84 9.95 10.77 12.48 12.92 14.84 17.97 18.59 19.37 Final 20.34 20.77 18.75 22.09 20.54 19.59 18.50 19.41 20.18 20.02 Volume increase, percent 7.71 5.55 5.03 4.30 3.48 3.30 2.15 1.38 0.75 Swelling pressure, psf 9,500 9,500 12,500 9,500 9,000 10,500 7,000 7,500 9,000 9,333 and the same a m o u n t of dead load pressure will be required to insure zero v o l u m e change. High moisture c o n t e n t soils will experience less uplift, b u t t h e pressure required t o maintain c o n s t a n t volume will n o t be altered. This also indicates t h a t t h e c o m m o n l y accepted p r o c e d u r e of prewetting t h e f o u n d a t i o n excavation t o eliminate t h e swelling characteristics is n o t a reliable p r o c e d u r e . Wetting of t h e f o u n d a t i o n soil, if it can be accomplished, can only serve t o decrease the a m o u n t of swelling. A f o u n d a t i o n placed on such soil will still require t h e same a m o u n t of dead load pressure. Soil engineers are often deceived b y the low volume change of a high m o i s t u r e c o n t e n t soil. If such soils possess a high swelling pressure, t h e y will cause severe damage t o structures if allowed t o dry and are s u b s e q u e n t l y w e t t e d . Stratum thickness L a b o r a t o r y research has been further e x t e n d e d t o explore t h e effect of s t r a t u m thickness on the a m o u n t of v o l u m e change and swelling pressure. In this series of tests, t h e sample thickness ranged from sample. As could be p r e d i c t e d , from t h e results s h o w n in figure 2 4 , t h e m a g n i t u d e of t h e v o l u m e change is p r o p o r t i o n a l t o t h e sample thickness and the percentage of v o l u m e increase remains c o n s t a n t . T h e shape of t h e e-log ρ curve remains almost identical for various sample thicknesses (fig. 2 3 ) and t h e swelling pressure is c o n s t a n t (table 9 ) . This series of tests indicates t h a t if t h e weight of a s t r u c t u r e is capable of exerting pressure t o various d e p t h s b e n e a t h t h e footing w i t h equal i n t e n s i t y , t h e n t h e v o l u m e increase can be arrested. U n f o r t u n a t e l y , d e a d load pressure e x e r t e d on t h e footing can only c o n t r o l v o l u m e change of t h e near surface soils. A t lower d e p t h s , pressure exerted on t h e footing is distributed 1/2 t o 1-1/2 inches. Again, t h e samples were c o m p a c t e d t o uniform m o i s t u r e c o n t e n t and density and sufficient t i m e was allowed for c o m p l e t e s a t u r a t i o n of t h e thickest MECHANICS OF SWELLING 53 over a larger area and is n o t effective in preventing v o l u m e change. D e e p seated swelling is controlled o n l y b y t h e weight of t h e o v e r b u r d e n soil and n o t b y dead load pressure e x e r t e d o n the foundation system. Table 9—. Effect of varying sample thickness on volume change and swelling pressure for constant density and moisture content sample. Initial density, pcf 105.20 106.33 105.31 106.05 106.05 Avg. 105.78 Moisture content, percent Initial 10.10 10.10 10.10 10.10 10.10 10.10 Final 22.30 20.92 21.14 20.49 20.58 21.08 Sample thickness, in. 0.504 0.748 1.007 1.250 1.500 Volume increase, percent 5.66 5.75 5.15 5.60 5.60 5.54 Volume increase, in. 0.0285 0.0430 0.0520 0.0700 0.0840 Swelling pressure, psf 11,000 11,500 11,000 15,000 12,500 12,200 Figure 23. Relationship between sample thickness and volume increase for constant density and moisture content samples. 54 FOUNDATIONS ON EXPANSIVE SOILS Ο 0.50 0.75 Sample Thickness 1.00 (Inches) 1.25 1.50 Figure 24. Effect of varying sample thickness on volume change for constant density and moisture content samples. MECHANICS OF SWELLING 55 Initial density Initial d e n s i t y , w h e t h e r u n d i s t u r b e d or r e m o l d e d , is t h e o n l y e l e m e n t t h a t affects t h e swelling pressure. As seen in figure 25 and table 10 for c o n s t a n t m o i s t u r e samples, t h e v o l u m e change increases w i t h dry d e n s i t y , as does t h e swelling pressure. T h e family of curves have t h e same shape and are a p p r o x i m a t e l y parallel t o each o t h e r . A similar relationship was found b y Kassiff and Shalom [ 3 9 ] . Figure 2 6 establishes a straight line relationship b e t w e e n dry d e n s i t y and v o l u m e change. T h e relationship b e t w e e n t h e d r y density and swelling pressure can b e p l o t t e d either in semi-log scale o r in rectangular scale. F o r t h e semi-log scale, t h e curve is a straight line as s h o w n in figure 2 7 . T h e curve can be expressed as: log y = a x - b where: y= swelling pressure, χ = d r y d e n s i t y , and a and b = c o n s t a n t s d e p e n d i n g o n soil p r o p e r t y and " a " is t h e slope of t h e curve. D r y density and swelling pressure relationship w h e n p l o t t e d in rectangular scale are s h o w n o n figure 2 8 . T h e curve can be expressed b y t h e following e x p o n e n t a l f o r m : y = kcx where: k = 10" c= 10a b It is seen t h a t w h e n d r y d e n s i t y decreases, swelling pressure rapidly a p p r o a c h e s zero. W h e n d r y d e n s i t y increases, swelling pressure rapidly increases and a p p r o a c h e s infinity. T h e soil engineer is i n t e r e s t e d , only w i t h i n a n a r r o w range of d r y d e n s i t y , ranging from 100 t o 130 pcf. Table 1 0 - . Effect of varying density on volume change and swelling pressure for constant moisture content samples. Moisture content, percent Initial density, pcf 94.3 99.4 100.2 103.3 109.1 110.8 114.5 118.9 Average Initial degree of saturation, percent 45.0 48.1 52.1 56.3 65.4 64.7 71.6 81.2 Initial 12.93 12.20 12.93 12.93 12.93 12.20 12.20 12.20 12.55 Final 21.27 24.92 19.93 20.51 20.56 19.03 19.17 17.08 21.08 Volume increase, percent 2.7 3.8 4.2 5.1 6.7 7.3 8.2 8.6 Swelling pressure, psf 2,600 4,600 5,000 7,000 13,000 14,000 21,000 35,000 The swelling pressure of a clay is independent of the surcharge pressure. For remolded soil. At this density the swelling pressure is 12. the standard Proctor density is 108.56 FOUNDATIONS ON EXPANSIVE SOILS Figure 25.4 pcf. the dry density of the soil is 110 pcf corresponding to a swelling pressure of 14. The swelling pressure increases with the increase of initial dry density. and the thickness of the stratum. Conclusions 1. the swelling pressure of remolded clay can be defined as the pressure required to keep the volume of a soil at its Proctor density constant. In other words. It is intresting to note that at the site where the sample is taken in its undisturbed state. Relationship between density and volume increase for constant initial moisture content samples. Therefore. the swelling pressure at the in situ dry density can be used directly to describe the swelling characteristics. . initial moisture content. For undisturbed soil. dry density is the in situ characteristic. In this particular case. swelling pressure can be conveniently used as a yardstick to measure the swelling characteristics of the soils. 2. the swelling pressure varies with degree of compaction. degree of saturation. Since the foregoing established that the swelling pressure of a given soil is a constant and varies only with the dry density. It is useful to introduce maximum Proctor density as a guide.000 psf.000 psf. MECHANICS OF SWELLING 57 90 95 100 105 110 115 120 Dry Density (pcf) Figure 26. Effect of varying density on volume change for constant moisture content samples. . . Effect of varying density on swelling pressure for constant moisture content samples.58 FOUNDATIONS ON EXPANSIVE SOILS 40000 2000 100 105 Dry Density (pcf) Figure 27. Effect of varying density on swelling pressure for constant moisture content samples.000 0 20 40 60 80 DRY DENSITY (pcf) 100 120 140 Figure 28. .000 60.MECHANICS OF SWELLING 59 70. Vol. D. 33 to the Federal Housing Administration.60 FOUNDATIONS ON EXPANSIVE SOILS 3." Proceedings. "Aging Effects on Swell Potential of Compacted Clay.. University of Colorado and Bureau of Public Roads. Highway Research Board. Vol.. State of Colorado." Quarterly of the Colorado School of Mines. Volume Change and Layer Thickness of Soils to the Behavior of Engineering Structures. C . and Baker.. 4. and Whitman. A. [38] Kassiff. 1970. "Interrelationship of Load. "Experimental Relationship Between Swell Pressure and Suction. Vol." International Research and Engineering Conference on Expansive Clay Soils. B. Texas A & M Press. 35. E. No. 1965. E. J. G. G. Report No.... 1953. R. "The Theory and Practice of Construction on Partially Saturated Soils as Applied to South Africa Conditions. and Richards. 5." 5th Edition. [30] "A Review of Literature on Swelling Soils. "Soil Moisture Suction Properties and their Bearing on the Moisture Distribution in Soils. Swelling pressure can be used as a yardstick for measuring swelling soil. October. F o r r e m o l d e d soil the swelling pressure can be defined as t h e pressure required t o k e e p t h e volume of a soil at its m a x i m u m P r o c t o r density c o n s t a n t . and Knight." Second International Research-and Engineering Conference on Expansive Clay Soils. Third International Conference on Soil Mechanics and Foundation Engineering." Building Research Advisory Board. V. and Coleman. Texas A & M Press. J. R. B. 1956." 1957-58 Symposium on Expansive Clays. 54. G." Butterworth. I. Swelling pressure reflects only t h e swelling characteristics of the soil and will n o t b e changed b y p l a c e m e n t c o n d i t i o n s or e n v i r o n m e n t a l c o n d i t i o n s . 1969. T. t h e swelling pressure can be defined as the pressure required t o k e e p t h e v o l u m e of a soil at its n a t u r a l dry density c o n s t a n t ." "Engineering Effects of Moisture Changes in Soils. K. Proc. W. "The Role of Effective Stress in the Behavior of Expansive Soils. 1968. [31] Aichison. South African Institution of Civil Engineers. 1971. March. . [34] "Special Procedures for Testing Soil and Rock for Engineering Purposes. Vol. Australia. Johannesburg. ASTM. 1971. [36] McDowell. [32] Jennings. 4. 97. "The Fundamental Mechanics Involved in Heave and Soil Moisture Movement and the Engineering Properties of Soils which are Important in Such Movement. STP 479. J. F o r u n d i s t u r b e d soil. [37] "Criteria for Selection and Design of Residential Slab-on-Ground. [33] Lambe. Zurich. Vol.1970. [39] Kassiff." Proceeding. 1965. SM 3.. G." Geotechnique. REFERENCES [28] "Moisture Equilibria and Moisture Changes in Soils Beneath Covered Areas. XXI." Journal of the Soil Mechanics and Foundation Division. 1959. No." Department of Highways. [29] Croney. "The Prediction of Total Heave from the Double Oedometer Test. and Shalom. D. D. ASCE. 3. September. [35] Jennings. However. . O n e i m p o r t a n t aspect is t h e selection of a b e n c h m a r k .Chapter 3 FIELD AND LABORATORY INVESTIGATIONS INTRODUCTION T h e stability of a s t r u c t u r e f o u n d e d o n expansive soil d e p e n d s u p o n t h e subsoil c o n d i t i o n s . and m a k i n g a reconnaissance survey. for larger projects a t o p o g r a p h i c survey is available. This is necessary in the investigation of soil for a building a d d i t i o n . Generally. F o r very favorable sites. t h e b e n c h m a r k should be referenced t o establish d a t u m such as an existing building. and possibly even t h e meteorological variations. Topography T h e t o p o g r a p h i c c o n d i t i o n is an essential part of t h e site investigation. g r o u n d surface features. excavating test pits. SITE I N V E S T I G A T I O N Before initiating t h e site investigation. T h e elevation of each test hole should be r e c o r d e d . However. care m u s t be t a k e n t o insure t h a t t h e survey is correct. Since t h e floor level governs t h e selection of f o u n d a t i o n t y p e . this aspect c a n n o t be overemphasized. G r o u n d surface features are controlled b y surface geology and physiography. seismic surveying. This can be accomplished b y reviewing available d a t a . extensive soil investigation will b e necessary even for very m i n o r s t r u c t u r e s . Whenever possible. m a n h o l e invert. s t u d y i n g t o p o g r a p h i c and geologic m a p s . Subsoil c o n d i t i o n s can be e x p l o r e d b y drilling and sampling. cross on a sidewalk. if t h e area is suspected of having swelling soil p r o b l e m s . surficial geology. Generally. t o p of fire h y d r a n t and so forth. A s t u d y of existing structures in t h e i m m e d i a t e vicinity of t h e site can be of prime i m p o r t a n c e in d e t e r m i n i n g t h e t y p e of c o n s t r u c t i o n in an expansive soil area. t h e soil engineer should o b t a i n i n f o r m a t i o n regarding site t o p o g r a p h y . t y p e of c o n s t r u c t i o n . it is t h e small building w i t h i n a d e q u a t e funding. Elaborate site investigation o f t e n t i m e s c a n n o t be c o n d u c t e d d u e t o limited assigned c o n s t r u c t i o n costs. and low-bidding contractor who unwisely economizes in constructing the building t h a t presents t h e m o s t p r o b l e m s . insufficient planning. Many times. T h e soil engineer should n o t accept j o b s in an expensive soil area w h i c h will n o t allow a t h o r o u g h subsoil investigation. e l a b o r a t e site investigation m a y n o t be w a r r a n t e d . site grading has c o m p l e t e l y altered t h e survey. and existing s t r u c t u r e s . Every effort should be m a d e t o tie in t h e elevations w i t h t h e a r c h i t e c t ' s reference p o i n t . and b y s t u d y i n g existing d a t a . soils. 2. Slope stability d e p e n d s u p o n t h e slope angle of t h e r o c k and soil f o r m a t i o n . T h e l o c a t i o n and elevation of t h e d i t c h should be included as a p a r t of t h e field records. The vegetative cover on t h e slope. R e p o r t s concerning geologic characteristics of the area t h a t w o u l d significantly affect t h e land use and t h e d e t e r m i n a t i o n of t h e i m p a c t of such characteristics on t h e p r o p o s e d subdivision. should also b e observed. t h e m a p p i n g of surficial geologic features is highly desirable. Erecting a s t r u c t u r e across a n a t u r a l gully always poses a future drainage p r o b l e m . Water leaking from t h e d i t c h e s can also cause o t h e r p r o b l e m s such as b a s e m e n t d a m a g e . t r i b u t a r y valleys. sinkholes. Such preliminary i n f o r m a t i o n can usually be o b t a i n e d from t h e U. O f particular interest. is t h e e x t e n t of flood plains. geology. etc. T o insure a d e q u a t e p l a n n e d d e v e l o p m e n t of a subdivision. Similar bills exist in m a n y o t h e r states in response t o e n v i r o n m e n t a l zeal of t h e n a t i o n . T h e steepness of valley slopes is of special c o n c e r n for sites t o be located in m o u n t a i n o u s areas. local slope failure. and drainage features. and vegetation. Senate Bill 35 of C o l o r a d o requires the subdivider t o s u b m i t i t e m s such as: 1. exposed rock sections. evidence of past slope m o v e m e n t . [40] . S.) provide evidence of t h e past geologic activity. T h e w a t e r level in a n y n e a r b y streams and rivers should be m e a s u r e d and r e c o r d e d . T e x t u r e of surficial deposits. In m o u n t a i n areas. S. A n engineering geologist should identify and describe all geologic f o r m a t i o n s visible at t h e surface and n o t e their t o p o g r a p h i c positions. o r sliding of t h e e m b a n k m e n t . landslides. Streams and n e a r b y rivers naturally are of i m p o r t a n c e in t h e site investigation. and the nature of the unconsolidated overburden. S o m e e n v i r o n m e n t a l agents classify valley slopes in excess of 30° as p o t e n t i a l hazard areas.62 FOUNDATIONS ON EXPANSIVE SOILS T h e l o c a t i o n of n a t u r a l and m a n m a d e drainage features is also of considerable i m p o r t a n c e . in avoiding flooding. J o h n s o n suggested t h a t t h e features t o be s h o w n o n t h e m a p should i n c l u d e : 1. A n inspection of u p l a n d and valley slopes m a y provide clues t o t h e thickness and sequence of f o r m a t i o n s and rock s t r u c t u r e . R e p o r t s concerning streams. alluvial. D e p a r t m e n t of Agriculture soil survey r e p o r t s . springs and seeps. excessive w a t e r loss. lakes. and t h e behavior of any neighboring s t r u c t u r e s Surficial geology General surficial geology of t h e area includes t h e s t u d y of slopes. Pier uplift d u e t o t h e infiltration of w a t e r from an irrigation d i t c h is n o t u n c o m m o n . shape of t h e trees. t o p o g r a p h y . 3 . T h e shape and c h a r a c t e r of channels and t h e n a t u r e of t h e soil (residual. Maps and tables concerning suitability of t y p e s of soil in t h e p r o p o s e d subdivision. Geological Survey and t h e U. colluvial. T h e field engineer should be aware of the possible slope p r o b l e m s associated w i t h landslides. m u d flow o r o t h e r p r o b l e m s . Irrigation d i t c h e s carry large a m o u n t s of w a t e r d u r i n g irrigation season. Water leaking from t h e d i t c h can s u p p l y m o i s t u r e t o t h e f o u n d a t i o n soil and cause swelling of footings and slabs. T h e local dip and strike of t h e formations should be d e t e r m i n e d and n o t e m a d e of any stratigraphie relationships or s t r u c t u r a l features t h a t m a y cause p r o b l e m s of seepage. the possibility of a high-water table c o n d i t i o n in t h e area exists. test pit investigation b e c o m e s useless. even if t h e existing s t r u c t u r e is in excellent c o n d i t i o n . If t h e existing s t r u c t u r e e x h i b i t s cracks. 4 . T h e existing s t r u c t u r e could have b e e n originally overdesigned. and w e a t h e r e d zones. t h e m o s t a c c u r a t e subsoil investigation m e t h o d is b y t h e opening of test pits. F o r i n s t a n c e . I n q u i r y should be m a d e as to t h e c o n d i t i o n of t h e s t r u c t u r e . Auger drilling t h r o u g h b o u l d e r s and cobbles is difficult. springs. W y o m i n g . swelling will be critical. If adjacent existing s t r u c t u r e s have experienced water problems. schistosity. this d o e s n o t m e a n t h a t t h e existing f o u n d a t i o n system should be d u p l i c a t e d in new c o n s t r u c t i o n . If t h e cracks are old and n e w cracks have n o t a p p e a r e d in t h e p a t c h e d areas. 3 . generally 12 feet. S t r u c t u r e of b e d r o c k . Area of m o d e r n d e p o s i t (result of accelerated e r o s i o n ) . All possible i n f o r m a t i o n should be o b t a i n e d concerning s t r u c t u r e s in t h e i m m e d i a t e p r o x i m i t y . t h e red siltstone f o r m a t i o n in Laramie. and landslides. should Geophysical always be e q u i p m e n t has b e e n c o m m o n l y used in d e t e r m i n i n g t h e surface of b e d r o c k . including d i p and strike. However. Existing structures T h e behavior of existing s t r u c t u r e s has an i m p o r t a n t bearing o n t h e selection of t h e p r o p o s e d s t r u c t u r e . and drainage. this usually indicates t h a t the existing f o u n d a t i o n system is n o t a d e q u a t e . observable w a t e r table. layers and lenses. However. slips. stratification. In l o c a t i o n s w h e r e t h e subsoils consist essentially of large b o u l d e r s and cobbles. DRILLING AND SAMPLING Geophysical t e c h n i q u e s are generally used for preliminary subsoil investigation. the field engineer can e x a m i n e in detail t h e subsoil strata. B o t h t h e seismic subsoil and the resistivity geophysical m e t h o d s have b e e n successfully and ground-water depth. s u p p l e m e n t e d b y drilling. Unstable slopes. . will n o t pose a swelling p r o b l e m while a few miles t o t h e east w h e r e t h e claystone of t h e Pierre F o r m a t i o n is e n c o u n t e r e d . and t y p e of the foundation. T h e age of t h e building should also be considered.FIELD AND LABORATORY INVESTIGATIONS 63 2.w a t e r features. including seeps. t h e d e p t h of t h e test pit is limited t o the reach of a b a c k h o e . G r o u n d . major changes in conditions. However. Of special i m p o r t a n c e is t h a t t h e geologist should b e able t o recognize a p o t e n t i a l swelling soil p r o b l e m . F a u l t s and fault z o n e s . p o r o s i t y and p e r m e a b i l i t y . This is especially t r u e for old s t r u c t u r e s w h e r e massive f o u n d a t i o n s y s t e m s were usually used. age. faults or features. if n o t impossible. t h e use of test pit investigation is m o s t favorable. and in a d d i t i o n t h e cost of r o t a r y drilling m a y n o t be w a r r a n t e d for small projects. and 6. such a survey used. Also w h e n t h e w a t e r table is high. In a test pit. as well as taking samples at the desired location. 5. t h e f o u n d a t i o n m o v e m e n t m a y have been stabilized. P r o b a b l y . stratification. A n experienced field engineer can readily identify t h e t y p e of distress w h i c h has occurred in t h e existing buildings and t h e n d e t e r m i n e if it is associated w i t h swelling soils. it is q u i t e possible t h a t claystone b e d r o c k containing high swelling p o t e n t i a l might have been revealed. even in only o n e test hole. C o n s e q u e n t l y . t h e r e is a t e n d e n c y to drill as few holes as possible. logs of these test holes c a n n o t b e considered as representative of t h e overall site subsoil c o n d i t i o n . T h e client s o m e t i m e s has the m i s c o n c e p t i o n t h a t drilling of test holes is t h e major cost of the subsoil investigation. m o r e a t t e n t i o n should b e directed to t h e u p p e r soil if a shallow f o u n d a t i o n is likely. and t o s o m e e x t e n t t h e i m p o r t a n c e of t h e s t r u c t u r e . t h e n it will be desirable t o drill shallow test holes close t o g e t h e r t o b e t t e r evaluate t h e subsoil c o n d i t i o n s within t h e loaded d e p t h of the footings. O f t e n t i m e s . Closely spaced test holes are especially i m p o r t a n t w h e r e t h e presence of expansive soils is suspected. say. After t h e c o m p l e t i o n of t h e d e e p test h o l e . If preliminary investigation indicates t h a t shallow spread footings will be t h e m o s t likely t y p e of f o u n d a t i o n . and t h e d e p t h . it is advisable t o drill a few holes i n t o t h e b e d r o c k . t h e r e f o r e . Samples in t h e h o l e should be t a k e n at frequent vertical intervals. If. a d e e p f o u n d a t i o n system is c o n t e m p l a t e d . T h e presence of swelling soil. Probably 95 percent of all site investigations are c o n d u c t e d b y drilling test holes. Therefore. T h e d e p t h of test holes required is generally governed b y t h e t y p e of f o u n d a t i o n . If. However. can change t h e f o u n d a t i o n r e c o m m e n d a t i o n s completely. if m o r e test holes had b e e n drilled. if s a n d s t o n e and siltstone b e d r o c k are e n c o u n t e r e d at a shallow d e p t h . however. however. T h e risk involved in such an u n d e r t a k i n g is e n o r m o u s . u n i f o r m i t y of subsoil c o n d i t i o n s . t h e first hole t h e field engineer drills should b e a d e e p h o l e . t h e d e p t h t o t h e t o p of shale b e d r o c k is e x t r e m e l y erratic. and c o n s e q u e n t l y . t h e d e p t h t o b e d r o c k is a criterion as t o t h e d e p t h of test holes. Test holes T h e n u m b e r of test holes required for a project d e p e n d s u p o n t h e t y p e of f o u n d a t i o n system. test holes should be spaced at a distance of 5 0 t o 100 feet. t h e shallow f o u n d a t i o n system is n o t feasible and a d e e p f o u n d a t i o n system is likely t o be required. In C o l o r a d o . Before drilling. In n o case should the test holes b e spaced m o r e t h a n 100 feet apart w h e n in an expansive soil area. preferably n o t m o r e t h a n 5 feet apart. By physically entering t h e caisson h o l e . percussion drilling. or other methods. d e e p e n o u g h t o provide i n f o r m a t i o n p e r t i n e n t to b o t h shallow and d e e p f o u n d a t i o n systems. a logical r e c o m m e n d a t i o n is t o found t h e s t r u c t u r e directly on t h e b e d r o c k w i t h spread footings designed for high pressure. Where b e d r o c k is w i t h i n e c o n o m i c a l reach.64 FOUNDATIONS ON EXPANSIVE SOILS A n o t h e r possible investigation m e t h o d is t o drill a large-diameter caisson hole to t h e required d e p t h . t h e subsoil strata can be clearly e x a m i n e d . r o t a r y drilling. As a rule of t h u m b . either b y auger drilling. t h e field engineer should have a fairly good idea of t h e possible f o u n d a t i o n system. preferably only o n e . within 4 0 feet. Erratic subsoil c o n d i t i o n s can exist b e t w e e n widely spaced test h o l e s . t h e n t h e n u m b e r of test holes can b e decreased and t h e spacing increased. drilling t o b e d r o c k would be necessary for all holes. F o r instance. t h e t y p e of f o u n d a t i o n c a n n o t be c o m p l e t e l y envisioned. for t h e s u b s e q u e n t holes. T h e d r a w b a c k of b o t h t h e test pit m e t h o d and t h e caisson h o l e m e t h o d is t h a t standard penetration tests c a n n o t be p e r f o r m e d . and u n d i s t u r b e d samples can b e o b t a i n e d at t h e desired d e p t h . Penetration test In performing t h e standard p e n e t r a t i o n test. at 20 feet b e l o w t h e g r o u n d surface w i t h Ύ = 110 pcf. S. T h e n u m b e r of h a m m e r blows required t o drive 12 inches is called t h e standard p e n e t r a t i o n resistance Ν w h i c h represents t h e n u m b e r of b l o w s p e r foot. or t h e blow c o u n t . This will have t o b e t a k e n i n t o consideration w h e n d e t e r m i n i n g t h e l o c a t i o n and d e p t h of test holes. A t a great d e p t h . and ρ = Effective o v e r b u r d e n pressure. Bureau of R e c l a m a t i o n t h r o u g h actual e x p e r i m e n t and has b e e n extensively used all over t h e w o r l d . P e n e t r a t i o n resistance is reliable o n l y if t h e driving c o n d i t i o n is n o t abused. d e e p holes are required. n o t from t h e subsoil r e q u i r e m e n t b u t for the d e t e r m i n a t i o n of t h e w a t e r table elevation. t h e calculated values are r a t h e r high. Driving a s t a n d a r d barrel i n t o gravelly soils p r e s e n t s a p r o b l e m . Using t h e Bureau of the adjusted penetration Reclamation e q u a t i o n and a value of N ' = 12. These t w o halves are held t o g e t h e r b y a c u t t i n g shoe at t h e lower end and a coupling w h i c h c o n n e c t s t h e sampler t o the drill rod. N ' = S t a n d a r d p e n e t r a t i o n resistance as actually r e c o r d e d . In s o m e cases. T h e split s p o o n is driven 18 inches i n t o t h e g r o u n d b y m e a n s of a 140-pound h a m m e r falling a free height of 3 0 inches. a soil sampler k n o w n as a split s p o o n is used.FIELD AND LABORATORY INVESTIGATIONS 65 t o b e d r o c k can increase as m u c h as 30 feet w i t h i n a short distance of 100 feet. T h e h a m m e r should b e entirely free falling w i t h o u t being subject t o u n d u e friction. This results in t h e designing engineer using a p e n e t r a t i o n resistance value larger t h a n required for a safe design. T h e a u t h o r found t h a t while t h e increase in p e n e t r a t i o n resistance at greater d e p t h c a n n o t be ignored. T h e value of Ν in cohesionless soils is influenced b y t h e d e p t h at w h i c h t h e test is m a d e . resistance is almost d o u b l e d . T h e s t a n d a r d p e n e t r a t i o n test has been and will c o n t i n u e t o be an i m p o r t a n t and practical field test. 2. T h e influence of o v e r b u r d e n pressure can be a p p r o x i m a t e d b y t h e following e q u a t i o n : Ν = N' \P+10/ where: Ν = Adjusted value of s t a n d a r d p e n e t r a t i o n resistance. T h e barrel will b o u n c e w h e n driving on cobbles. t h e same soil w i t h t h e same relative d e n s i t y w o u l d give a high p e n e t r a t i o n resistance. It is an open-ended steel cylinder w h i c h is split longitudinally i n t o halves. t h e d e p t h of test holes should be at least 2 0 feet t o preclude t h e possibility of ground w a t e r b e c o m i n g a p r o b l e m in t h e lower floor. . T h e standard p e n e t r a t i o n barrel should n o t be p a c k e d b y overdriving since this forces t h e soil against t h e sides of t h e barrel and causes incorrect readings. psi T h e above e q u a t i o n was derived b y t h e U. F o r d e e p b a s e m e n t c o n s t r u c t i o n . T h e following should be considered in using t h e p e n e t r a t i o n test: 1. A n increase in b l o w c o u n t b y as m u c h as 50 p e r c e n t can s o m e t i m e s be caused b y a p a c k e d barrel. particularly the p e r c e n t a g e of recovery. t h u s substantially increasing t h e b l o w c o u n t . are of i m p o r t a n c e t o f o u n d a t i o n design and c o n s t r u c t i o n . a small piece of gravel will j a m in t h e barrel t h e r e b y preventing t h e e n t r a n c e of soil i n t o t h e barrel. say. Field tests have been c o n d u c t e d c o m p a r i n g t h e results of t h e p e n e t r a t i o n resistance of t h e California sampler w i t h t h a t of t h e s t a n d a r d p e n e t r a t i o n tests. such as l i m e s t o n e and granite. as this is generally t h e site of shallow f o u n d a t i o n s . Core samples are b r o u g h t u p b y t h e drill and can be visually e x a m i n e d . b e c o m e critical w h e n a d e e p f o u n d a t i o n system is r e q u i r e d . By c o m b i n i n g p e n e t r a t i o n resistance test w i t h t h e sampling device. T h e tests indicate t h a t t h e results are (N < 4 ) and very stiff or dense soil c o m m e n s u r a b l e w i t h t h e e x c e p t i o n of very soft soil (N > 3 0 ) . LABORATORY TESTING Soil testing is essential in establishing t h e design criteria. Instead of assigning t h e field engineer definite instructions as t o the frequency of sampling. visual e x a m i n a t i o n of t h e samples. n o useful value can be o b t a i n e d . F o r a practicing engineer. p e n e t r a t i o n tests. Sampling and p e n e t r a t i o n tests at l o w e r d e p t h . This m a y prove unnecessary. F o r o t h e r t y p e s of b e d r o c k . Samples in t h e u p p e r 10 or 15 feet are i m p o r t a n t . S o m e t i m e s . for d e e p b a s e m e n t c o n s t r u c t i o n . Considerable economy can be achieved by c o m b i n i n g p e n e t r a t i o n resistance with sampling. m o r e tests can be m a d e and u n d i s t u r b e d samples can be o b t a i n e d w i t h o u t resorting t o the use of Shelby tubes. t h e p u r p o s e of l a b o r a t o r y testing is m a i n l y to confirm his preconceived c o n c e p t derived from field drilling. T h e field engineer should use his j u d g m e n t t o guide t h e frequency of sampling and avoid unnecessary sampling so t h a t t h e cost of investigation can be held t o a m i n i m u m . e x c e p t w h e n friction piers are u n d e r c o n s i d e r a t i o n . n o sampling or p e n e t r a t i o n tests will be necessary for t h e first 15 feet b e l o w g r o u n d level. T h e general characteristics. Sampling above t h e p r o p o s e d floor level should be limited since this material will be e x c a v a t e d . Distinction m u s t be m a d e b e t w e e n t h e needs of t h e practicing engineer and t h o s e of the research engineer. Sampling S o m e c o n t r a c t s call for a p e n e t r a t i o n test for every 5 feet and sampling for t h e same interval o r every change of soil s t r a t u m . Sampling and testing at an i n t e r m e d i a t e d e p t h generally is n o t t o o critical. S o m e t i m e s . also. T h e modified barrel is c o m m o n l y referred t o as a California sampler. Auger drilling in m o s t cases can be successfully c o n d u c t e d in shale b e d r o c k . it w o u l d be b e t t e r to leave the m a t t e r u p to his discretion. in b e d r o c k . A reasonably good sample can be o b t a i n e d w h e n driving i n t o shale b e d r o c k . and personal experience in the . A slight m o d i f i c a t i o n in t h e design of t h e barrel will allow t h e insertion of thin wall lining in t h e barrel and provide a b l o w c o u n t as well as an u n d i s t u r b e d sample.66 FOUNDATIONS ON EXPANSIVE SOILS and h e n c e . 3 . r o t a r y drilling will be necessary and rock cores o b t a i n e d . soil characteristics at this level govern t h e slab-on-ground c o n s t r u c t i o n a n d earth-retaining s t r u c t u r e s . T h e load imposed o n t h e specimen is m e a s u r e d b y t h e scale b e a m and a dial gage is provided t o m e a s u r e t h e vertical movement. E x o t i c l a b o r a t o r y e q u i p m e n t and refined analysis are in t h e realm of t h e research engineer. T h e advantage of such a p p a r a t u s is t h a t it is possible t o hold t h e u p p e r loading b a r at a c o n s t a n t v o l u m e and allow t h e m e a s u r e m e n t of t h e m a x i m u m uplift pressure of t h e soil w i t h o u t Figure 29. N e i t h e r t i m e n o r b u d g e t will allow t h e practicing engineer t o follow t h e researcher's p r o c e d u r e . Platform scale type consolidometer. P o r o u s stones are provided at each end of t h e specimen for drainage or s a t u r a t i o n . Swell test T h e m o s t i m p o r t a n t l a b o r a t o r y test on expansive soils is t h e swell test. is s h o w n on figure 29 and is o f t h e fixed-ring t y p e [ 4 1 ] . This a p p a r a t u s .25 inches in thickness. 2 5 inches in d i a m e t e r and from 0. T h e s t a n d a r d one-dimensional c o n s o l i d a t i o n test a p p a r t u s is similar t o t h a t used in m o s t soil laboratories for consolidation studies.FIELD AND LABORATORY INVESTIGATIONS 67 area. It can a c c o m m o d a t e a r e m o l d e d o r u n d i s t u r b e d sample from 2 t o 4 . . T h e sample u n i t is placed on a platform scale table and t h e load is applied b y a y o k e a c t u a t e d b y a screw jack.75 t o 1. k n o w n as a c o n s o l i d o m e t e r . L a b o r a t o r y test results are reliable only t o the extent of t h e c o n d i t i o n of t h e sample. Testing of a few samples on a single project and basing t h e final analysis o n such testing is n o t only undesirable b u t s o m e t i m e s d a n g e r o u s . A m o r e advanced scheme is t o use t h e c o n s o l i d o m e t e r w i t h a triaxial frame along w i t h t h e a t t a c h m e n t s and electrical circuits [ 4 2 ] . A simple c o n s o l i d o m e t e r m a d e locally w i t h low cost. T o o little testing is worse t h a n n o testing at all. Figure 30. .68 FOUNDATIONS ON EXPANSIVE SOILS volume change. Simplified lever-type consolidometer. Results of testing o n badly d i s t u r b e d samples or samples n o t representative of t h e strata n o t only are useless b u t add confusion t o t h e c o m p l e t e analysis. By testing only a few samples. T h e c o n s o l i d o m e t e r can also b e used t o m e a s u r e t h e a m o u n t of e x p a n s i o n u n d e r various loading c o n d i t i o n s . Since swelling pressure can b e evaluated b y loading t h e swelled sample t o its original can be volume as explained in chapter 2. T h e average soil l a b o r a t o r y should have a train consolidometers t o speed u p testing p r o c e d u r e s . it is simple to convert the platform scale of c o n s o l i d o m e t e r i n t o a lever-type c o n s o l i d o m e t e r as s h o w n o n figure 3 0 . especially w h e n evaluating swelling soils. t h e high swelling sample m a y have been missed and e r r o n e o u s conclusions d r a w n . Equally i m p o r t a n t t o testing of representative samples is t h e frequency of testing. Such a device will allow t h e a u t o m a t i c load i n c r e m e n t and m e a s u r e swelling pressure w i t h o u t allowing v o l u m e change to take place. and l a b o r a t o r y swell tests m u s t be c o n d u c t e d . This requires a c o n s t a n t load a d j u s t m e n t b y an o p e r a t o r . Interpretation of test results and u n d i s t u r b e d soils can b e p e r f o r m e d in m o s t soil Laboratory testing of disturbed laboratories b y an efficient l a b o r a t o r y technician. T h e p o t e n t i a l of swelling generally c a n n o t be d e t e r m i n e d visually. P. "Soil Testing for Engineers. I. and w h e n average values should be used. S. W. A. Inc. Israel. ASTM stp 479. . "Suggested Method for Geologic Reconnaissance of Construction Site" Special procedures for testing soil & rock for engineering purposes. [42] Agarwal.. K. 1970. T." Proceedings of the Third International Conference on Expansive Soils. REFERENCES [40] Johnson. [41 ] Lambe. "A Method for Measuring Swelling Pressure of an Expansive Soil.FIELD AND LABORATORY INVESTIGATIONS 69 I n t e r p r e t a t i o n of test results should be c o n d u c t e d b y an experienced engineer. Haifa. having t h e ability t o screen t h e test results and e x c l u d e t h e d u b i o u s ones." John Wiley & Son. and Sharma. t o d e t e r m i n e w h e n t h e m i n i m u m o r m a x i m u m values are t o be used. C . " t h e design b y e x p e r i e n c e a p p r o a c h is usually found t o b e . T h e drilled pier f o u n d a t i o n is used t o transfer t h e s t r u c t u r a l load from t h e u p p e r u n s t a b l e soil t o t h e lower stable soil. " PIER CAPACITY Piers bearing o n b e d r o c k or shale have b e e n designed using m o s t l y empirical c o n s i d e r a t i o n s derived from e x p e r i e n c e . "Where and are relatively u n i f o r m . piers s u p p o r t i n g a c o l u m n load in excess o f 1. have b e e n designed using strictly empirical c o n s i d e r a t i o n s which are derived conditions are well from regional e x p e r i e n c e .000 kips are c o m m o n l y designed and c o n s t r u c t e d in t h e R o c k y M o u n t a i n area w i t h satisfactory performance. T h e use of drilled pier f o u n d a t i o n covers a wide range of possibilities. F r i c t i o n piers b o t t o m e d on stiff clays for s u p p o r t i n g light s t r u c t u r e s . and 4 . t h e design and c o n s t r u c t i o n m u s t b e closely controlled. all t h e time on a project. " T h e y further stated t h a t . N o n e t h e l e s s . W o o d w a r d . t h e n t h e r e w o u l d be n o defective p i e r s . o r drilled caisson. because of its erratic characteristics. or caisson f o u n d a t i o n is widely used in t h e R o c k y M o u n t a i n area. 3 . inspection always c o n t i n u o u s . w h e n m a d e w i t h an enlarged base. " M a n y piers. supervision always c o m p e t e n t and a d e q u a t e . Piers drilled i n t o hard b e d r o c k for s u p p o r t i n g high c o l u m n load. and t h e behavior of t h e existing s t r u c t u r e s . particularly w h e r e r o c k bearing is used. As stated b y Richard W o o d w a r d [ 4 3 1 . G a r d e n e r and G r e e n [ 4 3 ] stated o n empirical design of piers. e x p e r i e n c e d and alert. Drilled piers. some of w h i c h follow: 1. small-diameter piers drilled i n t o a z o n e unaffected b y m o i s t u r e change in swelling soil areas. are c o m m o n l y referred t o as belled piers and w h e n m a d e w i t h o u t an enlarged base are referred t o as straight-shaft piers. and t h e c o n t r a c t o r ' s p e r s o n n e l always e x p e r t and conscientious—if all t h e s e c o n d i t i o n s prevailed. " I f investigation and design were always p e r f e c t . T h e drilled pier f o u n d a t i o n is a rational s o l u t i o n t o c o m b a t t h e p r o b l e m of expansive soils. subsurface of past established and t h e p e r f o r m a n c e c o n s t r u c t i o n s well d o c u m e n t e d satisfactory. however. F o r instance.Chapter 4 DRILLED PIER FOUNDATIONS INTRODUCTION Drilled pier. Belled piers b o t t o m e d o n sand and gravel for s u p p o r t i n g m e d i u m c o l u m n load. t h e u l t i m a t e load carrying capacity of t h e D e n v e r Blue Shale has never been d e t e r m i n e d . limited load test d a t a . Long. 2. Allowable bearing capacity derived b y p e n e t r a t i o n resistance d a t a should be checked b y b o t h the u n c o n f i n e d compressive s t r e n g t h test and the consolidation test.72 FOUNDATIONS ON EXPANSIVE SOILS T h e bearing capacity of drilled piers b o t t o m e d o n b e d r o c k is a c o m b i n a t i o n of the end-bearing capacity and t h e skin friction developed b e t w e e n the pier wall and b e d r o c k . w i t h Ν = 4 0 . Bearing capacity T h e bearing capacity of b e d r o c k for piles or piers can be found in m o s t building codes. b u t t h e presence of slickensides and o t h e r effects of d i s t u r b a n c e limit the validity of such tests. Consolidation tests o n b e d r o c k core sample u n d e r high load. In the . 0 0 0 t o 4 0 . High capacity c o n s o l i d a t i o n test o n a reasonably good sample can s o m e t i m e s be used t o d e t e r m i n e t h e a m o u n t of pier s e t t l e m e n t . all of t h e above a p p r o a c h e s have their limitations. Therefore. t h e soil engineer should convert t h e blow c o u n t t o allowable bearing capacity. t o o b t a i n and accurate d e t e r m i n a t i o n of p e n e t r a t i o n resistance. and Ν = blow count F o r instance. U n f o r t u n a t e l y . T h e m e t h o d s for d e t e r m i n i n g t h e bearing capacity are as follows: 1. One widely used m e t h o d is: Ν %= where: 2 q a = allowable bearing capacity in ksf. T h a t is. Probably. 2. the actual p e n e t r a t i o n resistance of t h e s t r a t u m c a n n o t b e accurately d e t e r m i n e d . Menard p r e s s u r e m e t e r test on b e d r o c k in t h e test holes. T h e values given are generally conservative and t h e t y p e of b e d r o c k usually n o t clearly defined. P e n e t r a t i o n resistance o n hard b e d r o c k involves blow c o u n t s in excess of 100. 0 0 0 psf. After t h e representative p e n e t r a t i o n resistance value has been d e t e r m i n e d . and 6. m a n y tests. Samples o b t a i n e d from large d i a m e t e r cores o b t a i n e d from core drilling are far m o r e reliable. if t h e representative blow c o u n t is 4 0 . Unconfined compressive strength test at best can only represent t h e lower limit of t h e actual bearing capacity of b e d r o c k . Unconfined compressive s t r e n g t h p e r f o r m e d o n drive samples can only be used for comparative purposes. 4 . the allowable bearing capacity should be m o r e nearly 3 0 . U n f o r t u n a t e l y . When a soft seam or a very hard lens is e n c o u n t e r e d . such records are scarce. This a p p r o a c h is q u i t e conservative. the most reliable method of estimating bedrock bearing capacity is the observation of t h e behavior of existing structures. T h e actual s e t t l e m e n t value t a k e n as a percentage of l a b o r a t o r y consolidation value should b e left to t h e j u d g m e n t of t h e soil engineer. P e n e t r a t i o n resistance of b e d r o c k . U n c o n f i n e d compressive s t r e n g t h or triaxial shear s t r e n g t h of u n d i s t u r b e d b e d r o c k core sample. 5. t h e allowable bearing capacity selected will be 2 0 . Extensive l a b o r a t o r y testing indicates t h a t the m o r e realistic value for q a should be in t h e range of Ν t o 0 . 7 5 N . A c t u a l record of s e t t l e m e n t of existing building f o u n d e d o n similar b e d r o c k . 3 . Load test o n b e d r o c k . should be c o n d u c t e d . possibly m o r e t h a n 5 0 . 0 0 0 psf. the skin friction value developed in the o v e r b u r d e n soils can be considerable. a t t e m p t s have b e e n m a d e t o provide a shear ring in t h e p o r t i o n of the pier in b e d r o c k . Actually. w i t h e x c e p t i o n of t h o s e drilled i n t o oily shale.t e n t h of the end bearing value of b e d r o c k . especially for large-diameter piers.DRILLED PIER FOUNDATIONS 73 Denver area. t h e skin friction developed in t h e p o r t i o n of pier e m b e d d e d in the o v e r b u r d e n soil is usually ignored. 0 0 0 psf have p e r f o r m e d satisfactorily w i t h long-term s e t t l e m e n t of less t h a n 1 inch. This value obviously has exceeded t h e cohesion of c l a y s t o n e and t h e t o t a l h o r i z o n t a l pressure against the shaft surface. piers bearing of Denver Blue Shale F o r m a t i o n designed for a bearing capacity of 6 0 ." Soil engineers in t h e R o c k y M o u n t a i n area chose t h e e s t a b l i s h m e n t of skin friction value b y t h e following t w o a s s u m p t i o n s : 1. Assuming t h a t skin friction value in b e d r o c k is o n e . In designing t h e pier load. t h e unit skin friction value b e t w e e n t h e c o n c r e t e pier and t h e granular soil is at least 4 0 0 to d e t e r m i n e as discussed later u n d e r " F r i c t i o n . 0 0 0 psf. C o n s e q u e n t l y . T h e skin friction value is u n f o r t u n a t e l y difficult Piers. 2. T h e m o s t direct and r e c o m m e n d e d m e t h o d in d e t e r m i n i n g bearing capacity is b y t h e insertion of a Menard p r e s s u r e m e t e r i n t o t h e b o t t o m of t h e test hole and evaluating t h e actual bearing capacity. Load tests o n piers are costly and t i m e c o n s u m i n g . A n e x p e r i e n c e d o p e r a t o r will be required t o c o n d u c t t h e tests. Shear rings are m a d e b y t h e use of grooving tools w h i c h cut slots a r o u n d t h e circumference of t h e h o l e . 0 0 0 psf. T h e circular slots are a b o u t 1-inch d e e p and 3/4-inch thick at intervals of 8 inches. t h e skin friction value is 6 . t h e surface of the hole can t h u s be r o u g h e n e d artificially. If it is f o u n d t h a t a pier hole is t o o s m o o t h . The walls of a pier hole are seldom s m o o t h . a d e q u a t e d e v e l o p m e n t of skin friction can be assured. for long piers. Limited d a t a on t h e testing of small-diameter piers indicates t h a t the c o n v e n t i o n a l a p p r o a c h e s are e x t r e m e l y conservative. Assuming t h e drilling of a 42-inch-diameter pier t h r o u g h 2 0 feet of dense sand and gravel i n t o t h e b e d r o c k . It is believed t h a t for small d i a m e t e r piers. Such values are considered t o be a d d e d factors of safety in t h e pier system. It is generally t h o u g h t t h a t t h e skin friction value m a y n o t d e v e l o p fully along t h e surface of the shaft b u t t h a t a lubricated surface m a y exist b e t w e e n t h e soil and shaft. a t o o t h p r o t r u d i n g a b o u t one-half inch can be inserted o n t o an auger and w i t h several t u r n s . E x p e r i e n c e shows t h a t such an installation is seldom justified. Skin friction of shale T h e load-carrying capacity of a pier d e p e n d s n o t only u p o n its end bearing value b u t t o a great e x t e n t o n t h e skin friction or side shear b e t w e e n c o n c r e t e and its s u r r o u n d i n g soils. Load tests o n small d i a m e t e r piers ( 1 2 inches) drilled i n t o b e d r o c k w i t h n o e n d bearing (pier p o u r e d on elastic material) indicate t h a t t h e skin friction value exceeds t h e design value b y several times. t h e n for an end bearing value of 6 0 . skin friction has actually t a k e n all t h e load e x e r t e d o n t h e pier w i t h o u t transferring t h e load t o t h e pier b o t t o m . It is believed t h a t b y grooving t h e hole. Oftentimes.03 E L ) + 0. ft. d e e p excavation will e x p o s e t h e t o p of shale and t h e material will be subject t o severe disintegration. The increase of bearing capacity w i t h d e p t h of p e n e t r a t i o n has its limitations. t h e t o p of shale is highly w e a t h e r e d and only a low value can be assigned. and t h e n increases t h e bearing value for additional p e n e t r a t i o n . Therefore.03 E L ) C L in w h i c h : Q = t o t a l pier carrying capacity (kips) A = end area of pier (sq. F o r major s t r u c t u r e s . Design capacity t o allow for unforeseen construction After end bearing and skin friction values have b e e n established.74 FOUNDATIONS ON EXPANSIVE SOILS psf. Obviously. S o m e t i m e s . This added factor of safety t o t h e drilled pier system is r e c o m m e n d e d . Total skin friction value can exceed 9 0 kips. T h e bearing capacity of the t o p of b e d r o c k should be carefully evaluated. 0 0 0 psf. and it was assumed t h a t t h e end bearing value of t o p of b e d r o c k is 5 0 . it is i m p o r t a n t t h a t an a d e q u a t e factor of safety be i n c o r p o r a t e d contingencies. the rate of 3 percent per foot. Extensive experience in t h e R o c k y M o u n t a i n area indicates t h a t t h e a b s o l u t e m a x i m u m of end bearing value for piers in shale is 1 0 0 .) Ε = end bearing capacity of pier (psf) L = d e p t h of p e n e t r a t i o n i n t o b e d r o c k (ft. 2.1 (E + 0. a higher load-carrying capacity can be assigned. Experience w i t h shale b e d r o c k in the R o c k y M o u n t a i n area generally indicates t h a t t h e hardness of b e d r o c k increases w i t h d e p t h .) C = p e r i m e t e r of pier (ft. t h e bearing capacity of the pier will be d o u b l e d w h e n the d e p t h of p e n e t r a t i o n exceeds 3 0 feet. 0 0 0 psf or a b o u t 7 0 0 psi. T h e r e f o r e . t h e load-carrying capacity of a straight-shaft pier can be expressed as follows: Q = A ( E + 0. for piers p e n e t r a t i n g d e e p i n t o b e d r o c k . Experience indicates t h a t b o t h t h e end-bearing capacity and t h e skin friction value increases w i t h d e p t h at Using this a s s u m p t i o n .) Figure 31 was p r e p a r e d using t h e above e q u a t i o n . It should be p o i n t e d o u t t h a t t h e above e q u a t i o n has its limitations as n o t e d : 1. it is usually safe t o assign a bearing capacity value for t h a t p o r t i o n of pier 2 t o 4 feet below t h e surface of b e d r o c k . This has been verified by penetration resistance and by pressuremeter tests. it is n o t difficult t o calculate t h e total load-bearing capacity of a pier. According t o t h e formula. a load-carrying capacity of such m a g n i t u d e c a n n o t be assigned t o the pier. . t h e principle of t h e use of drilled piers is t o provide a relatively inexpensive w a y of transferring t h e s t r u c t u r a l loads d o w n t o stable material or t o a stable z o n e w h e r e m o i s t u r e changes are i m p r o b a b l e . t h e w i t h h o l d i n g force m u s t balance t h e uplifting force.DRILLED PIER FOUNDATIONS 75 2.800 1. 0 0 0 kips w i t h reasonable p e n e t r a t i o n . Design load versus depth into bedrock.000 8 10 PENETRATION 12 INTO 14 BEDROCK · FEET Figure 31. F o r safe pier design. MECHANICS OF PIER UPLIFT As stated earlier in t h e c h a p t e r . It is seen from figure 3 2 t h a t t h e uplifting forces w h i c h t e n d t o pull t h e pier o u t of t h e g r o u n d are a direct function of t h e swelling pressure. Such load is of sufficient m a g n i t u d e t o a c c o m m o d a t e m o s t c o l u m n loads of a high-rise s t r u c t u r e . t h e load-carrying capacity of a single 60-inch-diameter pier can easily reach 3 . These forces are analyzed as follows. Figure 3 2 is a sketch of t h e grade b e a m and pier s y s t e m . Using t h e above a p p r o a c h in designing drilled piers. T h e w i t h h o l d i n g force consists of t h e dead load pressure e x e r t e d o n t h e pier a n d t h e skin friction along t h e u n w e t t e d p o r t i o n of t h e pier. There should be n o direct c o n t a c t b e t w e e n the soil and t h e s t r u c t u r e w i t h the e x c e p t i o n of t h e soils s u p p o r t i n g t h e piers. .200 < 1. (lbs.) D = t o t a l length of t h e pier.76 FOUNDATIONS ON EXPANSIVE SOILS Dead load pressure Reinforcement for . of P a r c h e r and Liu [441 have s h o w n t h a t c o m p a c t e d clay soils e x h i b i t e d greater u n i t swelling in t h e h o r i z o n t a l direction t h a n in t h e vertical d i r e c t i o n . (ft.) u = swelling pressure. U = t o t a l uplifting force. Grade beam and pier system. O u r swelling is small.) d = d e p t h of t h e z o n e of soils unaffected b y w e t t i n g .PIER FOUNDATION Figure 32.tension Grade Beam Air space beneath Grade Beam Uplifting Pressure U = uf Swelling Pressure Skin friction Assumed circular plane of failure 2r STRAIGHT SHAFT-PIER FOUNDATION BELL . With t h e m o d e l pier test described in t h e s u b s e q u e n t section. Uplifting force T h e t o t a l uplifting force of t h e soils s u r r o u n d i n g t h e pier can be w r i t t e n as follows: U = 27rrfu(D-d) where: r = radius of t h e pier (ft. we can assume t h a t t h e vertical swelling pressure can be used in estimating t h e . The coefficient of uplift b e t w e e n t h e soil a n d surface of t h e c o n c r e t e pier is n o t k n o w n . it was a t t e m p t e d t o establish a rational value for t h e coefficient of uplift for design p u r p o s e s . (ft.) The soils s u r r o u n d i n g t h e pier e x p a n d b o t h vertically and h o r i z o n t a l l y u p o n wetting. (psf) f = coefficient of uplift b e t w e e n c o n c r e t e and soil. It should be p o i n t e d o u t here t h a t t h e uplifting pressure of t h e pier m a y n o t b e c o n t r o l l e d b y t h e shear s t r e n g t h of t h e clay. Since t h e m a g n i t u d e of t h e difference uplifting force o n t h e piers. Surface w a t e r seeps t h r o u g h seams and fractures in t h e stiff fissured clays and clay shales. 5 feet can be arbitrarily assigned as t h e p r o b a b l e d e p t h of w e t t i n g . F o r t h e above reasons. (ft. (lbs. T h e c o n t a c t surface b e t w e e n t h e c o n c r e t e and t h e soil r e m a i n e d clean. Model test for pier uplift forces T h e soil sample used for c o n d u c t i n g t h e test was typical of Southeast Denver's highly expansive clay. Field tests indicate t h a t expansive soils are so i m p e r m e a b l e t h a t surface w a t e r can p e n e t r a t e o n l y a b o u t 2 feet i n t o t h e soil.) r = radius of t h e pier. p e r c e n t = 50. Water t e n d s t o seep along the walls of t h e pier shaft i n t o t h e s u r r o u n d i n g soils.DRILLED PIER FOUNDATIONS 77 e x p e r i m e n t discussed later u n d e r " M o d e l Test for Pier U p l i f t " indicates t h a t t h e pier actually slips o u t of t h e soil in very m u c h t h e same m a n n e r as e x t r a c t i n g a pile. in actual cases. (psf) W = total w i t h h o l d i n g force. T h e source of w e t t i n g m a y n o t b e derived from surface w a t e r . According t o M o h a n and C h a n d r a [ 4 5 ] " T h e frictional resistance of b o r e d c o n c r e t e piles in m e d i u m t o h a r d clays is a b o u t half t h e average u n d i s t u r b e d shear s t r e n g t h of t h e clay along t h e pile shaft and has the same value in loading and p u l l i n g . T h e value is conservative and d e p e n d s u p o n t h e roughness of t h e pier shaft and t h e compressive strength of t h e c o n c r e t e as previously discussed u n d e r " S k i n F r i c t i o n of S h a l e " . p e r c e n t Plastic limit. " E x p e r i e n c e w i t h piers e m b e d d e d in shale indicates t h a t the skin friction along t h e shaft of t h e pier is equal t o at least 1/10 of t h e end bearing value of t h e pier. This is possibly d u e t o t h e following reasons: 1.3 . T h e physical p r o p e r t i e s of t h e soil were as follows: Liquid limit. A b r o k e n w a t e r m a i n o r sewer pipe can cause a m u c h m o r e severe w e t t i n g c o n d i t i o n t h a n surface w a t e r . 2. However. it is n o t possible t o precisely define t h e d e p t h of t h e z o n e of w e t t i n g .0 = 24. Withholding force T h e w i t h h o l d i n g force t h a t k e e p s a pier from pulling o u t of t h e g r o u n d can be w r i t t e n as: W = πι2 ρ + 2 π Γ sd w h e r e : ρ = unit dead-load pressure.) T h e skin friction of t h e soils s u r r o u n d i n g t h e pier is again a factor w h i c h c a n n o t be fully evaluated. 3 .) d = d e p t h of z o n e of soils unaffected b y w e t t i n g . Zone of wetting In pier design it is i m p o r t a n t t o d e t e r m i n e t h e d e p t h of t h e z o n e of w e t t i n g . (psf) s = skin friction s u r r o u n d i n g t h e pier. F o r design p u r p o s e s . it was found t h a t soils s u r r o u n d i n g piers were w e t t o a d e p t h of as m u c h as 15 feet b e l o w t h e g r o u n d surface. (ft. 2 0 0 sieve. An adjustable screw was placed b e n e a t h t h e ball bearing. as s o o n as uplifting m o v e m e n t was observed on Dial A. t h e only pressure acting on these piers was at t h e base. was carefully observed for m o v e m e n t .0 p e r c e n t . p e r c e n t M a x i m u m d r y d e n s i t y . pcf = 25. b u t were bearing directly on t h e soil at t h e b o t t o m of the h o l e . the adjustable screw was t u r n e d t o k e e p t h e pier in its original position t h r o u g h o u t t h e test. C o n s e q u e n t l y . Apparatus for the determination of the coefficient of uplift . T h e uplifting m o v e m e n t of Pier A was recorded by placing a dial gauge directly on t h e t o p of t h e pier. Holes 2-1/2 inches in d i a m e t e r and 6 inches d e e p were drilled in t h e soil sample. any uplift pressure exerted on the pier w o u l d be only along t h e pier shaft and n o n e o n the base. Piers C and D were end-bearing piers. therefore. Dial A. T h e set u p for these t w o piers was identical t o Piers A and B. T h e proving ring w a s . Figure 33. Piers A and Β are friction piers. c e n t e r e d on t o p of t h e pier w i t h a ball bearing b e t w e e n t h e pier and proving ring. being o n l y 2 inches in d i a m e t e r . T h e piers w e r e 2 inches in d i a m e t e r and were b o t t o m e d o n t h e steel c o n t a i n e r . T h e uplifting pressure of Pier Β was m e a s u r e d w i t h a proving ring. T h e piers. did n o t have any direct c o n t a c t w i t h the soil along their circumference. T h e a r r a n g e m e n t of t h e tests is s h o w n on figures 3 3 and 3 4 .5 Optimum moisture content. p e r c e n t Passing n o . T h e stress registered b y t h e proving ring represents t h e t r u e uplifting pressure of t h e pier [ 4 6 ] . w h i c h was placed on t o p of t h e pier. T h e first set of t w o piers was p r e p a r e d for s t u d y i n g t h e behavior of friction piers. T h e second set of t w o piers was p r e p a r e d for s t u d y i n g t h e behavior of end-bearing piers. T w o sets of holes were drilled in the soil. T h e length of the pier was e m b e d d e d in t h e full d e p t h of soil w h i c h is 9 i n c h e s .7 = 82.4 = 96.78 FOUNDATIONS ON EXPANSIVE SOILS Plasticity i n d e x .0 T h e soil was p a c k e d i n t o a steel c o n t a i n e r t o a d e p t h of 9 inches in thin layers and at a m o i s t u r e c o n t e n t of 14. percent = 21. DRILLED PIER FOUNDATIONS 79 Friction Pier A Strain Gage ο ο ο End Bearing Pier C Strain Gage ο ο ο ° ο φ ο ο ° \ Drain Pipes ° r> (φ) ο ° ο Dial Support / 7 ί/ Friction Pier Β Stress Gage ο ο ο End Bearing Pier D Stress Gage ο ο ο — ^ ο LJ] r ο (| ι V \ ι JJ ο / Μ— Proving Ring / 0 (ι_4 \ > .^ 0 σ Dial A -^ZT^ ο ο PIER UPLIFTING TEST APPARATUS PLAN VIEW Load Frame SECTION A A Figure 34. Pier uplifting test apparatus—plan and section. .y Ο J Dial A . t h e total uplifting force is: U = 27rrfuD T h e t o t a l uplifting force exerted on an end-bearing pier w i t h t h e pier free from the s u r r o u n d i n g soils can be w r i t t e n as: 2 U =7 r r u Δ 0. Results of t h e time-strain and time-stress relationships are s h o w n on figures 35 and 3 6 respectively. Water was a d d e d t o t h e soil t h r o u g h t h e perforated pipes t o o b t a i n saturated c o n d i t i o n s .16 - 77. pounds Maximum upward movement.^ 0 ο — t-END BEAR! IG PIER • ί 1.0 - 0. T h e test was carried o u t for a period of 2 weeks.20 - _ 195. inches 0. A Β C D of pier Friction Friction End Bearing End Bearing M a x i m u m uplifting pressure. Time-strain relationship of friction and end-bearing piers in clay. T h e results of t h e tests can be s u m m a r i z e d as follows: Type Pier No.80 FOUNDATIONS ON EXPANSIVE SOILS Perforated plastic pipes were inserted a r o u n d t h e p e r i p h e r y of t h e piers as s h o w n o n figure 3 4 . B o t h stress and strain readings were t a k e n at frequent intervals. .20 FRICTK >N P I E R . 4/17 4/18 4/19 4/20 4/21 DATE 4/22 4/23 4/25 4/26 4/27 Figure 35.0 F o r c o m p l e t e w e t t i n g c o n d i t i o n s and w i t h o u t pressure being exerted o n t h e b o t t o m of t h e piers. Assuming t h a t t h e u p p e r 5 feet of t h e soil b e c o m e s w e t t e d .45 psi. F o r piers spaced o n a b o u t 10-to-l 5-foot c e n t e r s . Time-stress relationship of friction and end-bearing piers in clay S u b s t i t u t i n g t h e actual uplifting pressure m e a s u r e d in t h e m o d e l t e s t . t h e n t h e t o t a l . F o r end-bearing piers: 77 = π ( I ) 2 u u = 24. C o m p e n s a t i o n m u s t 10 feet in expansive soils having a swelling pressure of 1 0 . This value is of significant m a g n i t u d e and can b e t h e governing factor in t h e design of grade b e a m and pier systems. T h e t o t a l dead load pressure e x e r t e d o n a 12-inch-diameter pier is 15. t h e n fu = 3. 1 5 ) ( 1 0 . F r o m t h e results of t h e e x p e r i m e n t . 7 0 0 = 7 . 1 4 . Assuming t h a t t h e vertical swelling pressure is equal t o t h e h o r i z o n t a l swelling pressure. from w h i c h t h e value f is d e t e r m i n e d t o equal 0 . 0 0 0 psf. 9 0 0 lbs. T h e u n b a l a n c e d uplifting pressure is 2 3 .5 psi F o r friction piers: 195 = 2 π (l)fu(9) fu = 3. 6 0 0 lbs. and from e x p e r i e n c e w i t h pier systems in this area. 0 0 0 ) (10-5) = 2 3 . T h e following e x a m p l e will illustrate t h e i m p o r t a n c e of uplifting pressure along t h e surface of t h e pier. it appears t h a t t h e uplifting pressure along t h e surface of t h e c o n c r e t e e x e r t e d b y t h e soil in soil-concrete pier s y s t e m s is a b o u t 15 p e r c e n t of t h e vertical swelling pressure.1 5 . A s s u m e a 12-inch-diameter pier e m b e d d e d uplifting force e x e r t e d o n t h e pier will b e : U = 27rrfu(D-d) = 2π ( 0 .45 psi. 0 0 0 psf. t h e dead load pressure n o r m a l l y assigned t o t h e piers is a b o u t 2 0 .DRILLED PIER FOUNDATIONS 81 300 250 FRI CTION ο ο ο PIERο " Ô Ο _ 0 / •· ΕND BEARING PIER " Λ Λ 4/17 4/18 4/19 4/20 4/21 DATE 4/22 4/23 4/24 4/25 Figure 36.700 lbs. 6 0 0 . 5 ) ( 0 . 27rrfu(D-d) = 7 r r p + 27rrsd 2 then: where p = ~ [ f u ( D . T h e soil is u n i f o r m for t h e full length of t h e pier. T h e swelling pressure acting on the pier for soils w i t h high degree of e x p a n s i o n is a b o u t 1 0 . . w e found t h a t after t h e c o m p l e t i o n of t h e test. By c o n v e n t i o n . T h e following a s s u m p t i o n s are m a d e t o simplify t h e computation: 1. a d i s t a n c e a p p r o x i m a t e l y equal t o the vertical m o v e m e n t of t h e pier. the cone is assumed t o be 6 0 ° . 3. U n d e r this c o n d i t i o n . It is interesting t o k n o w t h e shape of t h e failure plane w h e n t h e pier is lifted from the g r o u n d . Surface w e t t i n g will affect only the u p p e r 5 feet of the pier. s. If shale b e d r o c k is e n c o u n t e r e d at t h e b o t t o m of the pier. it is possible t o assign values of t h e soil properties in the a b o v e e q u a t i o n and o b t a i n rational solutions. it is necessary to assign values t o u. b e t w e e n the pier and the soil is 0 . F o r usual design purposes. In e x t r e m e cases. Pier A had actually pulled from the s u r r o u n d i n g soils leaving a gap b e t w e e n the b o t t o m of the pier and the surface of the container. t h e error will be on the safe side. uplifting of t h e pier is inevitable and structural A rational pier formula can be derived b y e q u a t i n g t h e total uplifting force and t h e total w i t h h o l d i n g force acting o n a pier. 0 0 0 psf and for soils w i t h m e d i u m degree of e x p a n s i o n it is a b o u t 5 . a cone of soil is carried b y t h e pier as s h o w n on figure 37a. T h e total uplifting pressure t h e n will be 4 7 . In o u r m o d e l test. and D-d.sd] ρ = unit dead load pressure T o solve the above e q u a t i o n . It was c o n c l u d e d from t h e above described m o d e l pier tests t h a t the uplifting pressure along t h e surface of c o n c r e t e was a b o u t 15 p e r c e n t of t h e vertical swelling pressure as previously discussed. 4 . O n e t h e o r y is t h a t w h e n the pier is being lifted. 2 0 0 lbs.. 5. Skin friction. f.d ) . This indicated t h a t the failure t o o k place along t h e interface of soil and c o n c r e t e as i n d i c a t e d o n figure 3 7 b . 0 0 0 psf. of t h e soils s u r r o u n d i n g t h e pier in t h e u n w e t t e d z o n e is a b o u t 5 0 0 psf. s. T h e expansive soils u s u a l l y e n c o u n t e r e d in this area belong t o t h e categories of m e d i u m or high degree of expansion ( t a b l e 4 ) . as indicated o n the strain gauge. T h e o r e t i c a l analysis of t h e m a g n i t u d e of the uplifting pressure is n o t presently feasible since t h e m e c h a n i c s of swelling have n o t y e t b e e n translated i n t o a suitable m a t h e m a t i c a l m o d e l . 1 5 . A n o t h e r t h e o r y is t h a t the p i e r is pulled o u t from t h e g r o u n d relative t o t h e s u r r o u n d i n g soils as s h o w n o n figure 3 7 b . it can be considered t h a t t h e entire length of soil s u r r o u n d i n g t h e pier is w e t t e d . and t h e u n b a l a n c e d c r a c k i n g results. f. 5 0 0 lbs. Rational pier formula uplifting p r e s s u r e will be 3 1 . 2. This is accomplished by t h e pier in t h e lower u n w e t t e d p o r t i o n of the soil w h e r e friction along t h e surface of t h e pier will provide a restraint t o uplift. T h e coefficient of uplift.82 FOUNDATIONS ON EXPANSIVE SOILS b e m a d e for this u n b a l a n c e d uplifting pressure. ). " t h e w i t h h o l d i n g force d e p e n d s u p o n t h e skin friction of t h e pier in t h e z o n e unaffected b y w e t t i n g . figure 3 8 was p r e p a r e d . With belled piers. (lbs. Most drillers are capable of providing bells w i t h d i a m e t e r s equal t o three times t h e d i a m e t e r o f the shaft. H e n c e . and t h e r e is a s t r o n g possibility of d e v e l o p m e n t of a perched w a t e r table c o n d i t i o n . t h u s increasing t h e t o t a l load-carrying c a p a c i t y . Advantage of belled piers It is seen in figure 3 2 t h a t t h e uplift forces e x e r t e d on t h e belled pier system are as follows: U=P + F where: W + F S U = t o t a l uplifting force d u e t o t h e swelling of t h e soils s u r r o u n d i n g the pier shaft in t h e u n w e t t e d z o n e .). Ρ = total vertical pressure exerted o n t h e pier. or u n d e r r e a m e d piers. T h e sloping side of t h e bell should be at an angle at least 6 0 degrees w i t h t h e h o r i z o n t a l . will n o t b e affected b y m o i s t u r e change. (lbs. F w. such as t h e rise of g r o u n d w a t e r . T h e advantages and disadvantages of t h e belled system are p r e s e n t e d in t h e following t w o paragraphs. t h e n pier uplift is unavoidable. as s h o w n on figure 3 2 .DRILLED PIER FOUNDATIONS 83 Based o n t h e above simplified a s s u m p t i o n s . t h e r e is always an added factor of safety o f this system against uplift. In areas w h e r e t h e u p p e r soils are highly expansive. t h e skin friction is lost.). t h e use of a belled pier s y s t e m should be favorably considered. (lbs. T h e ideal bell is in t h e shape of a frustum w i t h a vertical side at t h e b o t t o m . t h e greatest advantage of t h e belled pier is t h a t t h e resistance against uplift will n o t be affected b y loss of friction in t h e z o n e unaffected b y w e t t i n g . t h e dead load pressure. Such piers are c o m m o n l y referred t o as belled piers. t h e total weight of t h e soil above t h e bell. T h e vertical side m a y b e 6 t o 12 i n c h e s high. b e d r o c k is shallow. it is possible for t h e s t r u c t u r a l engineer t o select t h e size of t h e pier. BELLED PIERS Piers drilled i n t o materials o t h e r t h a n b e d r o c k are often enlarged at t h e b o t t o m of t h e h o l e for t h e p u r p o s e of increasing t h e bearing area. (lbs. F s = t o t a l shearing resistance along t h e assumed circular line of failure in t h e u n w e t t e d z o n e . F r o m t h e figure.) P r o b a b l y . d i a m e t e r of t h e t o p of frustum 2 R . and t h e required t o t a l l e n g t h of t h e pier. . If. for s o m e reason. F w = t o t a l weight of t h e soil above t h e bell. As previously discussed u n d e r "Pier U p l i f t . it is estimated t h a t in a favorable drilling c o n d i t i o n . Soil failure plane resulting from pier uplift. When it is necessary t o send a m a n d o w n the hole w i t h a j a c k h a m m e r to c o m p l e t e t h e bell. . If t h e r e i n f o r c e m e n t is i n a d e q u a t e . the cost of belling can easily exceed t h e cost of drilling an e x t r a 10 feet of pier shaft. t h e uplift pressure exerted o n t h e pier is several times higher t h a n if the smaller 12-ineh-diameter shaft is used. In hard b e d r o c k .84 FOUNDATIONS ON EXPANSIVE SOILS Uplift Uplift a Original position ^^"'^ of pier bottom b Figure 37. P r o p e r cleaning of a belled pier can only be accomplished b y sending a m a n d o w n t h e hole w i t h a shovel. T h e m i n i m u m shaft size is 2 4 inches. a mechanical device m a y be unsuccessful. It is nearly impossible t o bell in granular soil. Cleaning a belled pier is m u c h m o r e difficult t h a n cleaning a straight-shaft pier. T h e m a g n i t u d e of dead load pressure exerted o n t h e pier is n o t a factor. With present safety regulations. Bells can be formed mechanically w i t h a special belling device. a l t h o u g h 3 0 inches is desirable. The difficulty of belled pier construction increases m a r k e d l y where the materials immediately overlying b e d r o c k are subject t o caving. Since a belled pier system relies entirely u p o n t h e anchorage of t h e lower p o r t i o n of the pier in t h e z o n e unaffected b y w e t t i n g against uplift. T h e advantage of anchorage in a bell pier system can easily be offset b y drilling an additional 10 feet i n t o b e d r o c k with a straight-shaft pier. Disadvantage of belled piers T h e shaft of a belled pier m u s t be sufficiently large t o allow c l e a n o u t and inspection. b u t t h e c o n d i t i o n of t h e bell c a n n o t be inspected from g r o u n d level. T h e m o s t p r o m i n e n t disadvantage of belled piers is the cost and t h e difficulty of inspection. T h e condition of t h e b o t t o m of the straight-shaft pier can b e inspected from t h e g r o u n d level with a t o r c h o r a m i r r o r . tension cracks can develop in t h e vicinity of t h e pier shaft and t h e bell. this m e a n s t h a t all pier shafts will have t o be cased. T h e p r o b l e m is further c o m p l i c a t e d if ground w a t e r is e n c o u n t e r e d . this t y p e of pier can b e used for c o l u m n s w i t h very light load. In t h e Denver area where the cost of pier drilling is exceedingly c o m p e t i t i v e . More r e i n f o r c e m e n t will be required in a belled pier t h a n in a small-diameter straight-shaft pier. t h e cost can be prohibitive. F o r straight-shaft piers w i t h a large shaft d i a m e t e r . elimination of t h e skin friction necessitates t h e use of large bells for s u p p o r t t o justify t h e use of such a system. o r straw. Since m o s t piers rely heavily o n t h e skin friction t o carry t h e c o l u m n load. T h e obvious m e t h o d is t o drill an oversized pier hole and provide soft. However. ash. N o lateral resistance is available for t h e system. t h e total load carrying capacity of t h e pier will be greatly r e d u c e d .FEET Figure 38. 3 . Isolation of pier uplift Since t h e m o s t d a m a g i n g action in a drilled pier system is t h e uplifting pressure e x e r t e d o n t h e p e r i m e t e r of t h e pier. and . compressible material a r o u n d t h e pier so t h a t t h e s u r r o u n d i n g soils c a n n o t exert uplifting pressure on t h e wall of t h e pier. t h e simple solution appears t o be isolating t h e pier shaft from t h e uplifting forces. Rational pier design chart.DRILLED PIER FOUNDATIONS 85 4 6 8 10 REQUIRED TOTAL 12 14 16 18 20 PIER LENGTH . b u t r e p e a t e d seasonal cycles of e x p a n s i o n and shrinkage will pack t h e filling and allow t h e s u r r o u n d i n g soils t o grip t h e pier shaft. T h e s h o r t c o m i n g s of this system are as follows: 1. 2. T h e load carrying capacity of t h e pier is r e d u c e d t o t h a t of a pad footing f o u n d a t i o n . t h e advantages of t h e belled pier can be achieved b y the use of straight-shaft piers w i t h a larger p e n e t r a t i o n i n t o a zone unaffected b y w e t t i n g . T h e a n n u l a r space m u s t be filled w i t h compressible m a t e r i a l such as s a w d u s t . T h e use of loose sand or pea gravel will b e satisfactory initially. b y so doing. With d u e c o n s i d e r a t i o n of t h e cost. 000 (or u p t o 1. and lastly. Texas [ 4 3 ] w h e r e highly expansive clays are found. With such subsoil c o n d i t i o n . T h e process is costly and in m o s t cases n o t effective in preventing the seepage of w a t e r . In the design of a friction pier. the annulus of c o n c r e t e outside t h e c o a t e d section b r e a k s in tension. and the upward force is limited b y t h e viscosity of t h e mastic. a small d i a m e t e r S o n o t u b e can t h e n be inserted i n t o the hole and t h e annular spaces filled w i t h sawdust. T h e outside of the pipe from the t o p d o w n to the b o t t o m of the p o t e n t i a l l y w e t t e d z o n e is coated w i t h a b i t u m i n o u s m a s t i c material. the m a x i m u m design value for skin friction of all cohesive soils should b e limited t o a b o u t 1. t h e infiltration of w a t e r t o t h e b o t t o m of t h e pier hole will n o t cause uplift. t h e p u r p o s e of placing t h e pier in a z o n e unaffected b y wetting is defeated. This s h o r t c o m i n g can be partially corrected b y sealing t h e u p p e r 3 feet of t h e a n n u l a r space w i t h c o m p a c t e d clay. unnecessary t o drill all piers i n t o b e d r o c k o r very h a r d f o r m a t i o n s . A n oversized pier hole can be drilled t o t h e t o p of t h e gravel s t r a t u m .86 FOUNDATIONS ON EXPANSIVE SOILS 4 . as long as t h e pier shaft is free from c o n t a c t w i t h t h e expansive soils. T h e end result of using such isolated-shaft belled piers is n o different from using individual p a d s . FRICTION PIERS Since t h e p u r p o s e of a drilled pier f o u n d a t i o n is t o transfer t h e building load to a z o n e where t h e m o i s t u r e c o n t e n t will n o t be affected b y surface wetting. however. a logical solution is t h e use of friction piers. T h e annular space filled w i t h compressible material affords a free p a t h for w a t e r t o reach t h e b o t t o m of t h e pier and heave t h e entire pier from t h e b o t t o m . T h e concrete-filled pipe is designed t o carry t h e c o l u m n load at t h e t o p . A pipe is i n t r o d u c e d i n t o the pier. it is. Where b e d r o c k is d e e p and t h e u p p e r overburden clays are expansive. t h e m a s t i c coating breaks the b o n d . A n o t h e r a p p r o a c h has b e e n devised t o break t h e b o n d b e t w e e n the pier and t h e clay. a rational value should be assigned t o t h e skin friction. therefore. In stiff or hard clays. When t h e pier is lifted by swelling soils. t h e S o n o t u b e filled w i t h c o n c r e t e . C o n s e q u e n t l y . U l t i m a t e pier shaft resistance can b e calculated as the sum of t h e u l t i m a t e shearing resistance imposed b y t h e various strata w h i c h are in c o n t a c t w i t h t h e pier. Figure 39 illustrates this system. A c c o r d i n g t o T e n g [ 4 7 ] . Skin friction It is generally recognized t h a t the skin friction b e t w e e n cohesive soils and the pier shaft c a n n o t exceed t h e cohesion of t h e soil. T h e u l t i m a t e shaft . O n e possible application of such a system is w h e r e t h e u p p e r expansive soils are underlain b y n o n e x p a n s i v e gravelly soils. Cohesion m a y be assumed to be equal t o one-half of t h e u n c o n f i n e d compressive s t r e n g t h of t h e soil. T h e c o n c e p t was initiated in San A n t o n i o . t h e b o n d b e t w e e n t h e c o n c r e t e and t h e soil m a y b e less t h a n cohesion. Such systems have been used w i t h great success in a n o t o r i o u s l y t r o u b l e s o m e area w h e r e t h e thickness of the u p p e r swelling soil is a b o u t 20 feet.500) psf. San Antonio. Consulting Engineers. Tex. Design of belled pier for relief of uplift due to expansion of upper clay layer. Note that the outer annulus of concrete is expected to break in tension near the bottom of the expansive clay layer.DRILLED PIER FOUNDATIONS 87 Figure 3 9 . (Raba and Associates.) . 62 0. Material Properties Material Stiff clay Stiff clay Massive shale Glacial till Stiff clay Stiff clay Moisture content W n. this length should be excluded in calculating t h e pier load capacity. Sufficient field p e n e t r a t i o n resistance tests should b e p e r f o r m e d n o t only t o establish the p r o p e r friction value b u t also t o insure t h a t soft soils are n o t e n c o u n t e r e d within the length of t h e pier. Gardner. percent 23 25 15 12 . Since t h e u p p e r 5 feet of soil around t h e pier is subject t o surface w e t t i n g and uplifting.44 0. Plasticity Index I . Shear strength reduction factors (a) for drilled piers.2 5. Table 11—. " T h e following considerations should be given: 1. 3.49 0. . are listed in table 1 1 . Shear value can also be interpreted indirectly from field p e n e t r a t i o n resistance tests or c o n e p e n e t r a t i o n resistance data. ft. the end bearing capacity of such piers can generally be neglected. O t h e r m o r e direct m e t h o d s in evaluating shear strength such as the use of a p r e s s u r e m e t e r inserted i n t o t h e drill h o l e at t h e desired d e p t h can be used. .2 1. With t h e use of small d i a m e t e r piers. and tills. +Sandy gravel with cobbles and approximately 50 percent silty clay. percent 35-55 20-60 7-16 2-16 .52 0..88 FOUNDATIONS ON EXPANSIVE SOILS resistance of a pier drilled in uniform clay strata can be expressed in terms of the u n d r a i n e d shear strength S u and a r e d u c t i o n factor α as follows: S =n The undrained shear strength of r2aSu clays can be d e t e r m i n e d in the laboratory from the unconsolidated-undrained triaxial test. Design of friction piers T h e principle for t h e design of friction piers is given in a previous section u n d e r " R a t i o n a l Pier F o r m u l a .4 a 0. F r i c t i o n piers generally bear o n stiff clays.0 2. Shear strength. an u n c o n f i n e d compression test will generally serve the p u r p o s e . tons per sq. Water migrating from t h e c o n c r e t e t o t h e pier wall also reduces t h e shear strength.1 0. T h e shear strength r e d u c t i o n factor. G a r d e n e r and Greer [ 4 3 ] is also influenced b y c o n s t r u c t i o n effects and t h e m o i s t u r e sensitivity of the s u p p o r t i n g materials. (After Woodward. 2. A vane shear test is n o t applicable for use in stiff clays b u t can be valuable in soft clays.30 Reference Whitaker and Cooke Reese and O'Neill Matich and Koziki Matich and Kozicki Woodward et al.. and Greer) . O t h e r elements such as t h e d u r a t i o n of e x p o s u r e of o p e n shafts can also be i m p o r t a n t factors. according t o W o o d w a r d . shales.64*+ 0.9 1. F o r stiff clays. Ν > 45 blows per ft.64* 0. T h e shear strength r e d u c t i o n factors a as derived from analysis of p r o t o t y p e field load tests in clays.5 1. . 1. Mohan and Jain 19 36-46 *Failure was not reached. or incorrectly c o n s t r u c t e d . F r i c t i o n piers should n o t be used at a site w h e r e g r o u n d w a t e r is high or w h e r e t h e r e is t h e possibility of future high g r o u n d .000 psf 15.14 Using a factor of safety of 3 Design load factor of safety of 2 can b e used. e x p a n s i o n j o i n t . Average p e n e t r a t i o n resistance Average unconfined compressive strength Pier length Pier d i a m e t e r Shear s t r e n g t h r e d u c t i o n facto Pier capacity U l t i m a t e skin friction r 4. An e x a m p l e of typical friction pier design is as follows: Data Stiff clay t o d e p t h 4 0 feet. 6 kips.5 ) x 1.000 x0.w a t e r c o n d i t i o n . a building w i t h a pier f o u n d a t i o n is j u s t as vulnerable. t o m o v e m e n t t h a n a building f o u n d e d o n spread footings. grade b e a m .000 x 3. and floor system.15 = 2 3 .7 kips W h e n designing for a d e a d load plus full live load.5 ) x 1. 0 0 0 psf 2 0 feet 12 inches 0. = 1 0 . r e i n f o r c e m e n t .000 x 3 . 5 kips = 1. 1 4 = 47. if incorrectly designed. 1 kips T h e above calculations indicate t h a t t h e design is safe as t h e w i t h h o l d i n g force is larger t h a n t h e uplifting force. if n o t m o r e vulnerable. T h e n design load Uplifting force Swelling pressure Coefficient of uplift T o t a l uplift = 3. 0 0 0 χ 0. A typical detail is s h o w n o n figure 4 0 . . F A I L U R E O F T H E P I E R SYSTEM A p r o p e r l y designed drilled pier system involves t h e c o o r d i n a t i o n of t h e floor slab.15 χ 5 Withholding force U l t i m a t e skin friction U l t i m a t e w i t h h o l d i n g force = ( 2 0 . 0 0 0 psf = 0. n o ground w a t e r e n c o u n t e r e d .1 kips = 2 3 . a 4 7 . T h e grade b e a m and pier system offers t h e m o s t logical s o l u t i o n for lightly loaded s t r u c t u r e s founded on expansive soils.14 χ 1 0 .DRILLED PIER FOUNDATIONS 89 4.000 psf T o t a l load carrying capacity = ( 2 0 .5 4 . void space.5 : : 25 b l o w s p e r foot 1. However. t h e first sign of pier m o v e m e n t will be reflected as cracks t h a t develop in the brick wall as s h o w n o n figures 41 and 4 2 .90 FOUNDATIONS ON EXPANSIVE SOILS Figure 40. . the t y p e of cracking d e p e n d s u p o n the structural configuration of the building. Masonry walls and cinderblock walls are m o s t sensitive t o m o v e m e n t . When severe diagonal cracks a p p e a r in t h e b a s e m e n t as s h o w n o n figure 4 3 . Typical detail of grade beam and pier system. O f t e n t i m e s . Generally. Basement walls are structurally m o r e resilient to differential m o v e m e n t t h a n m a s o n r y walls. pier uplifting can be considered a certainty. the cracks are caused b y slab m o v e m e n t as discussed in c h a p t e r 6. Cracks often are wider at t h e t o p and n a r r o w e r at t h e b o t t o m . Considerable experience is required to d e t e r m i n e the cause of cracking of a building founded on piers. C o n s e q u e n t l y . Typical pier uplift m o v e m e n t generally takes place a short distance from t h e pier and has a 4 5 degree p a t t e r n . Insufficient pier length is the cause. Figure 4 2 . Typical cracking caused by pier uplift.DRILLE D PIE R FOUNDATION S 91 Figure 4 1 . . Heaving of piers of a three-story structure. considerable difficulty can be e n c o u n t e r e d in cleaning t h e holes. N o r m a l l y . excessive pier diameter.92 FOUNDATIONS ON EXPANSIVE SOILS Figure 43. t h e safer t h e building. o r defective air space b e n e a t h t h e grade b e a m s . T h e m o s t c o m m o n errors in c o n s t r u c t i o n are excess c o n c r e t e on t o p of pier resulting in m u s h r o o m s at t h e t o p of t h e piers and t h e absence of. piers should have a m i n i m u m spacing of 12 feet. These are discussed in detail as follows: Excessive pier size M a n y l a y m e n have t h e impression t h a t t h e larger t h e d i a m e t e r of t h e pier. w i t h pier holes less t h a n 12 inches in d i a m e t e r . Note that crack is wide at the top and narrow at the bottom. T h e m o s t e c o n o m i c a l spacing of t h e piers is limited b y t h e a m o u n t of r e i n f o r c e m e n t in t h e grade b e a m s o r t h e e c o n o m i c a l size of t h e floor b e a m s . Typical cracks developing in the basement wall immediately beneath the window well. However. Auger sizes of 8 and 10 inches are also available. Actually. t o e x e r t e n o u g h dead load pressure o n t h e pier. or t h e absence of pier r e i n f o r c e m e n t . T h e m o s t c o m m o n errors in design are insufficient pier length. A pier hole w i t h as little as 2 inches of loose soil at t h e b o t t o m will experience excessive s e t t l e m e n t at a later . Most small drill rigs in t h e R o c k y M o u n t a i n area are e q u i p p e d t o drill 12-inch-diameter pier holes. it is necessary t o use small d i a m e t e r piers in c o m b i n a t i o n w i t h long spans. t h u s t h e skin friction along t h e pier providing t h e anchorage w o u l d b e lost and t h e pier w o u l d have t o d e p e n d u p o n dead load pressure alone t o resist uplifting. . This is illustrated on figure 4 4 . T h e dead load pressure for a lightly loaded building is n o t of sufficient m a g n i t u d e t o resist t h e uplifting. T h e use of 12-inch-diameter piers for residential and light c o m m e r c i a l c o n s t r u c t i o n is recommended. T h e r e is a good possibility of w e t t i n g of t h e entire length of such a pier and s u b s e q u e n t heaving of t h e pier. t h e function of a s h o r t pier actually is n o different or m o r e desirable t h a n individual pad footings. plus t h e uplifting pressure acting u p o n t h e p e r i m e t e r of t h e pier. insufficient pier length As explained in t h e previous section. the stability of t h e pier against uplift d e p e n d s u p o n t h e a m o u n t of dead load pressure exerted o n t h e pier and t h e anchorage provided in t h e l o w e r p o r t i o n of the pier. T h e r e f o r e . it is c o m m o n in this area t o specify a m i n i m u m b e d r o c k p e n e t r a t i o n of 4 feet. This is especially t r u e in t h e case of t h e interior. t h e uplifting pressure is t h e sum of t h e swelling pressure acting o n t h e b o t t o m of t h e pier. T h e practice of using piers of insufficient length is usually observed in areas w h e r e claystone shale is near g r o u n d surface and t h e engineer specifies only t h e d e p t h of p e n e t r a t i o n i n t o b e d r o c k . 4 5 ) is c o m m o n p l a c e . t h e possibility of the soil at t h e b o t t o m of t h e pier b e c o m i n g w e t t e d is great.DRILLED PIER FOUNDATIONS 93 d a t e . F o r short piers. If t h e length of t h e pier is short. Dead Dead load pressure load pressure ! \ \ Swelling p r e s s u r e on s u r f a c e of pier HUH Swelling pressure t t Swelling SHORT PIER pressure FOUNDATION PAD FOUNDATION Figure 44. Swelling pressure acting on short pier foundation and pad foundation. this results in a pier w i t h a total length of only 4 feet. Failure of t h e pier system d u e t o insufficient pier length (fig. In m a n y instances. lightly loaded piers. F o r e x a m p l e . excess c o n c r e t e is usually n o t removed from t h e t o p of t h e pier. Note: Adjacent Sonotube placed for underpinning. Uniform pier diameter t h a t t h e engineer also specify t h e m i n i m u m total pier length in the f o u n d a t i o n system to insure t h a t t h e piers are a n c h o r e d sufficiently d e e p in t h e u n w e t t e d zone of After t h e pier hole is drilled. t h e m u s h r o o m has been k n o w n t o have a d i a m e t e r t h r e e times t h e d i a m e t e r of t h e pier. resulting in a m u s h r o o m occurring at the t o p of t h e pier as indicated o n figures 4 6 and 4 7 . Typical example of short pier foundation (insufficient pier length). F o r a 12-inch-diameter pier w i t h a 36-inch-diameter m u s h r o o m . and d u r i n g t h e placing of c o n c r e t e . A t times.94 FOUNDATIONS ON EXPANSIVE SOILS Figure 45. Pier drilled into highly weathered claystone. Soil b e n e a t h t h e grade b e a m s will exert direct uplifting pressure on t h e underside of t h e m u s h r o o m . the total area . It is i m p o r t a n t the bedrock. T h e m a x i m u m . and t h e uplifting pressure is acting u p o n t h e u p p e r w e t t e d p o r t i o n of t h e pier.DRILLED PIER FOUNDATIONS 95 GRADE BEAM Reinforcement Void beneath grade beam Uplifting pressure exerted on mushroom of pier. a m o u n t of tensile force developed can roughly be calculated as follows: T = 27rrsd -P w h e r e : Τ = total tensile force in lbs. Pier reinforcement Since t h e lower p o r t i o n of t h e pier is a n c h o r e d i n t o b e d r o c k b y skin friction. Figure 46. 0 0 0 psf. It is r e c o m m e n d e d t h a t S o n o t u b e s . and Ρ = t o t a l dead load pressure exerted o n t h e pier in lbs. b e used t o form t h e u p p e r p o r t i o n of t h e pier t o assure u n i f o r m pier d i a m e t e r . Effect of mushroom on the uplifting of the pier. . subjected t o uplifting is 6. This pressure alone will be sufficient t o lift a pier provided it is n o t a d e q u a t e l y a n c h o r e d .25 square feet. tensile stress develops w i t h i n t h e pier. or a similar p r o d u c t . t h e t o t a l uplifting pressure exerted o n t h e m u s h r o o m is a b o u t 6. F o r a m o d e r a t e l y expansive soil having a swelling pressure of 1 0 .2 kips. 6 t o 1. however. F o r a 12-inch-diameter pier w i t h a 15-kip dead load pressure having 4 feet of u n w e t t e d length. A typical tension crack in a pier is s h o w n on figure 48.0 p e r c e n t r e i n f o r c e m e n t is sufficient. Generally. T h e required void space can be formed b y t h e use of sand. Air space T o prevent t h e lower soils from exerting uplifting pressure o n t h e grade b e a m s . o r o t h e r similar material which can be removed after t h e grade b e a m . 0.96 FOUNDATIONS ON EXPANSIVE SOILS Figure 47.1 kips o r 89 psi. This stress can be t a k e n b y lightly reinforced piers. c a r d b o a r d . R e i n f o r c e m e n t of t h e full length of t h e pier is essential t o avoid tensile failure. t h e r e f o r e . b u t t h e r e are cases w h e n it is necessary t o use as m u c h as 7 p e r c e n t . This z o n e generally is located at least 3 feet b e l o w grade b e a m . t h e n o r m a l d o w e l bars used in t h e piers will n o t provide t h e required resistance t o tension. it is essential t h a t t h e r e b e n o c o n t a c t b e t w e e n soil and grade b e a m s . and assuming t h a t t h e skin friction of c l a y s t o n e is 2 . w i t h o u t r e i n f o r c e m e n t t h e pier will fail in tension. 0 0 0 psf. T h e location of t h e tension cracks is usually at t h e b o u n d a r y of t h e w e t t e d and u n w e t t e d p o r t i o n of t h e pier. Typical mushroom 26 inches in diameter on top of a 12-inch-diameter pier. t h e t o t a l possible uplifting force is 10. DRILLED PIER FOUNDATIONS 97 Figure 48. T h e cardboard form material also p r o t e c t s t h e backfill soils from plugging u p t h e void space. T h e c a r d b o a r d material is w r a p p e d in plastic and has a d e q u a t e strength t o s u p p o r t t h e c o n c r e t e b u t will d e t e r i o r a t e after t h e plastic is p u n c t u r e d as s h o w n on figure 4 9 . is p o u r e d . It is n o t necessary t o r e m o v e t h e c a r d b o a r d material after t h e c o m p l e t i o n of t h e grade b e a m s . Tension crack developed at approximately 3 feet below grade beams. T h e m o s t convenient m e t h o d is b y t h e use of a void-forming c a r d b o a r d form k n o w n as " V e r t i c e l " . Figure . however. h o w e v e r . In e x t r e m e cases. excessive s e t t l e m e n t can occur. Bearing capacity of t h e pier is usually m u c h larger t h a n t h e design bearing value. T h e thickness of air space provided b y t h e cardboard ranges from 3 t o 4 inches. If the pier is n o t b o t t o m e d on b e d r o c k b u t instead o n the u p p e r stiff clays. t h e n excessive s e t t l e m e n t can occur. Deterioration of Vertical (void-forming cardboard) beneath the grade beam. However. If. Pier settlement If piers are drilled d e e p i n t o b e d r o c k t o provide t h e necessary anchorage in a swelling soil area. It is assumed t h a t the a m o u n t of e x p a n s i o n of t h e soils b e n e a t h the grade b e a m will n o t exceed 3 inches. and a m i s t a k e n identification of t h e bearing soil occasionally occurs. it m a y be necessary t o provide a 6-inch air space. S e t t l e m e n t of a single pier can usually be bridged b y t h e adjacent piers and is usually u n n o t i c e d . 50 indicates the l o c a t i o n of t h e c a r d b o a r d material. pier s e t t l e m e n t should n o t pose a p r o b l e m . Stiff clays s o m e t i m e s have a strong resemblance t o claystone b e d r o c k . These cases c o m m o n l y o c c u r in small projects w h e r e t h e r e is n o c o n s t r u c t i o n c o n t r o l and t h e pier driller d e t e r m i n e s t h e length of t h e pier and the a m o u n t of p e n e t r a t i o n . A small a m o u n t of d r y or plastic c u t t i n g s o n t h e b o t t o m of a pier hole will n o t affect the bearing capacity of t h e pier. An . t h e r e are cases w h e r e excessive s e t t l e m e n t of the pier has t a k e n place resulting in severe cracking of t h e building. U n d e r n o r m a l pier load. there have been installations where a 4-inch air space was c o m p l e t e l y closed b y highly expansive soils. t h e m a g n i t u d e of pier s e t t l e m e n t in c o m p e t e n t b e d r o c k should b e b e t w e e n 1/4 and 1/2 inches.98 FOUNDATIONS ON EXPANSIVE SOILS Figure 49. there are 1 or 2 inches of soft soils at the b o t t o m of t h e pier hole and c o n c r e t e is p o u r e d o n t h e soft m u d . Such p h e n o m e n o n usually takes place in small-diameter piers (10 t o 12 inches in d i a m e t e r ) w h e r e t h e driller is u n a b l e t o remove t h e m u d a c c u m u l a t e d at the b o t t o m of the pier hole b y spinning the auger. DRILLED PIER FOUNDATIONS 99 Figure 50. 111. voids are caused b y collapse of t h e . Voids o r discontinuities in t h e pier shaft often result w h e n a c o n c r e t e which is t o o stiff is used. In a d d i t i o n . S e t t l e m e n t of o n e pier can cause visible lifting and separation of t h e c h i m n e y as s h o w n o n figure 5 1 . Void in pier shaft Voids in pier shafts have b e e n of great c o n c e r n t o f o u n d a t i o n engineers since t h e p r o b l e m developed in a major s t r u c t u r e in Chicago. e x c e p t i o n is t h e c h i m n e y of a h o u s e . The use of Verticel (void-forming cardboard) beneath the grade beam. C h i m n e y f o u n d a t i o n pads are generally s u p p o r t e d b y t w o t o t h r e e piers. casing. However. piers drilled in expansive soil areas often e n c o u n t e r stiff clays and the p r o b l e m of squeezing of t h e soft formation seldom takes place. w i t h a carefully designed grade b e a m and pier s y s t e m . there should be n o m o v e m e n t of a building even u n d e r severe w e t t i n g c o n d i t i o n s . F o r t u n a t e l y . or b y hang-up of c o n c r e t e in t h e casing while being pulled. b y squeezing of t h e soft f o r m a t i o n . . Uplifting of foundation walls Theoretically. an i m p o r t a n t factor c o n t r i b u t i n g t o t h e m o v e m e n t of a building w h i c h is generally ignored is t h e possibility of uplifting pressure e x e r t e d o n t h e e x t e r i o r surface of t h e b a s e m e n t walls and grade b e a m s . However. cases are k n o w n where reinforcing cage was inserted i n t o cased small-diameter piers w i t h stiff c o n c r e t e resulting in voids.100 FOUNDATIONS ON EXPANSIVE SOILS Figure 51. Separation between chimney and house caused by pier settlement. and at t h e same t i m e . T h e e x c e p t i o n being t h e rise of g r o u n d water w h e r e t h e o n c e dry p o r t i o n of soil s u r r o u n d i n g t h e pier b e c o m e s c o m p l e t e l y s a t u r a t e d . Several h o u s e s were e x a m i n e d where t h e long side of t h e b a s e m e n t wall actually assumed a b o w shape w i t h m a x i m u m deflection in excess of 3 inches and all of t h e a n c h o r b o l t s were b e n t and p u s h e d in. It should be n o t e d t h a t t h e lateral pressure d u e t o swelling differs from active e a r t h pressure in t h a t it is of u n i f o r m i n t e n s i t y . a 20-foot-long. Lateral pressure on foundation walls Backfill against t h e b a s e m e n t wall n o t o n l y exerts uplifting pressure o n t h e wall.000 χ 0. Rise of ground water T h e t h e o r y b e h i n d t h e drilled pier system is t h a t t h e r e must be sufficient d e a d load pressure and t h e pier m u s t be long e n o u g h so t h a t t h e lower p a r t of t h e pier is e m b e d d e d in a z o n e unaffected b y m o i s t u r e change. T h e above calculation is based on an e x t r e m e case. Assuming t h a t surface w a t e r will n o t p e n e t r a t e m o r e t h a n a b o u t 15 feet. T h e skin friction used for pier w i t h h o l d i n g is n o w c o m p l e t e l y lost. selected backfill material is desirable. d e p e n d i n g o n l y u p o n t h e d e p t h of w e t t i n g . T o r e d u c e such lateral m o v e m e n t . should consist of n o n e x p a n s i v e soils t o m i n i m i z e t h e risk of wall uplift. well c o m p a c t e d . in expansive soil areas. t h e n t h e c o n t a c t area b e t w e e n t h e soil and c o n c r e t e is 8 square feet per foot of wall.DRILLED PIER FOUNDATIONS 101 Assuming t h a t t h e soil has a swelling pressure of 1 0 . Lateral m o v e m e n t of t h e b a s e m e n t walls is p r e v e n t e d at t h e t o p b y floor joists w h i c h are a n c h o r e d t o t h e wall b y m e a n s of w o o d e n sills and a n c h o r b o l t s and at t h e b o t t o m of floor slabs. 0 0 0 psf. In e x t r e m e cases w i t h c o m p l e t e w e t t i n g . b u t also exerts full h o r i z o n t a l e x p a n s i o n pressure against t h e wall. and t h e pier will lift. T h e F H A Specifications call for t h e use of 1 /2-inch bolts e m b e d d e d n o t less t h a n 6 inches w i t h a m a x i m u m spacing n o t less t h a n 4 feet. However. All backfill material a r o u n d t h e b a s e m e n t walls. With piers at 15-foot intervals. such an uplifting pressure seldom occurs because t h e backfill a r o u n d t h e b a s e m e n t walls is loosely c o m p a c t e d and c o m p l e t e w e t t i n g of t h e backfill rarely takes place. . heavily loaded pier should theoretically b e free from a n y possible movement.15 χ 8 = 1 2 . t h e uplifting pressure exerted o n each pier w o u l d be a b o u t 180 kips. swelling pressure acting along t h e face of t h e b a s e m e n t walls c a n n o t be ignored. T h e size and spacing of b o l t s is insufficient t o p r e v e n t lateral wall m o v e m e n t . t h e a m o u n t of lateral pressure e x e r t e d o n t h e wall can r e a c h as high as 8 0 kips per r u n n i n g foot of t h e wall. In actual c o n d i t i o n s . 0 0 0 psf and backfill is in c o n t a c t w i t h 8 feet of height of t h e e x t e r i o r wall. 0 0 0 lbs per r u n n i n g foot of t h e wall. This pressure equals at least t h e full swelling pressure of t h e soil. N o n e x p a n s i v e material will m i n i m i z e t h e expansive pressure e x e r t e d o n t h e wall. t h e uplfiting pressure along t h e face of t h e c o n c r e t e b a s e m e n t wall will be 10. and w i t h an e x p a n s i o n pressure of 1 0 . In all cases. impervious backfill will p r e v e n t surface w a t e r from seeping t h r o u g h t h e backfill i n t o t h e f o u n d a t i o n soils. In an 8-foot b a s e m e n t backfilled w i t h expansive soils. backfill material should b e n o n e x p a n s i v e and impervious. In 1 9 6 5 . W.w a t e r level in the surrounding area rose. R.. C . and Greer. and Chandra. H. Vol. D. 1972. K. B. and Liu. ASCE. 1962. Mitchell. C. houses generally were f o u n d e d w i t h spread footings and had n o e x p a n s i o n p r o b l e m s for m a n y years. "Some Swelling Characteristics of Compacted Clays. Gardner. W." Prentice-Hall. 1-17. pp.102 FOUNDATIONS ON EXPANSIVE SOILS Rising of g r o u n d w a t e r can s o m e t i m e s cause expansion of soil in an otherwise relatively stable soil area. M.. "Drilled Pier Foundation. pp. 9 1 . m a n y cases of cracked houses were r e p o r t e d . Such f o u n d a t i o n m o v e m e n t was directly a t t r i b u t e d t o t h e rise of g r o u n d water. D. V. P. K. [46] Seed." Journal of the Soil Mechanics & Foundation Division. REFERENCES [43] Woodward. 294-301." Geotechnique. Vol. Inc. S u b s e q u e n t l y . C . No. "Foundation Design." Highway Research Board Bulletin 313. during t h e area flood. and Chan.. S. t h e w a t e r level at Cherry Creek D a m reached an all time high and t h e g r o u n d . "Frictional Resistance of Bored Piles in Expansive Clays. S. [45] Mohan. In t h e Cherry Creek D a m area of Denver. XI. [44] Parcher." McGraw-Hill Book Company. 4. "Studies of Swell and Swelling Pressure Characteristics of Compacted Clays. J. [47] Teng... . J. CONTINUOUS FOOTINGS T h e m o s t c o m m o n t y p e of f o u n d a t i o n for lightly loaded structures is the continuous footings.000 t o 1. T o insure t h a t a d e a d load pressure of at least 1. t h e w i d t h of the footing should b e as n a r r o w as possible. T o c o n c e n t r a t e sufficient dead load pressure o n expansive soils. 2. Sufficient dead load pressure is e x e r t e d on t h e f o u n d a t i o n . Wall footings Engineers often specify t h e erection of b a s e m e n t walls directly o n t h e soil w i t h o u t t h e use of footings. Such a c o n c e p t is sound from the expansive soil . per ft. per ft.000 psf is exerted on t h e soil. It should be n o t e d t h a t c o n t i n u o u s spread footings c a n n o t be e x p e c t e d t o function well in highly expansive soil areas. T h e s t r u c t u r e is rigid e n o u g h so t h a t differential heaving will n o t cause cracking. per ft. Generally. it will be necessary t o use very n a r r o w footings. in m o s t cases less t h a n 12 inches wide. 1. This reduces t h e bearing w i d t h t o a b o u t 9 inches and increases considerably t h e u n i t dead load pressure e x e r t e d on the soils.500 lb. t h e dead load pressure e x e r t e d o n a c o n t i n u o u s f o u n d a t i o n is low and in t h e following range: Single story schools Basement h o u s e Butler t y p e building 2 . 0 0 0 to 4 . 0 0 0 lb. or 3 . T h e swelling p o t e n t i a l of the f o u n d a t i o n soils can be eliminated o r r e d u c e d . < 5 0 0 lb.Chapter 5 FOOTING FOUNDATIONS INTRODUCTION F o o t i n g f o u n d a t i o n s can be successfully placed o n expansive soil provided one or m o r e of t h e following criteria are m e t : 1. Local building codes s o m e t i m e s specify the m i n i m u m allowable w i d t h of footing as 2 0 inches w h i c h is n o t applicable for footings which are t o be placed on expansive soils. those having a swelling p o t e n t i a l of less t h a n 1 p e r c e n t and a swelling pressure of less t h a n 3 . T h e use of this system should be limited t o soils w i t h a low degree of e x p a n s i o n . 0 0 0 psf. T h e s t r u c t u r e did n o t exhibit damage after 17 years despite t h e considerable differential m o v e m e n t s of u p t o 5 inches b e t w e e n t h e corners of individual b o x e s . 2. Each u n i t will t h e n act i n d e p e n d e n t l y and differential m o v e m e n t can be confined t o t h e j o i n t s . and 3. and change of elevation. Masonry bricks and cinder blocks c a n n o t w i t h s t a n d m o v e m e n t and should n o t be used for f o u n d a t i o n walls. M a k e sure t h a t t h e walls are p r o p e r l y restrained against e a r t h pressure. care should b e exercised t o insure t h e rigidity of t h e system b y checking t h e following c o n d i t i o n s before c o n s t r u c t i o n begins: 1. T h e a r r a n g e m e n t is s h o w n o n figure 54. T h e average height of a c o n c r e t e b a s e m e n t wall is 6 feet. T h e small saving derived from using m a s o n r y c o n s t r u c t i o n instead of c o n c r e t e f o u n d a t i o n walls m a y later result in heavy loss of p r o p e r t y in t h e event of f o u n d a t i o n m o v e m e n t . Israel. Shraga and A m i r c o n c l u d e d t h a t b o x c o n s t r u c t i o n can structurally w i t h s t a n d m o v e m e n t and tension w i t h o u t cracking. Box construction T h e use of heavy r e i n f o r c e m e n t in t h e f o u n d a t i o n wall can p r o t e c t t h e s t r u c t u r e from cracking d u e t o differential heaving. such c o n s t r u c t i o n is m o r e difficult. Consideration should t h e n be given t o t h e use of a c o n s t r u c t i o n j o i n t t o separate t h e s t r u c t u r e i n t o t w o o r m o r e u n i t s . F o r split-level residential h o u s e s o r b a s e m e n t s with walk-out d o o r s . T h e u p p e r wall heaved and i m p a r t e d h o r i z o n t a l pressure t o t h e b a s e m e n t wall resulting in heavy b o w i n g of t h e wall even before t h e h o u s e was c o m p l e t e d (figs. Such walls can span an u n s u p p o r t e d length of at least 10 feet. PAD F O U N D A T I O N S T h e pad f o u n d a t i o n system consists essentially of a series of individual footing pads placed o n t h e u p p e r soils and s p a n n e d b y grade b e a m s . t h e difference being t h a t t h e pads bear o n t h e u p p e r soils and skin friction is n o t involved. D e t e r m i n e if t h e r e are any soft p o c k e t s in t h e excavation t h a t m a y i n t r o d u c e s e t t l e m e n t . where t h e s t r u c t u r e consists of t w o reinforced c o n c r e t e b o x e s each a b o u t 2 2 b y 35 feet in dimension. therefore. such as d o o r s . . However. Weak p o i n t s do a p p e a r at p o i n t s of d i s c o n t i n u i t y . t h e r e are n o weak sections. T h e principle of a pad f o u n d a t i o n system is similar t o t h a t of a drilled pier f o u n d a t i o n in t h a t t h e load of t h e s t r u c t u r e is c o n c e n t r a t e d at several p o i n t s . S. d e e p w i n d o w s . A m i r [ 4 8 ] r e p o r t e d t h e use of b o x c o n s t r u c t i o n in K i b u t z G a t . Reinforced b r i c k w o r k has b e e n widely used in S o u t h Africa. and can therefore tolerate considerable differential m o v e m e n t w i t h o u t exhibiting cracks. 52 and 5 3 ) . Shraga and D. A n e x t r e m e case recently occurred w h i c h involved a wall bearing directly u p o n expansive soil. Webb [ 4 9 ] r e p o r t e d t h e use of r e i n f o r c e m e n t in t h e e x t e r n a l wall panels b e t w e e n j o i n t s t o resist b e n d i n g stresses and shear stresses resulting from f o u n d a t i o n m o v e m e n t . L. Insure t h a t t h e r e is sufficient c o n t i n u o u s r e i n f o r c e m e n t in t h e f o u n d a t i o n wall t o provide rigidity.104 FOUNDATIONS ON EXPANSIVE SOILS s t a n d p o i n t . Box c o n s t r u c t i o n is based u p o n t h e principle t h a t t h e r e is n o d i s c o n t i n u i t y of s t r u c t u r e . Box c o n s t r u c t i o n is e c o n o m i c a l for s t r u c t u r e s having simple configurations. D. Note pressure distribution. Plan and section of foundation wall bearing directly on expansive soil.FOOTING FOUNDATIONS 105 Swelling Pressure SECTION t 1 Side Wall^ Basement 1 PLAN VIEW Figure 52. I . the practical dead load pressure t h a t can be applied t o t h e pad is a b o u t 3 . 2. m a x i m u m allowable soil pressure will n o t pose a p r o b l e m . Design By loading an expansive soil so t h a t the pressure e x e r t e d on the soil is greater t h a n the swelling pressure of t h e soil. it is theoretically possible t o exert any desirable dead load pressure. if t h e pads are placed on stiff swelling clays. 3 .106 FOUNDATIONS ON EXPANSIVE SOILS Figure 53. C o n s e q u e n t l y . Where b e d r o c k o r bearing s t r a t u m is deep and c a n n o t be e c o n o m i c a l l y reached b y drilled piers. pads f o u n d e d on clay are designed t o w i t h s t a n d a dead . By using an individual pad f o u n d a t i o n system. 0 0 0 psf. 0 0 0 psf (considering the ratio of dead and live load t o be a b o u t 2 t o 3). Actually. However. Where t h e u p p e r soils possess m o d e r a t e swell p o t e n t i a l . Generally. t h e m a x i m u m bearing capacity should be a b o u t 5 . heaving m o v e m e n t can be p r e v e n t e d . Where t h e bearing capacity of t h e u p p e r soils is relatively high. t h e m a x i m u m bearing capacity of t h e pad is limited b y the u n c o n f i n e d compressive strength of clay. Foundation wall bearing directly on expansive soil without footings. Occasionally. U n d e r t h e following c o n d i t i o n s . and 4. the use of a pad f o u n d a t i o n system can be advantageous: 1. If a pad is f o u n d e d directly on b e d r o c k . Where t h e w a t e r table or a soft layer exists preventing t h e use of a friction pier. t h e capacity of t h e pad is limited b y t h e allowable bearing capacity of t h e f o u n d a t i o n soils. Heaving has pushed the side wall toward the basement wall resulting in heavy bowing of the basement wall. H a n s o n & T h o r n b u r n stated in t h e second edition of F o u n d a t i o n Engineering t h a t . Section through externally reinforced brick wall.FOOTING FOUNDATIONS 107 Figure 54. " This is s h o w n o n figure 5 7 . Figure 56 shows a grade b e a m and pad f o u n d a t i o n system t h a t failed because the dead load pressure was n o t sufficient t o prevent t h e heaving of t h e f o u n d a t i o n soils. "Swelling can b e p r e v e n t e d only in a localized z o n e b e n e a t h t h e footings or piers w h e r e the stressed i n d u c t e d b y t h e f o u n d a t i o n are c o n c e n t r a t e d . an individual pad f o u n d a t i o n system can only be used in those areas where t h e soils possess only a m e d i u m degree of e x p a n s i o n w i t h v o l u m e change—on t h e o r d e r of 1 t o 5 p e r c e n t and a swelling pressure in t h e range of 3 . (After D. 0 0 0 psf. Webb) load pressure as high as 5 . Peck. 0 0 0 t o 5. T o allow for t h e c o n c e n t r a t i o n of dead load pressure on t h e individual p a d s . L. a void space is required b e n e a t h the grade b e a m and should b e c o n s t r u c t e d in the same m a n n e r as grade b e a m s and pier system (fig. . 5 5 ) . With this limitation.000 psf. even if it is entirely prevented above. . it is desirable t o place all footing pads o n uniform nonswelling soils. T h e excavation should be larger t h a n t h e footing pad and t h e space b e t w e e n t h e c o n c r e t e and t h e soil filled w i t h loose backfill. In such cases. Grade beams and pads constructed with void space between pads. T h e use of a d e e p pad system usually applies t o c o n s t r u c t i o n areas where the p r o b l e m soil ranges in thickness from 0 t o 5 feet. Deep pads In areas w h e r e t h e layer of swelling soils is relatively t h i n . A typical e x a m p l e is w h e r e 2 t o 3 feet of swelling clays are underlain b y sand and gravel o r b y nonswelling b e d r o c k such as granite or s a n d s t o n e . t h e intensity of a d d e d stress is small and swelling m a y occur b e l o w this level. swelling is u n d i m i n i s h e d . In t h e area b e t w e e n t h e footings. In t h o s e parts of t h e world w h e r e hand labor is inexpensive and drilling e q u i p m e n t n o t readily available. A t a comparatively shallow d e p t h b e n e a t h t h e f o u n d a t i o n . d e e p individual pads placed on nonswelling soil can be e c o n o m i c a l l y used. Pads placed as d e e p as 5 feet b e l o w t h e g r o u n d surface can b e economically used in areas where drill rigs are unavailable. Care should be exercised t o insure t h a t uplifting pressure will n o t b e exerted on t h e sides of t h e pad.108 FOUNDATIONS ON EXPANSIVE SOILS Figure 55. t h e use of a d e e p pad system can be an advantage from a cost consideration. Interrupted footing I n t e r r u p t e d footings are used in c o n j u n c t i o n w i t h a wall footing system. t h u s preventing further f o u n d a t i o n m o v e m e n t s . By i n t r o d u c i n g some void space b e n e a t h the footings. (See Case S t u d y IV for illustrations) FOOTINGS ON SELECTED FILL T h e removal of n a t u r a l expansive soils and their r e p l a c e m e n t w i t h n o n e x p a n s i v e soil is t h e m o s t obvious m e t h o d of preventing structural d a m a g e d u e t o soil heaving.FOOTING FOUNDATIONS 109 Figure 56. Dead load pressure was not sufficient to prevent pad uplift. In a few cases. t h e dead load pressure exerted on t h e soil can b e easily d o u b l e d . By placing a void space at intervals. the m a x i m u m unit dead load pressure exerted on t h e soil is a b o u t 2 . Typical crack which developed in the basement of house founded with grade beams and individual pads. t h e expansive material e x t e n d s t o t o o great a d e p t h t o allow c o m p l e t e removal and . it m a y be possible t o c o m p l e t e l y r e m o v e t h e expansive strata. p e r ft. t h u s increasing t h e dead load pressure. With f o u n d a t i o n walls bearing directly o n swelling soil. This principle of i n t e r r u p t e d footings has b e e n successfully applied to t h e c o r r e c t i o n of cracked buildings having a c o n t i n u o u s footing f o u n d a t i o n . t h e bearing area will be decreased. 0 0 0 lbs. In m o s t cases. t h u s eliminating t h e heaving p r o b l e m . t h e dead load pressure can b e substantially increased. In this m a n n e r . If t h e excavation is w e t t e d pressure against t h e selected fill resulting in severe damage excessively before t h e p l a c e m e n t of t h e selected fill. T h e fill should consist of n o n e x p a n s i v e soil. Many school buildings have been successfully placed on selected fill. mainly because t h e site was flooded during c o n s t r u c t i o n . t h e imbibed m o i s t u r e in t h e soil will cause t h e soil t o swell and heave and exert t o t h e s t r u c t u r e . Diagram illustrating influence on swelling of high contact pressure beneath footing. care should be t a k e n t o avoid t h e excessive w e t t i n g of n a t u r a l soils. Coincidentally. t h e r e have also been failures w h e n such c o n s t r u c t i o n has b e e n used. 2. Hanson & Thornburn) backfill. T h e fill should e x t e n d b e y o n d the building line for a distance of at least 10 feet in every direction. T h e r e should be at least 3 feet of selected fill b e n e a t h t h e b o t t o m of footings and slabs.000 Ib/sq ft and swelling pressure at zero volume change is 2. " Probably t h e m o s t i m p o r t a n t single factor affecting t h e success of footings placed on selected fill is the drainage c o n t r o l used during c o n s t r u c t i o n . T h e fill should be c o m p a c t e d t o at least 9 0 p e r c e n t standard P r o c t o r density for s u p p o r t i n g slabs and 100 p e r c e n t standard P r o c t o r density for s u p p o r t i n g footings. 5. Detail discussion is given in c h a p t e r 8 u n d e r "Soil R e p l a c e m e n t . preferably impervious and granular. F o r success in placing footings and slabs on selected fill. Before t h e p l a c e m e n t of fill. . If net pressure at base of footing is 8. T h e p r o b l e m is t h e n t o d e t e r m i n e t h e a m o u n t of excavation and t h e t y p e of backfill required to prevent heaving. 3 . swelling will be prevented within shaded areas only. (After Peck.110 FOUNDATIONS ON EXPANSIVE SOILS Figure 57. t h e following p r e c a u t i o n s should be observed: 1.000 Ib/sq ft. 4. b o t h for the entire system and for slabs alone. A s t u d y of w e a t h e r d a t a indicates t h a t the yearly annual p r e c i p i t a t i o n . d i s t r i b u t i o n of p r e c i p i t a t i o n . 4. It would be difficult t o apply such c o n s t r u c t i o n t o b a s e m e n t houses w i t h an a t t a c h e d garage o r split level houses. t h e n slab m o v e m e n t will n o t affect t h e stability of t h e s t r u c t u r e . and the v o l u m e change of the soil is affected b y m o i s t u r e c o n t e n t . National Weather Service has developed is selected. there could be tilting of t h e m a t . T h e first step in t h e design is t o d e t e r m i n e the s u p p o r t i n d e x . Negative m o m e n t consists mainly of those pressures caused b y t h e swelling of the underslab soils. b o t h t h e swelling and the s e t t l e m e n t of the soil will be affected b y climate. Single level c o n s t r u c t i o n is required. t h e U. w . the designer m u s t develop a s t r u c t u r e capable of satisfying the shear. 2. F r o m figure 5 8 . Past p e r f o r m a n c e has b e e n limited t o residential c o n s t r u c t i o n . T h e slab receives and t r a n s m i t s all t h e structural load t o t h e underslab soils. bending m o m e n t . negative m o m e n t consideration generally controls t h e design of t h e m a t f o u n d a t i o n . Configuration of building m u s t be relatively simple. Since t h e swelling pressure in an expansive soil area can reach m a n y t h o u s a n d p o u n d s per square foot. S. t h e climate rating is b e t w e e n 2 0 and 2 5 . and a m o u n t of each precipitation all affect the consistency of climate. the climate rating C instance. Such c o n c e p t i o n has been studied b y t h e "Building Research Advisory B o a r d " [ 5 0 ] . and length-to-width ratio of the f o u n d a t i o n . T h e slab should be designed t o resist b o t h t h e positive and the negative m o m e n t . F o r i n f o r m a t i o n which has been transformed i n t o frequency isolines on a m a p of t h e C o n t i n e n t a l United States as shown on figure 5 8 . T h e dead and live load acting on the slab. frequency of p r e c i p i t a t i o n . Design T h e design of a m a t f o u n d a t i o n is generally based o n t h e following p a r a m e t e r s : 1. 3 . are considered to be b o t h a load s u p p o r t i n g as well as a separating e l e m e n t . A s t u d y on the w o r k in t h e R o c k y M o u n t a i n areas indicates t h a t there are limitations o n the use of such a system as follows: 1. and 3 . If all structural e l e m e n t s are t o be placed o n a stiffened slab. d u r a t i o n of precipitation. T h e success of such system so far is limited to m o d e r a t e swelling soil areas. It is assumed t h a t t h e m o i s t u r e c o n t e n t in t h e soil is affected b y climatic c o n d i t i o n s .FOOTING FOUNDATIONS 111 MAT FOUNDATION Mat foundations. F r o m the above p a r a m e t e r s . plasticity i n d e x . Positive m o m e n t includes t h a t induced by b o t h dead and live load pressure e x e r t e d o n t h e slab. sometimes referred to as structural slab-on-ground or reinforced and stiffened slabs. Based on d a t a o b t a i n e d from 122 w e a t h e r stations. Slab d i m e n s i o n s . 2. C o n s e q u e n t l y . in C o l o r a d o . and deflection c o n d i t i o n s . b u t t h e p e r f o r m a n c e of t h e building would n o t be structurally affected. T h e s u p p o r t index is based u p o n t h e climatic rating. However. T h e s u p p o r t index. T h e load exerted o n t h e f o u n d a t i o n m u s t be light. L y t t o n and . T h e design of the stiffened slab section will b e based o n the value of t h e s u p p o r t i n d e x . t h e s u p p o r t i n d e x can t h e n be d e t e r m i n e d from figure 5 9 . With t h e swell i n d e x or t h e p e r c e n t of swell u n d e r specific loading c o n d i t i o n and the climate rating d e t e r m i n e d . W o o d b u r n [51] of T e x a s A & M University have performed considerable research o n t h e design p r o c e d u r e for stiffened m a t s o n expansive clay. A. R. T o o b t a i n t h e swell i n d e x . Figure 59 shows t h e relationship of t h e various p r o p e r t i e s .112 FOUNDATIONS ON EXPANSIVE SOILS Figure 58. T h e u n d i s t u r b e d samples should be o b t a i n e d u n d e r soil m o i s t u r e c o n d i t i o n s representative of c o n d i t i o n s prevailing at the time of c o n s t r u c t i o n . T h e s u p p o r t index is directly related t o t h e climate factor and the soil p r o p e r t i e s . Also. L. and swell i n d e x . T h e soil p r o p e r t i e s are related t o t h e A t t e r b e r g limits. t h e swell index is t h e m o s t reliable factor of t h e three for predicting p o t e n t i a l v o l u m e change of t h e f o u n d a t i o n soils. L y t t o n and J. t h e PVC m e t e r is based o n testing soils in a r e m o l d e d state which can materially differ from t h a t in the u n d i s t u r b e d state. O f these. Soils w i t h identical plasticity index exhibit greatly varying swell p o t e n t i a l . percent swell in t h e P V C m e t e r . (after Federal Housing Administration) T h e second major factor necessary for design is t h e s u p p o r t index. Climatic ratings C w for Continental United States. the percentage swell for a specific soil s t r a t u m should be o b t a i n e d t h r o u g h swell tests using c o n v e n t i o n a l c o n s o l i d o m e t e r test e q u i p m e n t on undisturbed soil samples and pressure c o r r e s p o n d i n g t o t h e in situ o v e r b u r d e n pressure plus the average of the t o t a l dead and live loads o n t h e slab. In t h e same year. N o n e of these houses exhibited n o t i c e a b l e cracks. 5 2 houses were built using waffled slabs in West Field Park. Jefferson C o u n t y .0 Swell Index (%) 1 Figure 59.I I 1.0 1 50 1 60 70 7. b u t distress.0 1 4.0 1 6. t h e design of t h e m a t f o u n d a t i o n is within the realm of a structural engineer. T h e soils are considered t o have m o d e r a t e swell p o t e n t i a l . such f o u n d a t i o n system was first considered in 1 9 7 0 .5 p e r c e n t u n d e r l o a d s ranging from 5 0 0 t o 1. With t h e s u p p o r t i n d e x d e t e r m i n e d .000 psf. subgrade m o d u l u s . (after Federal Housing Administration) W o o d b u r n d e t e r m i n e d t h e s u p p o r t index algebraically from t h e average f o u n d a t i o n pressure.0 20 1 30 1 1 1 3. A survey of these h o u s e s was m a d e in 1974 and their c o n d i t i o n was excellent.0 1 40 1 64 1 4.A.0 1 5.000 psf.8 3.0 7. In Denver.0 1 no I 80 1 "Ρ 9p PI PVC 12. Behavior Stiffened slab c o n s t r u c t i o n has been widely used in s o u t h e r n Texas w h e r e m o d e r a t e swelling soils are e n c o u n t e r e d .0 2. either in t h e interior or t h e e x t e r i o r . Differential elevation b e t w e e n t h e o p p o s i t e corners of t h e h o u s e in s o m e cases reached 1 inch. T h e swell p o t e n t i a l ranged from a low of 1 to a high of 5. exhibiting swelling pressure as high as 10. has n o t . Support index C based upon criterion for soil sensitivity and climatic rating C w. m a x i m u m e x p e c t e d differential heave of t h e soil and t h e m o u n d e d area.7 1 8. T h e soils are generally stiff clays w i t h a plasticity i n d e x ranging from a low of 3. Typical m a t f o u n d a t i o n design is s h o w n o n figure 6 1 . A l a b a m a .H. S u b s e q u e n t l y .9 1 10.6 t o a high of 3 2 .FOOTING FOUNDATIONS 113 ΙΟ I I.0 9. Typical design and c o n s t r u c t i o n details of these houses are given in figures 62 t h r o u g h 6 8 .-sponsored c o n s t r u c t i o n in M o n t g o m e r y . T h e soils in t h e Lake A r b o r area possess m u c h higher swell p o t e n t i a l t h a n t h a t of West Field Park. T h e so-called waffle slabs have b e e n in use in San A n t o n i o for m o r e t h a n 25 years and are also required for F. 12 h o u s e s were built using t h e stiffened slab system in Lake A r b o r Subdivision in N o r t h Denver. 1 . L y t t o n and W o o d b u r n p r e p a r e d a n o m o g r a p h for d e t e r m i n i n g t h e s u p p o r t i n d e x as s h o w n o n figure 6 0 . 114 FOUNDATIONS ON EXPANSIVE SOILS Figure 60. A. Woodburn). L. Support index nomograph (After R. Lytton and J. . As stated in t h e opening r e m a r k s of the advisory b o a r d [ 5 0 1 . This a m o u n t s t o an increase in cost of a b o u t $ 7 5 0 per h o u s e w h i c h is negligible w h e n c o m p a r e d t o t h e p r o b l e m s and rehabilitation costs e n c o u n t e r e d w h e r e t h e stiffened slab system had n o t b e e n used.Figure 61. some o t h e r houses have required r e p l a c e m e n t of their b a s e m e n t floor slab t h r e e times within 4 years. " I t is recognized that experience and t h e state of engineering k n o w l e d g e are such t h a t precise answers t o m a n y of t h e p r o b l e m s posed m u s t . In t h e Denver area d u r i n g 1970 t o 1 9 7 1 . (Sheet 1 of 2) t a k e n place. t h e increased cost of using stiffened slabs r a t h e r t h a n c o n v e n t i o n a l pier and grade b e a m system was a b o u t 50 cents p e r square foot. This indicates t h a t t h e stiffened slab system used for t h e 12 h o u s e s appears t o b e highly successful. of necessity. In t h e same Lake A r b o r area. Typical mat foundation design. be considered b e y o n d a t t a i n m e n t in t h e i m m e d i a t e foreseeable . Typical mat foundation design.FOUNDATIONS ON EXPANSIVE SOILS SECTION A SECTION Β -ONE 3 /β β STRAND SECTION C SECTION D TOTA L 3 TENDONS IN F I R E P L A C E TWO **5 BAR S 6"xl8" CURTAIN W AL SECTION Ε Figure 6 1 . (Sheet 2 of 2) . FOOTIN G FOUNDATION S 117 Figure 62. Trenching the cross-beams. Figure 63. Placing reinforcement. 118 FOUNDATIONS ON EXPANSIVE SOILS Figure 64. Post-tensioning. Figure 65. Placing concrete. FOOTING FOUNDATIONS 119 Figure 66. Completed mat. Figure 67. Interior partitions. 120 FOUNDATIONS ON EXPANSIVE SOILS Figure 68. Completed residence founded on mat foundation foreseeable future. Nevertheless, the approach recommended herein is considered to be sufficiently valid t o w a r r a n t application n o w . " Based on o u r experience and p e r f o r m a n c e relating t o t h e Denver project, t h e stiffened slab system of c o n s t r u c t i o n can be successfully applied t o low t o m o d e r a t e swelling soil areas. Much research will be required t o d e t e r m i n e and u n d e r s t a n d t h e m a n y variables, especially the relationship, of swelling characteristics w i t h s u p p o r t i n d e x . As discussed previously in " D e s i g n " , t h e s u p p o r t i n d e x should be related t o swelling pressure. T h u s , t h e loading c o n d i t i o n can be eliminated from the design as well as t h e climatic rating. REFERENCES [48] Shraga, S., Amir, D., and Kassiff, G., "Review of Foundation Practice for Kibbutz Dwelling in Expansive Clay." Proceedings of the Third International Conference on Expansive Soils, 1973. [49] Webb, D. L., "Foundations and Structural Treatment of Buildings on Expansive Clay in South Africa," Second International Research and Engineering Conference on Expansive Clay Soils, Texas A & M Press, 1966. [50] "Criteria for Selection and Design of Residential Slabs-on-Ground," Building Research Advisory Board. [51] Lytton, R. L. and Woodburn, J. Α., "Design and Performance of Mat Foundations on Expansive Clay," Proceedings of the Third International Conference on Expansive Soils, 1973. Chapter 6 SLABS ON EXPANSIVE SOILS INTRODUCTION Slab-on-ground c o n s t r u c t i o n , w h e n o n expansive soils, is a very difficult aspect t o c o n t r o l . In t h e category of slabs are interior floor slabs, e x t e r i o r sidewalks or a p r o n s , and p a t i o slabs. Generally, floor slabs d o n o t s u p p o r t any appreciable live load, and t h e dead load actually exerted o n t h e slab is small. C o n s e q u e n t l y , m o v e m e n t of t h e slab is t o be e x p e c t e d w h e n t h e underslab m o i s t u r e c o n t e n t increases, and it should be designed accordingly. T h e m o v e m e n t of slabs n o t only presents unsightly cracks b u t , in m o s t cases, also directly affects t h e stability of the s t r u c t u r e . SLAB-ON-GROUND C o n c r e t e slabs, placed directly on t h e g r o u n d , are m u c h less expensive t h a n s t r u c t u r a l floor slabs o r "crawl s p a c e " t y p e c o n s t r u c t i o n . This is especially t r u e w h e r e b a s e m e n t c o n s t r u c t i o n is involved. Since 1940, m o s t of t h e residential h o u s e s , school buildings, industrial, and w a r e h o u s e structures call for t h e use of slab-on-ground c o n s t r u c t i o n . It was n o t until t h e discovery of t h e expansive soil p r o b l e m t h a t engineers began t o q u e s t i o n t h e w i s d o m of using slab-on-ground construction. Types of slab-on-ground Slab-on-ground, s o m e t i m e s referred t o as slab-on-grade, are c o n c r e t e slabs placed directly o n the ground w i t h little consideration given t o their structural r e q u i r e m e n t s . These slabs are c o n s t r u c t e d b o t h w i t h or w i t h o u t r e i n f o r c e m e n t . T h e unreinforced slabs are generally c o n s t r u c t e d in residential houses o r w h e r e light floor load is e x p e c t e d . T h e limits of t h e length of t h e unreinforced slab are based u p o n the a m o u n t of shrinkage cracking c o n t r o l desired. N o r m a l l y , shrinkage cracks are controlled weakened plane j o i n t s . A lightly reinforced slab is n o r m a l l y reinforced w i t h t e m p e r a t u r e c o n t r o l as a p r i m e design factor. T h e Portland C e m e n t Association [ 5 2 ] r e c o m m e n d e d t h e use of a 4-inch-thick slab reinforced w i t h 6 x 6 - 1 0 / 1 0 mesh or N o . 3 bar at 24 inches on c e n t e r each w a y for slabs placed in m o d e r a t e l y swelling soil areas. F o r high swelling soil areas, the Association r e c o m m e n d e d the use of 6 x 6 - 6/6 m e s h or N o . 3 b a r at 18 inches on center each w a y . T h e choice b e t w e e n an unreinforced slab and a lightly reinforced slab d e p e n d s u p o n t h e subsoil c o n d i t i o n s as well as t h e loading c o n d i t i o n s . R e i n f o r c e m e n t in t h e slab will r e d u c e t h e b y designed and this fault w e n t u n d e t e c t e d until extensive damage had taken place. b u t also to m a i n t a i n t h e structural integrity of t h e building. Slab-on-ground c o n s t r u c t i o n o n expansive soil will always pose a cracking and heaving p r o b l e m unless t h e subgrade soils are treated or replaced. is n o t u n c o m m o n as s h o w n on figure 6 9 .122 FOUNDATIONS ON EXPANSIVE SOILS opening of t e m p e r a t u r e cracks b u t will n o t prevent cracking of t h e slab caused b y heaving of t h e underslab soils. floor m o v e m e n t is invariably associated w i t h the increase of m o i s t u r e c o n t e n t of t h e underslab soils. F o r c o n c r e t e floors covering a large area. B r o k e n utility lines often c o n t r i b u t e w a t e r t o t h e underslab soils. Surface w a t e r enters the loose backfill and causes a wetting condition. soil heaving is unjustly blamed for all cracks t h a t develop in a floor. usually perched water. F l o o r cracking caused b y swelling soils m u s t be differentiated from t h a t caused by shrinkage of c o n c r e t e . In t h e absence of j o i n t s . Water and sewer lines buried in expansive soils are subject t o stress. In o n e case. 0 0 0 psf are e x p e c t e d . a r e p o r t concerning residential slab-on-ground c o n s t r u c t i o n was prepared by the Building Research Advisory Board [ 5 0 ] for use b y t h e Federal Housing A d m i n i s t r a t i o n which provided criteria for the selection and design of residential floor slabs. In an expansive soil area. Moisture migration d u e t o t h e r m a l differential as m e n t i o n e d in c h a p t e r 2 can also cause d a m a g e t o slab-on-ground w i t h o u t the observance of free water. Such leakage can c o n t i n u e for a long period of time w i t h o u t being d e t e c t e d . t h e c o n t r a c t o r neglected t o c o n n e c t interior sewer line to t h e street sewer. T h e source of w a t e r t h a t enters i n t o t h e underslab soils can generally be associated w i t h t h e following: 1. Rise of g r o u n d w a t e r . shrinkage cracks can t a k e place at a p p r o x i m a t e equally spaced intervals. 3. M i n o r floor cracking of slab-on-ground c o n s t r u c t i o n is difficult if n o t impossible to prevent. the Portland C e m e n t Association r e c o m m e n d s t h e installation of c o n t r o l j o i n t s at intervals of a p p r o x i m a t l e y 20 feet. n o t only from t h e s t a n d p o i n t of expansive soils. A m o s t c o m m o n source of m o i s t u r e entering t h e underslab soils is derived from irrigation. T h e r e p o r t r e c o m m e n d e d t h e use of unreinforced c o n c r e t e slabs for firm. 2. F l o o r cracks d u e t o heaving generally take place along t h e bearing wall as shown in figure 6 9 . Water m a r k s and severe floor cracks indicate t h e e x t e n t of d a m a g e . Both reinforced and unreinforced slabs are considered to have the limiting function of separating t h e g r o u n d from . special design will be required. In 1 9 6 8 . Isolation j o i n t s separating where the subgrade may undergo slight m o v e m e n t . lawn watering. and r o o f d o w n s p r o u t s . in excess of 6 inches. w h e r e floor loads as high as 3 . Differential heaving can break pipes and cause leakage. Slab movement In expansive soil areas. N o m i n a l r e i n f o r c e m e n t was recommended living space. n o n e x p a n s i v e soils. In commercial buildings such as warehouses and storage areas. The above sources of w a t e r t h a t e n t e r t h e underslab soils are t h e obvious ones. can cause excessive swelling. Figure 7 0 shows floor m o v e m e n t in a boiler r o o m . T h e floor drain t o t h e boiler r o o m b e c a m e plugged resulting in severe slab heaving from uplift. Heaving of slabs. SLABS ON EXPANSIVE SOILS 123 • Figure 69. Differential slab heaving of up t o 12 inches in a newly completed basement. . however. If. Heaving of a floor slab in a boiler room. T h e installation of a subdrainage system inside or outside of a c o m p l e t e d building is a major u n d e r t a k i n g . such as from a b r o k e n pipe or from an i m p r o p e r l y located d o w n s p o u t . this t h e o r y has n o t b e e n proven. T o d a t e . Uplift is 2 inches. there is n o easy m e t h o d of removing t h e water. however. Underslab gravel C o n v e n t i o n a l slab c o n s t r u c t i o n uses 4 inches of gravel b e n e a t h all c o n c r e t e floors. T h e m a i n advantage of using gravel b e n e a t h t h e floor slab. C o n c r e t e curling has a strong resemblance t o uplift of slabs d u e t o heaving of u n d e r s l a b soils. A widely accepted t h e o r y pertaining t o slab-on-ground c o n s t r u c t i o n in expansive soil areas is that w a t e r from a single source. is t o p r o t e c t t h e building from t h e rise of g r o u n d water. will travel w i t h o u t resistance t h r o u g h o u t t h e gravel bed and saturate t h e entire area u n d e r n e a t h the slab. or walls t o p e r m i t b o t h h o r i z o n t a l m o v e m e n t d u e t o v o l u m e changes and vertical m o v e m e n t caused b y differential s e t t l e m e n t or heaving are also recommended. footings.124 FOUNDATIONS ON EXPANSIVE SOILS Figure 70. t h u s reducing t h e shrinkage cracks and s o m e t i m e s t h e curling of c o n c r e t e . Therefore. Source of water derived from inadequate floor drain system. concrete slabs from c o l u m n s . free draining gravel has b e e n previously installed . m o r e extensive damage t o t h e floor will take place w h e n t h e gravel is used. If a perched w a t e r c o n d i t i o n develops b e n e a t h a b a s e m e n t which has n e i t h e r a subdrainage system n o r a gravel bed b e n e a t h t h e slab. Curling of c o n c r e t e slabs in large floor areas d u e t o i m p r o p e r curing is n o t u n c o m m o n . T h e use of gravel b e n e a t h t h e slab allows t h e uniform d i s t r i b u t i o n of floor load and the uniform curing of c o n c r e t e . t h e r e b y eliminating t h e need for void forming material. and the h o n e y c o m b system. T h e m o s t c o n v e n i e n t c o n s t r u c t i o n m e t h o d is t o provide a crawl space b e n e a t h t h e slab. t h e r e b y allowing the build-up of uplifting pressure. b u t also u p o n the construction technique. w h i c h is much more expensive than the conventional slab-on-ground m e t h o d . T h e systems n o w include t h e s t r u c t u r a l floor slab. T h e crawl space provides access for inspection. F o r m i n g materials are costly and t h e r e is n o assurance t h a t t h e material will c o m p l e t e l y d e t e r i o r a t e b e n e a t h t h e slab. t h e raised floor system. and can also serve as a convenient area for utility pipes and c o n d u i t s . C o m m e r c i a l prestressed. it is n o t possible t o c o n s t r u c t a crawl space. STIFFENED SLABS Slab-on-ground c o n s t r u c t i o n c a n n o t be safely used in an area w h e r e the subsoil possesses high swell p o t e n t i a l . T h e use of prestressed slabs in large q u a n t i t i e s can prove t o be e c o n o m i c a l . T h e p r o b l e m w i t h this t y p e of c o n s t r u c t i o n is t h a t of providing a forming material t o allow t h e placing of c o n c r e t e . O n e possible alternative is t h e use of balloons as a forming material w h i c h could b e deflated after t h e c o n c r e t e has reached its initial set. This is typical w h e r e a structural slab is to be c o n s t r u c t e d in a b a s e m e n t area. flat slabs are available in sufficient length to span 20 feet. and t h e s t r u c t u r a l floor m u s t t h e n be c o n s t r u c t e d w i t h only a few inches of air space b e t w e e n t h e slab and g r o u n d . . J-void. U n f o r t u n a t e l y . Either t i m b e r o r c o n c r e t e floors can be used in this t y p e of construction. can be ventilated. such a system has n o t b e e n devised t o d a t e . b o t h s t r u c t u r a l and soil engineers a t t e m p t e d t o devise an economical floor system w h i c h would c o m b a t t h e p r o b l e m of swelling soil. F o r m a n y years. it m a y only be necessary to install a s u m p p u m p in t h e b a s e m e n t as the w a t e r will flow t h r o u g h t h e gravel t o w a r d the s u m p . This can be readily provided in major structures such as schools and office buildings. Raised floor system T h e Portland C e m e n t Association [ 5 2 ] has a p p r o a c h e d t h e p r o b l e m of t h e c o n s t r u c t i o n of a structural floor slab on expansive soils b y utilizing a c o n c r e t e floor raised above grade b y intersecting c o n c r e t e ribs f o r m e d in a waffle p a t t e r n . O f t e n t i m e s . t h e advantages of providing gravel b e n e a t h t h e slab far exceeds any possible disadvantages. hollow core. T h e use of Verticel. T h e s h o r t c o m i n g s of this system lie n o t only in t h e cost of c o n s t r u c t i o n . In any event.SLABS ON EXPANSIVE SOILS 125 b e n e a t h the slab. o r o t h e r forming material similar t o t h a t used b e n e a t h the grade b e a m s in t h e pier f o u n d a t i o n is satisfactory. Structural floor slabs T h e best m e t h o d t o prevent floor m o v e m e n t is t o c o n s t r u c t a s t r u c t u r a l slab s u p p o r t e d o n each side b y grade b e a m s and provide a void b e n e a t h t h e slab t o prevent c o n t a c t b e t w e e n t h e soil and t h e slab. T h e system can also i n c o r p o r a t e utility r o u t e s . 2 1/2 si_AB I — SECTION A 1 FLOOR PLAN 1 Values and dimensions are i l l u s t r a t i v e purposes only. BOXES 1 1 — EVERY 3 6 " ON CENTER EA CH WAY .. A typical plan and cross-section is s h o w n on figure 7 1 . Raised concrete floor system (after Portland Cement Association). rational a p p r o a c h for t h e structural engineers.126 FOUNDATIONS ON EXPANSIVE SOILS T h e raised boxes) upon a floor level system is c o n s t r u c t e d by placing Verticel or J-void (waxed c a r d b o a r d subgrade. The f o u n d a t i o n consists of longitudinally split S o n o t u b e s t h a t are placed w i t h t h e openings t o w a r d the soil as s h o w n o n figure 7 2 . It was theorized t h a t as t h e clay swells. T h e S o n o t u b e forms stand u p well during placing of the c o n c r e t e b u t disintegrate after being w e t t e d . the sand runs o u t from u n d e r the joists. such as heating and cooling. This t y p e of c o n s t r u c t i o n is also expensive. T h e spacing of the ribs and the thickness of the slab d e p e n d s u p o n the swelling p o t e n t i a l of t h e surface soils and t h e dead load imposed on t h e waffle s t r u c t u r e . is placed over the s u p p o r t i n g ribs and c a r d b o a r d b o x e s in a m o n o l i t h i c c o n c r e t e p l a c e m e n t . This grading is an additional cost. After t h e t u b e distintegrates. T h e disadvantage of such a floor system is t h e inability t o e x e r t sufficient dead load pressure u p o n the ribs t o c o u n t e r a c t the swelling pressure. containing wire fabric. T h e system has been tried in a few limited cases in the Denver area with d o u b t f u l success. ' 32 SQ. and the formed voids provide a m e a n s of relieving u p w a r d swelling pressure. it could e x p a n d i n t o these openings and reduce the swelling pressure. In a d d i t i o n . t h r o u g h the floor. Honeycomb The system development of the honeycomb system was based upon the assumption that comparatively slight m o v e m e n t s of some clays reduce or relieve swelling pressures [ 5 3 ] . The spaces between the boxes contain reinforcing and form-supporting concrete ribs. . t h e floor area m u s t be very finely graded to provide a level base for t h e void forming material so t h a t uniform thicknesses result. ' TYPICAL i 1 ' . for Figure 71. t h e b o t t o m 2 inches of the space b e t w e e n S o n o t u b e s being filled w i t h sand. T h e advantage of such a system is t h a t it offers a clear. T h e actual floor slab. When the backfill exerts lateral pressure o n t h e grade b e a m . t h e slab is capable of moving i n d e p e n d e n t l y w i t h o u t being in c o n t a c t w i t h the s u r r o u n d i n g s t r u c t u r e . the expansion j o i n t is u n d e r compression and part of the uplift pressure b e n e a t h the slab is t h e n t r a n s m i t t e d t o t h e grade b e a m . . Slip joints Interior floor slabs should be totally separated from t h e grade b e a m s and interior c o l u m n s t o allow for free slab m o v e m e n t . T h e o r e t i c a l l y . the slab is separated from the grade b e a m s b y t h e use of asphalt felt e x p a n s i o n j o i n t s . If the slab is n o t separated from t h e grade b e a m . F L O A T I N G SLABS A floating slab refers t o a slab-on-ground c o n s t r u c t i o n in which t h e slabs are totally separated from t h e grade b e a m a n d building s t r u c t u r e . cracks appeared a b o u t 2 feet from and parallel t o t h e grade b e a m . Typical honeycomb form system.SLABS ON EXPANSIVE SOILS 127 8" SECTION A-A 8" Figure 72. as s h o w n on figure 7 3 . In m a n y cases. In nearly all b a s e m e n t buildings which have been subjected t o uplifting. In practice. heaving of t h e slab can t r a n s m i t pressure t o t h e grade b e a m and. lift t h e piers. in t u r n . the central p o r t i o n of the slab raised while t h e area along the p e r i m e t e r of t h e grade b e a m remained essentially in place. Naturally. b u t also tilts t h e e x t e r i o r wall. Theoretically. t h e uplifting pressure was relieved. This t y p e of j o i n t system is n o t affected b y lateral pressure t h e r e b y allowing free slab m o v e m e n t . as m u c h as 1 0 . 0 0 0 p o u n d s per linear foot of uplifting pressure can be t r a n s m i t t e d t o t h e grade b e a m . causing great structural d a m a g e . t h e initial uplift force is s o m e t i m e s sufficient t o cause heavy d a m a g e . if the swelling pressure of the underslab soil is 5 . heaving of the floor slab n o t only p r o d u c e s floor cracks.128 FOUNDATIONS ON EXPANSIVE SOILS Figure 73. Figure 76 indicates t h e results of faulty design. S o m e architects prefer t o e x t e n d the floor slab i n t o the e x t e r i o r f o u n d a t i o n wall as s h o w n o n figure 7 5 . T h e result is o b v i o u s . Floor cracks parallel to the foundation wall resulting from lack of slip joints. However. as soon as t h e slab cracked. This installation involves t h e use of t w o 1/8-inch m a s o n i t e strips w i t h silicone l u b r i c a n t b e t w e e n t h e m . A n improved c o n s t r u c t i o n m e t h o d is t h e installation of a lubricated slip j o i n t b e t w e e n the grade b e a m and the slab as s h o w n on figure 7 4 . . 0 0 0 psf. Heaving of the patio slab can cause severe cracking of the upper structure without any sign of movement being apparent in the basement portion of the structure. When the floor slab heaves. not nail. Do N.SLABS ON EXPANSIVE SOILS 129 2 . Coat smooth sides with silicone lubricant. See also "Aprons" for a subsequent discussion of similar problems. the patio slabs are tied into the top of the foundation wall with dowel bar as shown on figure 80. Partition wall The single largest factor that causes damage to structures founded on expansive soils is partition walls that bear directly on a slab.l / 8 " x 9 " continuous tempered masonite. Conventional practice calls for the exterior slab to be tied in with the grade beam by the use of dowel bars. Provide temporary support by taping to wall. Exterior slabs Exterior patio slabs can also transmit swelling pressure to the structure. This resulted in the flaking and damaging of the brick wall. Such type design is not recommended. full swelling pressure is transmitted into the foundation walls. everything resting on the . Oftentimes. 78). Figure 74. Figure 77 shows a typical case where the patio slab has transmitted pressure through dowel bars to the foundation wall causing considerable damage. In another case. as shown on figure 79. In this manner. the exterior sidewalk slab was extended about half an inch into the brick course for aesthetic reasons (fig. Typical slip joint detail between slab-on-ground and foundation wall. Tape smooth sides together. Tilting of exterior wall caused by slab heaving and improper slip joints.130 FOUNDATIONS ON EXPANSIVE SOILS τ Figure 75. . Slab heaving ranging from a fraction of an inch t o as m u c h as 12 inches has been observed. Sheet rock.SLABS ON EXPANSIVE SOILS 131 Figure 76. Exterior brick course buckled due to heaving of interior floor slab. floor will rise. F u r n a c e d u c t s . Water lines. D o o r frames. Wall paneling. and Shelves and b o o k c a s e s . T h e a m o u n t of floor heaving d e p e n d s u p o n t h e swelling p o r t e n t i a l of t h e underslab soils as well as t o t h e degree of w e t t i n g . T h e i t e m s affected b y floor heaving are: Stud walls. Cinder block p a r t i t i o n s . Staircase walls. vertical cracking will o c c u r b e t w e e n t h e partition wall and t h e exterior walls as s h o w n on figure 8 2 . . Patio slab dowelled into wall. Magnitude of swelling pressure transmitted is estimated to be about 10. it is relatively easy t o provide slip j o i n t s in t h e system so the wall is free t o m o v e w i t h o u t exerting pressure on t h e u p p e r s t r u c t u r e s . Crack appeared parallel to wall. A typical detail of such c o n s t r u c t i o n is s h o w n o n figure 8 1 . Patio Slab -Swelling Pressure Basement Wall Figure 77.132 FOUNDATIONS ON EXPANSIVE SOILS Brick Course Dowel bar Floor Joist t u t 4 . Figure 83 indicates t h e architectural detail of a school building in which t h e load-bearing walls are s u p p o r t e d b y piers and t h e interior m a s o n r y walls are placed o n t h e slabs. T h e disadvantage of using a floor-supported wall is t h a t w h e n t h e wall lifts. initially. or several. d o o r s will bind followed b y t h e occurrence of severe cracking. Almost every building investigated suffered some degree of d a m a g e d u e to t h e uplifting of t h e slab-bearing partition walls. of t h e above i t e m s can i m p a r t pressure t o the u p p e r floor joist or b e a m s . Heaving of t h e slab-on-ground has resulted in severe cracking of the slab-bearing p a r t i t i o n wall while t h e walls s u p p o r t e d b y t h e grade b e a m s and piers remain stable as s h o w n on figure 8 4 .000 pounds per running foot. F o r stud wall c o n s t r u c t i o n . The slip j o i n t s can either be installed at the t o p (floor s u p p o r t e d ) or at t h e b o t t o m ( h u n g p a r t i t i o n wall). A n y o n e . Flaking of brick course caused by slab heaving. .SLABS ON EXPANSIVE SOILS Figure 78. 134 FOUNDATIONS ON EXPANSIVE SOILS Figure 79. Sidewalk heaving caused flaking of the brick course as a direct result of the slab extending into the brick course. F r e q u e n t l y , the stud walls have been p r o p e r l y provided with slip j o i n t s ; b u t sheet rock, applied on b o t h sides of t h e s t u d s , resting on t h e floor as s h o w n on figure 85 negates the slip joint installation. Sheet r o c k is capable of t r a n s m i t t i n g sufficient pressure t o t h e floor j o i n t or ceiling resulting in great d a m a g e . A similar situation occurs w h e n b a s e m e n t walls are paneled. T h e studs m a y be free from the floor, b u t t h e paneling bears directly on t h e floor. If uplifting occurs, it m a y result in t h e p o p p i n g of t h e paneling as s h o w n on figure 8 6 . Staircase walls are t h e m o s t frequently neglected w h e n it comes to providing p r o p e r uplift precautions. When instructions are t o provide slip j o i n t s to all slab bearing walls, the staircase is usually neglected. O n e single 2 x 4 s h o w n on figure 8 7 . Figure 88 indicates a p r o p e r l y formed slab-bearing partition wall with slip j o i n t s at t h e bottom. can exert great uplift pressure and t h u s damage the u p p e r s t r u c t u r e , and t h e force m a y even extend to t h e second level in the case of split level buildings as SLABS ON EXPANSIVE SOILS 135 Figure 80. Patio slab attached to basement wall by dowel bar. Swelling pressure results in damage to brick course. Door frames and utilities D o o r frames should be h u n g from the t o p and n o t s u p p o r t e d on slabs. Slab heaving can t r a n s m i t high intensity pressure t h r o u g h t h e d o o r frame t o t h e u p p e r s t r u c t u r e s . A very c o m m o n distress in residential houses is t h e separation of garage d o o r frames from m a s o n r y walls. This is essentially caused by heaving of t h e garage slab w h i c h results from failure t o provide a grade beam, across the e n t r a n c e for the garage d o o r opening. Figure 89 indicates a c o m m o n sight of garage floor slab heaving in a swelling soil area. T h e central p o r t i o n of t h e slab heaved; however, t h e edges remained in place because t h e y were restrained b y t h e d o o r frame. Figure 9 0 indicates t h e crushing and d i s t o r t i o n of furnace d u c t s resulting from heaving of a b a s e m e n t slab. Figure 91 shows t h e severe b e n d i n g of w a t e r line also caused b y floor heaving. Such distress generally brings i m m e d i a t e alarm t o t h e h o m e o w n e r . If t h e utility lines above t h e slab are being d a m a g e d , t h o s e below t h e slab can also be seriously d a m a g e d . FOUNDATIONS ON EXPANSIVE SOILS FLOOR SUPPORTED PARTITION WALL HUNG PARTITION WALL Figure 81. Detail of slip joints used in a partition wall. (After Jorgensen and Hendrickson, Inc.) SLABS ON EXPANSIVE SOILS 137 Figure 82. The cracking of slab bearing partition wall at the junction of the exterior wall. Aprons C o n c r e t e sidewalk slabs a r o u n d a building will prevent surface w a t e r from entering t h r o u g h t h e backfill and i n t o t h e f o u n d a t i o n soils. However, t h e c o n c r e t e apron should n o t be doweled i n t o the f o u n d a t i o n wall for t h e reasons previously discussed. C o n c r e t e sidewalks or a p r o n s will heave and crack. Heaving of c o n c r e t e walks can s o m e t i m e s result in t h e drainage being directed toward t h e building, allowing surface w a t e r t o e n t e r t h r o u g h 138 FOUNDATIONS ON EXPANSIVE SOILS \Z2 PROJECT 2l'-IO" 0 CLASS 30-/0 RM. CRACKED SLAB BEARING WALL Figure 83. Architectural detail of school building. Heaving of interior slab-bearing partition. is supported by structural grade beams. Figure 85. Note the uncracked wall. on the right.SLABS ON EXPANSIVE SOILS Figure 84. 139 . Buckling of stud wall due t o floor heaving. . % χη „ 1 ι. Buckling of wall paneling due t o floor heaving. 0 2x4 s due to floor heaving. 3 Λ / .140 FOUNDATIONS ON EXPANSIVE SOILS Figure 86. Staircase 2x4 s rest directly on floor. Note bow of ~. . Ι ι ri ι Figure 87. Typical garage floor heaving at the central portion. Properly formed slab-bearing partition wall w i t h slip joint at b o t t o m .SLABS ON EXPANSIVE SOILS 141 Figure 88. . Figure 89. e i g D j s n tf oo wr a r t R J tp ge o iud pe ru t o floor heaving.! 42 FOUNDATIONS ON EXPANSIVE SOILS Figure 90. Crushing of furnace duct caused by heaving of basement floor slab. . Sidewalk slab heaving transmits pressure through slab bearing timber frame t o the building causing heavy structure cracking. 143 .SLABS ON EXPANSIVE SOILS Figure 92. Typical heaving of sidewalk. Figure 93. California. . 54. F r e q u e n t expansion j o i n t s will be necessary t o prevent excessive cracking of t h e slab.144 FOUNDATIONS ON EXPANSIVE SOILS t h e j o i n t b e t w e e n t h e a p r o n and t h e wall. Vol. Figure 9 2 shows t h e heaving of a c o n c r e t e slab.. it t r a n s m i t s pressure to t h e s t r u c t u r e t h r o u g h c o n n e c t i n g girders causing building d a m a g e . No. 4. E. R. "Buildings on Expansive Clay. [531 Means." Quarterly of the Colorado School of Mines. Los Angeles. II). 1959. Portland Cement Association. A n o t h e r serious m i s t a k e is t o place t h e posts of a p a t i o on a c o n c r e t e walk as s h o w n on figure 9 3 ." (Vol. As t h e c o n c r e t e walk heaves. the REFERENCES [521 "Recommended Practice for Construction of Residential Concrete Floors on Expansive Soils. Chapter 7 MOISTURE CONTROL INTRODUCTION Terzaghi stated t h a t . S o m e t i m e s a plank is installed along the edge of the m e m b r a n e . . b e c o m e w e t regardless of the presence of such m e m b r a n e because evaporation can n o longer take place. in t i m e . or flexible paving. However. V a p o r barriers. Even in t h e case of a perfect impervious m e m b r a n e . have b e e n used w i t h only a limited degree of success in impeding m o i s t u r e migration. Ever since t h e a c k n o w l e d g m e n t of expansive soil p r o b l e m s . Membranes A widely used h o r i z o n t a l m o i s t u r e barrier is a c o m b i n a t i o n of a p o l y e t h y l e n e m e m b r a n e extending beyond t h e limits of backfill and loose gravel placed o n t o p of t h e m e m b r a n e . his s t a t e m e n t can be accurately applied t o t h e b e h a v i o r of e x p a n s i o n soils. rigid paving. engineers have b e e n a t t e m p t i n g to isolate w a t e r from t h e f o u n d a t i o n structure. however. Surface w a t e r p o n d i n g in a depressed area will in time leak t h r o u g h t h e holes and edges of the m e m b r a n e and e n t e r t h e soil b e n e a t h .. Figure 9 4 indicates the typical design. it is difficult t o isolate t h e m i g r a t i o n of m o i s t u r e from an e x t e r i o r l o c a t i o n t o a covered area. " W i t h o u t any w a t e r t h e r e would be n o use for soil m e c h a n i c s . It is a relatively simple u n d e r t a k i n g t o remove free w a t e r w h i c h m a y seep i n t o a building f o u n d a t i o n b y providing a d e q u a t e surface drainage and p r o p e r l y installed subdrainage systems. m o i s t u r e migration d u e to t h e r m a l transfer as explained in c h a p t e r 2 will i n t r o d u c e additional m o i s t u r e to the f o u n d a t i o n soils. T h e thickness of t h e p o l y e t h y l e n e m e m b r a n e ranges from a b o u t 4 t o 2 0 mils. b o t h h o r i z o n t a l and vertical. " Terzaghi had only limited k n o w l e d g e of swelling soils. It should be realized t h a t the d r y soils b e n e a t h an impervious m e m b r a n e will. T h e use and effectiveness of these m o i s t u r e barriers are discussed below. F u r t h e r research is necessary in b o t h t h e field and l a b o r a t o r y t o establish a practical and e c o n o m i c a l m e t h o d of controlling m o i s t u r e migration. T h e m e m b r a n e tears easily and eventually develops holes. while evaporation and drying of t h e soil b e n e a t h t h e m e m b r a n e is impossible. T h e p u r p o s e of t h e h o r i z o n t a l barriers is t o prevent excessive i n t a k e of surface m o i s t u r e . T h e p u r p o s e of such installation is t o prevent surface w a t e r from seeping t h r o u g h t h e backfill i n t o t h e building and to prevent the g r o w t h of w e e d s . HORIZONTAL MOISTURE BARRIERS H o r i z o n t a l m o i s t u r e barriers can be installed a r o u n d a building in the form of m e m b r a n e s . Obviously. With p o o r l y constructed j o i n t s . T h e advantage of using c o n c r e t e aprons r a t h e r t h a n plastic m e m b r a n e s is t h a t t h e former offers a positive barrier t o water. it is easy to find t h a t t h e soil has a m o i s t u r e c o n t e n t greater t h a n t h e plastic limit. . care should be exercised in obtaining an effective seal b e t w e e n the aprons and the f o u n d a t i o n walls. w h i c h proved effective in controlling m o v e m e n t . w a t e r will e n t e r t h e j o i n t and seep into the f o u n d a t i o n soil. Concrete aprons T h e installation of concrete aprons or sidewalks has been found effective in controlling m o i s t u r e fluctuation. Impervious membrane along exterior walls. M o h a n and R a o [ 5 4 ] installed a 4-foot-wide c o n c r e t e apron a r o u n d distressed buildings f o u n d e d on black c o t t o n soils.146 FOUNDATIONS ON EXPANSIVE SOILS Foundation wall Figure 94. Swelling soils can heave an apron so t h a t surface drainage is t o w a r d the building rather t h a n away. T h u s . In t h e course of several years. By lifting t h e m e m b r a n e . Paving t h e entire non-building area is impractical and unsightly. it appears t h a t t h e q u e s t i o n a b l e advantage of using a m e m b r a n e around the building is t o increase t h e time required for m o i s t u r e p e n e t r a t i o n and m a k e t h e m o i s t u r e d i s t r i b u t i o n m o r e u n i f o r m . Nonetheless. within reason. t h e backfill soil b e n e a t h a m e m b r a n e will be totally s a t u r a t e d . T h e y claim that t h e function of t h e a p r o n is t o m o v e the marginal m o i s t u r e variation away from the building. While t h e use of concrete aprons a r o u n d t h e e x t e r i o r of the building m a y prove beneficial. it has b e e n observed t h a t f o u n d a t i o n m o v e m e n t d u e t o expansive soils seldom takes place in gasoline service stations where t h e ground surface is extensively covered. the wider the c o n c r e t e apron t h e m o r e p r o t e c t i o n it offers t o t h e building. less t h a n one-half inch thick.. compared to original) percent Solubility in C C I 4 . 32° F. 5 sec. 5 sec. Penetration. 50-60 30 Min.. pavement S u b s e q u e n t m o i s t u r e d e t e r m i n a t i o n s of t h e enclosed e m b a n k m e n t indicated very little change in content remained in a stable c o n d i t i o n .. 3 t o 4 feet w i d e .. 77° F..—Specification for catalytically-blown asphalt cement. 1. The asphalt shall be homogeneous. and t h e material. 77° F. T h e c o n c r e t e p a v e m e n t was placed u p o n an asphalt m a t 4 8 feet wide. 3. 100 gms. In t h o s e areas w h e r e c o n c r e t e a p r o n s are used. 115° F. Min. Asphalt used as a membrane shall be 50-60 penetration grade. Test Designation Flash point Softening point Penetration. t h u s preventing v o l u m e t r i c change of t h e fill 10-year period s u b s e q u e n t t o c o n s t r u c t i o n .0 Min. V a n L o n d o n [ 5 7 ] used t h e 50-60 p e n e t r a t i o n asphalt m e m b r a n e t o c o m p l e t e l y envelop t h e highway e m b a n k m e n t .0 Max.5 Min. t h e T e x a s Highway D e p a r t m e n t used asphalt m e m b r a n e s t o p r e v e n t surface w a t e r from entering t h e expansive clay subgrade [ 5 5 ] . Such material can b e c o n v e n i e n t l y from b y t h e Asphalt Institute [56] advocates t h a t asphalt membranes from catalytically b l o w n asphalt can be effective in preventing m o i s t u r e intruding i n t o subgrade soils. asphalt sheets. The use of iron chlorides or compounds thereof will not be permitted. 120 Max. 175°F. T h e p u r p o s e of t h e m e m b r a n e was t o m a i n t a i n a c o n s t a n t m o i s t u r e content moisture in the embankment over t h e soil. Asphalt membranes As early as 1 9 3 3 .-225°F. . Ductility. (100 gms. D 165 60. This material shall be prepared by the catalytic blowing of petroleum asphalt. c o n s t a n t care and m a i n t e n a n c e is required. 60 sec. 200 gms. A n o t h e r t y p e of asphalt m e m b r a n e consisted of prefabricated handled and easily placed. This m e m b r a n e installation m a y retard swelling b u t will n o t prevent it. in 5 hrs. Specifications for catalytically b l o w n asphalt c e m e n t are given in Table 12. 77° F. (5 cm per min) cm Loss on heating 325° F. Penetration. Penetration of residue.MOISTURE CONTROL 147 t h u s an a p r o n can cause m o r e damage t h a n g o o d . Research c o n d u c t e d constructed table 12.. 50 gms. free of water and shall not foam when heated to 347° F. 97.0 Min. and u p t o 2 0 feet long. It shall meet the above tabulated requirements. 5 sec. percent Test Method ASTM D D D D D D D 92 36 5 5 5 113 6 50-60 Penetration Grade 425° F. 3 gallons per square y a r d . T h e p a t h of m o i s t u r e migration w h e n using a vertical barrier is s h o w n on figure 9 5 . or o t h e r d u r a b l e . However. is a b o u t 1. rainfall. but can still absorb moisture with time. Moisture content relatively constant. T h e use of asphalt m e m b r a n e s in c o n n e c t i o n w i t h t h e c o n s t r u c t i o n of a swimming pool in an expansive soil area is particularly desirable. vertical barriers should b e m o r e effective t h a n h o r i z o n t a l barriers in minimizing seasonal drying and shrinking of the p e r i m e t e r f o u n d a t i o n soils. some moisture migrates below membrane depth. Where slab-on-ground c o n s t r u c t i o n is required. F u r t h e r research and field observation will be required. As seen from figure 9 5 . Theoretically. T h e a m o u n t of asphalt c e m e n t required to c o n s t r u c t a m e m b r a n e . c o n c r e t e . . With time. t h e installation of a vertical barrier prevents edge w e t t i n g d u e t o lateral m o i s t u r e migration w i t h i n t h e d e p t h t o which t h e m e m b r a n e e x t e n d s . VERTICAL MOISTURE BARRIERS Vertical m o i s t u r e barriers have been used a r o u n d t h e p e r i m e t e r of t h e building t o cut off t h e source of w a t e r t h a t m a y e n t e r t h e underslab soils. (After Woodward-Clyde-Sherard and Associates). Figure 95. impervious materials. such t r e a t m e n t can be very advantageous. over a period of t i m e . Path of moisture migration blocked by vertical barrier. and lawn irrigation w a t e r will a c c u m u l a t e near the b o t t o m Rainfall and yard watering Depth of seasonal moisture change Most migration "blocked" by membrane.148 FOUNDATIONS ON EXPANSIVE SOILS Asphalt m e m b r a n e s can be used t o cover t h e surface of expansive soils so t h a t nonexpansive fill can b e placed o n t o p of t h e m e m b r a n e s . as well as m a i n t a i n i n g long term uniform m o i s t u r e c o n d i t i o n s b e n e a t h t h e covered area. This will m i n i m i z e the infiltration of surface w a t e r into t h e underslab soils. according t o t h e Asphalt I n s t i t u t e . Installation Buried vertical barriers m a y consist of p o l y e t h y l e n e m e m b r a n e . melting snow. O t h e r s a t t e m p t t o consolidate t h e backfill b y p u d d l i n g . r a t h e r t h a n from capillary rise. backfill serves t h e same p u r p o s e as d o vertical m o i s t u r e barriers. Backfill A n i m p o r t a n t e l e m e n t involved in building c o n s t r u c t i o n w h i c h is usually slighted is t h e backfill a r o u n d a building. Surface w a t e r can t h e n e n t e r t h e backfill and seep freely i n t o t h e f o u n d a t i o n soils. Theoretically. t h e same degree of w e t t i n g of t h e f o u n d a t i o n soil could result w i t h or w i t h o u t t h e use of a barrier. it is d o u b t f u l t h a t such an installation is of sufficient m e r i t t o w a r r a n t t h e e x p e n s e . impervious slurries installed in a n a r r o w t r e n c h . vertical m o i s t u r e barriers have a distinct advantage over h o r i z o n t a l m o i s t u r e barriers. c a n n o t b e p e r f o r m e d b y large c o m p a c t i n g e q u i p m e n t . C o m p a c t i o n of backfill in restricted areas.MOISTURE CONTROL 149 of t h e m e m b r a n e and t h e m o i s t u r e will be sucked i n t o t h e mositure-deficient soils b e n e a t h t h e building. A s t u d y in S o u t h Africa [ 5 8 ] has s h o w n t h a t w e t t i n g of soils b e n e a t h a h o u s e occurred t o a d e p t h of at least 24 feet in a fissured-clay profile. s e t t l e m e n t of backfill a r o u n d a building as well as s e t t l e m e n t of utility t r e n c h e s is t h e rule. M a n y builders choose t o p u s h t h e loose soil i n t o t h e excavation w i t h n o further c o m p a c t i o n effort. Vertical m o s i t u r e barriers should be installed 2 t o 3 feet from t h e p e r i m e t e r f o u n d a t i o n t o p e r m i t m a c h i n e excavation of t h e t r e n c h for t h e m e m b r a n e . It is obvious t h a t p r o p e r c o m p a c t i o n of t h e backfill c a n n o t b e achieved b y such processes. Vertical m o i s t u r e barriers should be installed t o a d e p t h equal t o or greater t h a n t h e d e p t h of seasonal m o i s t u r e change. 2. Vertical m o i s t u r e barriers c a n n o t be effectively installed a r o u n d b a s e m e n t s t r u c t u r e s . T h u s .o p e r a t e d vibrating plates. U n f o r t u n a t l e y . However. t h e p o t e n t i a l for damage w o u l d b e less because of t h e slower rate of m o s i t u r e migration and t h e m o r e u n i f o r m m o i s t u r e c o n t e n t of t h e soil at any particular t i m e . E i t h e r c o n c r e t e or p o l y e t h y l e n e m e m b r a n e can b e used. 4. Because of p o o r c o m p a c t i v e effort. r a t h e r t h a n t h e exception. This is especially t r u e in b a s e m e n t building w h e r e p r o p e r l y c o m p a c t e d backfill can prevent surface w a t e r from entering t h e f o u n d a t i o n soils. When i m p r o p e r l y c o m p a c t e d . By installing a m o s i t u r e barrier. almost all of t h e backfill along t h e f o u n d a t i o n walls is in a loose state. backfill is seldom c o m p a c t e d p r o p e r l y . 3. It is also possible t o use semi-hardening. T h e following should be considered in t h e installation of a vertical m o i s t u r e barrier: 1. b y h a n d . T h e result of s e t t l e m e n t of loose backfill is s h o w n o n figure 9 6 . Most of this w e t t i n g was a t t r i b u t e d t o lateral migration of m o i s t u r e from seasonal rains. When p r o p e r l y c o n s t r u c t e d . especially w h e r e great d e p t h is required. b u t w o u l d o c c u r over a longer p e r i o d of t i m e . such as in utility t r e n c h e s . These areas should b e c o m p a c t e d . T h e m e m b r a n e should be of sufficient thickness and durability t o resist p u n c t u r e s d u r i n g backfilling of t h e trench. in view of the high cost involved in t h e installation of a vertical m o i s t u r e barrier. . T h e vertical barrier is s o m e t i m e s a t t a c h e d t o a h o r i z o n t a l barrier t o prevent w e t t i n g b e t w e e n t h e vertical barrier and t h e building. 5. in t h i n lifts of n o t m o r e t h a n 4 inches. Typical case of loose backfill around the building. Excess puddling. resulting in great d a m a g e . . N o t only does such practice m a k e it impossible t o o b t a i n the required density b u t it is s o m e t i m e s d a n g e r o u s . By using impervious clay c o m p a c t e d t o 85 percent or m o r e of standard P r o c t o r density at o p t i m u m m o i s t u r e c o n t e n t .150 FOUNDATIONS ON EXPANSIVE SOILS Figure 96. in fact. T h e so-called p u d d l i n g process has been widely used b y c o n t r a c t o r s in small j o b s w i t h the assumption t h a t t h e soils will consolidate w i t h o u t c o m p a c t i o n . t h e backfill acts as a very effective vertical m o i s t u r e barrier. puddling will cause t h e collapse of the e x t r e m e l y loose. such d a m a g e will be reflected in t h e s t r u c t u r e for m a n y years. In cohesive backfills. t h e c o m p a c t o r has p r o b a b l y a lift c o m p a c t i o n capacity of no m o r e t h a n 12 inches. puddling inevitably leads t o weakening and softening of the soil and to future loss of stability and subsidence. O n e recently advertised hydraulic-operated c o m p a c t o r claimed to have the ability to c o m p a c t backfill in a t r e n c h . from t h e ground surface. In u n i f o r m l y granular soils. in one effort. Such barriers are m o r e effective t h a n m e m b r a n e s . Note depressions which trap water. Loose backfill allows surface water to enter the foundation soils. unstable z o n e associated w i t h bulking. In expansive soil areas. w h i c h frequently results w h e n a hose is left discharging i n t o t h e backfill overnight. Such a claim appears t o be false while. can easily i n t r o d u c e w a t e r i n t o the f o u n d a t i o n soils and b e n e a t h t h e slab. Perched water As m e n t i o n e d previously u n d e r "Slabs on Expansive Soils. T o insure t h e i n t e r c e p t i o n of free w a t e r . Arrest t h e capillary m o i s t u r e m o v e m e n t and m o v e m e n t of m o i s t u r e in t h e v a p o r state. In tercep ting d rains Intercepting drains are effective in minimizing t h e w e t t i n g of t h e f o u n d a t i o n soils w h e r e t h e w e t t i n g is d u e t o t h e gravity flow of free w a t e r in a subsurface pervious layer such as a layer of gravel or fissured clay.MOISTURE CONTROL 151 SUBSURFACE DRAINAGE T h e purposes of a subsurface drainage system are as follows: 1. I n t e r c e p t t h e gravity flow of free w a t e r . INTERCEPT GRAVITATIONAL FREE WATER. and 3 ." a perched w a t e r table c o n d i t i o n can develop in areas w h e r e b e d r o c k is shallow. Typical function of an intercepting drain. Figure 97. (After Woodward-Clyde-Sherard & Associates). I n t e r c e p t i n g drains are also widely used for improving slope stability and preventing landslides. A perched w a t e r table can also be created b y a relatively i m p e r m e a b l e s t r a t u m of small areal e x t e n t and b y a z o n e of aeration above t h e main b o d y of g r o u n d w a t e r [ 5 9 ] as s h o w n o n figure 9 8 . When a s t r u c t u r e is located near an irrigation d i t c h or canal w i t h a leakage p r o b l e m . . Intercepting drains are m o s t effective w h e n located along t h e t o e of a slope w h e r e g r o u n d w a t e r leaves t h e d e e p strata and w h e r e it m a y emerge to t h e surface. Surface w a t e r a c c u m u l a t e d from yard irrigation will n o t p e r m e a t e t h e b e d r o c k and can create a local perched w a t e r c o n d i t i o n . T h e u p p e r soils are relatively pervious and surface w a t e r is capable of seeping t h r o u g h t h e u p p e r soils e n c o u n t e r i n g relatively low resistance. a perched w a t e r table c o n d i t i o n m a y develop d u e t o t h e following: 1. L o w e r t h e g r o u n d w a t e r or perched w a t e r . 2. t h e drain m u s t be c o m p l e t e l y filled w i t h gravel and t h e t r e n c h should be d e e p e n o u g h t o reach t h e water-bearing layer. Where b e d r o c k is situated at a slight d e p t h b e n e a t h t h e g r o u n d surface. t h e installation of an i n t e r c e p t i n g drain will p r o t e c t against t h e infiltration of seepage w a t e r . This is s h o w n o n figure 9 7 . 2. A large v o l u m e of w a t e r is capable of flowing in the fissures of t h e b e d r o c k . Wells tapping perched aquifers yield only t e m p o r a r y or small a m o u n t s of w a t e r . b u t also because of m o i s t u r e migration [ 6 0 ] . (After Todd). w h i c h o c c u r n o t only because of gravitational flow of free w a t e r . an intercepting drain should be installed at least 2 feet below t h e floor and should lead t o a suitable o u t l e t w h e r e w a t e r can be removed by gravity or b y a s u m p p u m p . These drains can be installed around either t h e interior or exterior of t h e building. T h e lower b e d r o c k is impervious and will n o t allow t h e infiltration of water. T h e subdrainage system is effective in minimizing general w e t t i n g of t h e f o u n d a t i o n soils. t h e surface w a t e r t e n d s t o seep t h r o u g h t h e u p p e r soil and a c c u m u l a t e on t o p of b e d r o c k . such w a t e r can e n t e r b a s e m e n t s and cause considerable d a m a g e . Part of t h e a c c u m u l a t e d w a t e r will flow on t o p of t h e b e d r o c k and p a r t of it will run t h r o u g h t h e fissures of t h e b e d r o c k . t h e b a s e m e n t excavation can cut t h r o u g h the fissures of b e d r o c k and t h e w a t e r can a c c u m u l a t e in t h e low b a s e m e n t area. Where t h e w a t e r table is d e e p . With surface irrigation and precipitation. When a d e e p b a s e m e n t is c o n s t r u c t e d . However. creating a perched w a t e r table c o n d i t i o n . T o be effective. Peripheral drains Figure 9 9 indicates t h e suggested location of peripheral drains. t h e r e are seams and fissures in t h e b e d r o c k w h i c h provide passage for w a t e r . 3. However. T h e installation of a subsurface drainage system a r o u n d t h e p e r i m e t e r of t h e lower level of a s t r u c t u r e can p r o t e c t against infiltration from perched w a t e r . .152 FOUNDATIONS ON EXPANSIVE SOILS Ground surface Perched water tables Water table Unconfined aquifer Figure 98. m o i s t u r e migration includes capillary m o i s t u r e m o v e m e n t in t h e liquid state and m o v e m e n t of m o i s t u r e in t h e vapor state d u e t o t e m p e r a t u r e differential. Perched aquifers. As explained in c h a p t e r 2. . Grade beam or Foundation wall J / 2 " Expansion joint material 4 Void Material Concrete Pier SECTION A A Figure 99. 5 % to sump Drain Trench •5 ï Sump 3 deep can be located at any convenient location in the basement. Typical sub-drain detail.MOISTURE CONTROL 153 Foundation Wall Grade down 0 . T h e possible p l a c e m e n t of a subdrain t o i n t e r r u p t m o i s t u r e m o v e m e n t is s h o w n o n figure 1 0 0 . In some instances. SUBDRAIN INTERRUPTS MOISTURE MOVEMENT Figure 100. Positive o u t l e t s should b e provided for t h e subdrainage s y s t e m . t h e vapor pressure in t h e drain should b e at a lower value t h a n t h e v a p o r pressure in t h e f o u n d a t i o n soils. it m a y b e permissible t o c o n n e c t t h e subdrain t o t h e gravel b e d b e n e a t h t h e street MOISTURE MOVEMENT WITHOUT SUBDRAIN Legend Gravitational flow of free water (in shrinkage cracks and fissures) Saturated soil at surface Capillary or vapor movement in soil mass. (After Woodward-Clyde-Sherard & Associates) . it m u s t b e designed as a capillary b r e a k .154 FOUNDATIONS ON EXPANSIVE SOILS capillary action and v a p o r transfer are p r o b a b l y t h e major causes of w e t t i n g of t h e moisture-deficient soils in a covered area. for a subdrainage system t o b e effective in preventing m o i s t u r e m o v e m e n t discussed above. Interruption of moisture movement by subdrain. It should b e noted t h a t . T h e gravel used t o fill t h e subdrain t r e n c h should have a gradation b e t w e e n 3 / 4 and 2 inches in size w i t h p e r c e n t o f fines less t h a n 5. a n d additionally. If a gravity o u t l e t is n o t possible. t h e drain should b e discharged t o a s u m p w h e r e w a t e r can b e removed b y p u m p i n g . t h e area a r o u n d the building had b e e n p r o p e r l y graded after c o n s t r u c t i o n . In m a n y cases. t h e peripheral drain should be placed at least 12 inches b e l o w t h e floor level. preferably 24 inches. cost. T h e r e f o r e . it is pleasing t o have shrubs and flower beds p l a n t e d adjacent t o buildings. all o u t l e t s in t h e system should b e pressure checked t o d e t e c t t h e presence of a n y possible o p e n o u t l e t u n d e r g r o u n d w h e r e w a t e r could flow u n c h e c k e d for a long period of t i m e w i t h o u t arousing suspicion. Nevertheless. since it is necessary t o irrigate flower b e d s and shrubs. However. As a result. This usually is n o t accomplished d u e t o negligence. it is n o t u n c o m m o n t o find buildings w i t h surface drainage d i r e c t e d toward t h e f o u n d a t i o n walls. such in a r r a n g e m e n t is n o t usually satisfactory. shrubs and flower b e d s were l o c a t e d adjacent t o t h e building.MOISTURE CONTROL 155 sewer line. . limited p r o p e r t y size and o t h e r reasons. Figure 101 indicates a typical case w h e r e drainage away from t h e building is o b s t r u c t e d b y t h e paved walk and i m p r o p e r c o m p a c t i o n results in a depression along t h e building creating a p o n d i n g c o n d i t i o n . It is d o u b t f u l w h e t h e r large tress will pose a p r o b l e m in high-swelling soil areas. Experience indicates t h a t t o be fully effective. Nozzles of t h e sprinkling system should never be directed t o w a r d a building. Before use. S h r u b s planted along a wall is typical of m a n y school buildings and residential houses. b y improving t h e drainage. b u t t h e grade was later changed t o improve t h e a p p e a r a n c e of t h e landscape. An a u t o m a t i c timing device should be provided for all sprinkling systems so t h a t excessive watering is avoided. a beneficial effect is inevitable. Vegetation F r o m an architectural s t a n d p o i n t . E x p e r i e n c e indicates t h a t in practically every investigation of a cracked building. because of t h e small gradient differential b e t w e e n sewer line i n t a k e and subdrain o u t l e t . However. Moisture change at t h e p e r i m e t e r of t h e building apears t o be t h e m o s t significant c o n t r i b u t o r t o d a m a g e . L a w n sprinkling systems should be installed at least 10 feet from t h e building. SURFACE DRAINAGE T h e ground surface a r o u n d a building should b e graded so t h a t surface w a t e r will drain away from t h e s t r u c t u r e in all directions. Sprinkling system Lawn sprinkling systems often create f o u n d a t i o n soil p r o b l e m s . M a n y studies have s h o w n t h a t large bushes and trees can cause differential drying [611 of t h e f o u n d a t i o n soils and result in d a m a g e t o t h e building from shrinkage. it is good practice t o p l a n t trees and shrubs at least 10 feet from a s t r u c t u r e . t h e excess w a t e r will p e n e t r a t e t h r o u g h t h e loose backfill i n t o t h e f o u n d a t i o n soils. Most of t h e d a m a g e caused b y shrinkage takes place in non-swelling or low-swelling soil areas. T h e downspouts should e x t e n d well b e y o n d t h e p e r i m e t e r of the f o u n d a t i o n and should discharge to an area w h e r e t h e surface drainage is a d e q u a t e to carry off t h e w a t e r rapidly and prevent any possible p o n d i n g of w a t e r . Plantation strip around a school building allowing the ponding of water between the sidewalk and the foundation wall. S o m e t i m e s an o p e n c o u r t y a r d is c o n s t r u c t e d in t h e central p o r t i o n of a building. t h e w a t e r from d o w n s p o u t s should b e carried in a closed pipe or lined d i t c h to t h e street. If necessary. and w a t e r from t h e r o o f drains freely t h r o u g h the loose backfill i n t o t h e f o u n d a t i o n soils. and trees. flower beds. .156 FOUNDATIONS ON EXPANSIVE SOILS Figure 101. T h e c o u r t y a r d is usually covered w i t h lawn. Such a c o u r t y a r d c o n s t i t u t e s a major drainage p r o b l e m because surface w a t e r will b e u n a b l e t o drain unless an a d e q u a t e subsurface drainage system is provided. M a n y m o d e r n buildings are c o n s t r u c t e d w i t h o u t d o w n s p o u t s . Roof drain R o o f d o w n s p o u t s m u s t be directed away from a s t r u c t u r e so t h a t w a t e r will n o t seep i n t o the foundation soils. Butterworth.. 1959. [60] "Remedial Methods Applied to Houses Damaged by High Volume Change Soils. Association of Asphalt Paving Technologists. Oakland. In o n e instance. "Waterproofing Value of Asphalt Membranes in Earth Fills for Gulf Freeway. No. it was found t h a t t h e plug on the drain trap b e n e a t h t h e s h o w e r stall was missing.. Soil Mechanics and Foundation Division. 1953." Proceedings. [56] "Asphalt Membranes and Expansive Soils. Sewer lines laid b e n e a t h t h e b a s e m e n t are subjected t o stress w h e n t h e s u r r o u n d i n g soils e x p a n d . M. Inc." Highway Research Board Proceedings. Australia. J. . Leakage from w a t e r lines is less frequent. B. [57] Van London. "Some Observations on the Movement of Buildings on Soils in Vereening and Odendaalsrus. while investigating a cracked h o u s e . 145 (IS-145) May 1968.." Symposium on Expansive Clays. "Moisture Variation and Performance of Foundations in Black Cotton Soils in India." John Wiley & Sons. [61] Hammer. B.. including sewer and w a t e r lines. As a result. 1945. D. 92." Journal ASCE. [59] Todd. California. REFERENCES [54] Mohan. "Ground Water Hydrology. [55] "Report of Committee on Warping of Concrete Pavements. "Foundation Clay Shrinkage Caused by Large Trees. shearing stress has caused pipe breakage resulting in flooding. SM 6.. [58] Collins. K. and in e x t r e m e cases. 1968. all t h e w a t e r from t h e shower drained i n t o t h e crawl space area for a period of at least 3 years." Moisture Equilibria and Moisture Change in Soils Beneath Covered Areas. 1966. Vol.. 25. L. and Rao. G." Woodward-Clyde-Sherard & Associates. Vol. W. Leaking w a t e r follows t h e loose backfill a r o u n d t h e w a t e r pipe i n t o the f o u n d a t i o n soils causing d a m a g e . South African Institution of Civil Engineers. Nov. Ο. Vol. S o m e t i m e s leakage is found n e a r t h e w a t e r m e t e r w h i c h generally is situated n e a r t h e sidewalk in front of t h e h o u s e ." Information Service No. and Thompson. D. E. 22. J. should be carefully checked for leakage.MOISTURE CONTROL 157 Interior plumbing Interior p l u m b i n g . In s o m e cases.t e m p e r a t u r e area t o a l o w . such as floor slabs. N o r m a l l y this m o i s t u r e evaporates at t h e surface and m o i s t u r e equilibrium is m a i n t a i n e d in t h e soil. Isolate t h e soil so t h e r e will be n o m o i s t u r e change. T h e p r e w e t t i n g t h e o r y is based o n t h e a s s u m p t i o n t h a t if soil is allowed t o swell b y w e t t i n g prior t o c o n s t r u c t i o n and if t h e high soil m o i s t u r e c o n t e n t is m a i n t a i n e d . o r similar s t r u c t u r e s which inhibit this evaporation increases t h e m o i s t u r e c o n t e n t of t h e f o u n d a t i o n soil w i t h resultant swell. Ponding T h e present p r e w e t t i n g practice usually involves direct flooding or p o n d i n g of t h e building area.Chapter 8 SOIL STABILIZATION INTRODUCTION In t h e o r y . 3 . or 5.o s m o s i s or o t h e r m e c h a n i s m s . T h e f o u n d a t i o n and floor area is flooded b y c o n s t r u c t i n g a small e a r t h b e r m a r o u n d t h e outside of t h e f o u n d a t i o n t r e n c h e s t o i m p o u n d t h e water. where t h e m o i s t u r e c o n t e n t at footing d e p t h is stable. PREWETTING A n old established c o n c e p t a m o n g engineers and c o n t r a c t o r s as well as l a y m e n in dealing w i t h swelling soils is p r e w e t t i n g . A n o t h e r practice includes first p r e w e t t i n g the f o u n d a t i o n t r e n c h e s . T h e presence of covered areas. F l o o d t h e in-place soil t o acheive swelling prior t o c o n s t r u c t i o n . 4. Decrease t h e d e n s i t y of t h e soil b y c o m p a c t i o n c o n t r o l . 2. Replace t h e swelling soils w i t h nonswelling soils. Change t h e p r o p e r t i e s of expansive soils b y chemical injection. t h e swelling p o t e n t i a l of an expansive clay can be m i n i m i z e d or c o m p l e t e l y eliminated b y o n e of t h e following m e t h o d s : 1. . t h e soil v o l u m e will remain essentially c o n s t a n t .t e m p e r a t u r e area b y m e a n s of t h e r m o . Moisture migration can also take place from a h i g h . achieving a no-heave state and therefore structural damage will n o t occur. Isolation of t h e soil has b e e n extensively discussed in c h a p t e r 7. As explained in c h a p t e r 2 . t h e n placing t h e f o u n d a t i o n w h i c h is used as a dike t o flood t h e floor area. m o i s t u r e can migrate from a moderate-depth w a t e r table t o an u p p e r moisture-deficient soil b y m e a n s of capillary rise. it is possible t o place c o n c r e t e footings and utilize t h e m as dikes so t h a t only the floor area is pre w e t t e d . p a v e m e n t s . climatic condition. . 2. and prior local experience. subsoil condition. The moisture content profile should be checked frequently by tests in the field to assure that the desired results are achieved. the ground surface must be kept moist until the slab is placed. after completing of the prewetting treatment. Experience in Southern California [63] indicates that pre wetting moderately expansive soils to a condition of 85 percent saturation at a depth of 2-1/2 feet is often satisfactory. Subgrade moisture movement on IH35. (After McDowell) McLennan Co. A gravel or sand bed 4 to 6 inches thick should be placed over the subgrade prior to the prewetting period. Figure 102. The moisture content achieved a significant penetration of only 4 feet below the pond during a period of 24 days. swelling potential. foundation system. ponding should extend approximately 30 days.160 FOUNDATIONS ON EXPANSIVE SOILS The effect of ponding or flooding on the moisture content at various depths has been investigated by the Texas Highway Department [ 6 2 ] . To obtain desirable moisture distribution at greater depths. For slab-on-ground construction. Texas. The moisture variation at specific depths beneath the ponding area is shown on figure 102. prewetting to as much as 3 feet may not be sufficient. The treatment should be based upon an engineering investigation and evaluation of the site. The following observations were made: 1. Texas was chosen for the experiment. A section of Interstate Route 35 north of Waco. In the case of highly expansive soils. The subgrade was ponded and the moisture content at various depths was taken.. The prewetting operation must not be at the discretion of a contractor or owner. The gravel layer prevents the clay from drying and shrinking. Bara [ 6 4 ] claimed t h a t if t h e dense clays w i t h a particular liquid limit could be e x p a n d e d t o densities at or above critical n a t u r a l density-liquid limit reference line. . Similarly. t o increase t h e soil m o i s t u r e t o a degree t h a t will prevent harmful heaving u p o n s u b s e q u e n t w e t t i n g . S o u t h Africa [ 6 5 ] . Large scale e x p e r i m e n t s of flooding of f o u n d a t i o n soil for building sites have been c o n d u c t e d in Vereeniging. A soil w i t h liquid limit of 70 intercepts the reference line at 9 0 pcf d e n s i t y . while t h o s e near liquid limit 100 require at least 37 p e r c e n t m o i s t u r e before t h e y are considered t o be relatively n o n e x p a n s i v e in situ. Here. Clays encountered along San Luis Drain. a stable or n e a r u l t i m a t e m o i s t u r e c o n d i t i o n w o u l d have been a p p r o a c h e d and future v o l u m e changes w o u l d be small. A t the end of 9 6 d a y s . Moisture c o n t e n t s above t h e reference line in figure 104 would assure t h a t the densities were Figure 103. has been used in the c o n s t r u c t i o n of t h e San Luis Drain on the San Luis Unit of t h e Bureau of R e c l a m a t i o n ' s Central Valley project in California. Figure 104 indicates t h a t clays at liquid limit 4 0 require only a b o u t 2 3 p e r c e n t m o i s t u r e . The liquid limit versus d r y d e n s i t y relationship is s h o w n o n figure 1 0 3 . (after Bara).SOIL STABILIZATION 161 Practice Ponding or sprinkling. t h e w a t e r content-liquid limit figure relationship was developed for soil liquid limit ranging b e t w e e n 4 0 and 100 as s h o w n on on the non-critical side of t h e reference line in figure 103. over 9 0 p e r c e n t of 104. t h e effect of w e t t i n g was accelerated b y a grid of vertical 4-inch-diameter wells each 20 feet d e e p . T h e effective migration of m o i s t u r e . (after Bara). Felt [ 6 6 ] discusses a p r e w e t t i n g project in w h i c h t h e soil m o i s t u r e c o n t e n t did n o t increase appreciably after t h e first m o n t h of p r e w e t t i n g . t h e trenches again filled w i t h water. Prewetting practice is m u c h m o r e complicated t h a n assumed b y m o s t l a y m e n . It was c o n c l u d e d b y D a w s o n [ 6 7 ] t h a t it is e x t r e m e l y difficult t o s a t u r a t e high plasticity clays within a reasonable period of t i m e . t h e expansive soil b e n e a t h t h e f o u n d a t i o n was p r e w e t t e d b y filling t h e f o u n d a t i o n t r e n c h w i t h water. and the soil k e p t w e t t e d thereafter. t i m e required for s a t u r a t i o n . Minimum water content required for soil liquid limit. E. Evaluation Most highway engineers strongly endorse t h e use of p r e w e t t i n g t o m i n i m i z e subgrade heaving. This h o u s e heaved b o t h during and after c o n s t r u c t i o n . it is d o u b t f u l if p r e w e t t i n g can be successfully used w i t h lightly loaded s t r u c t u r e s . It was suggested t h a t t h e first infiltration of w a t e r was p r o b a b l y t a k e n b y seams and fissures present in t h e clay and. A t a housing project near Austin. full soil e x p a n s i o n did n o t occur. Texas. It is c o n c l u d e d b y t h e a u t h o r s t h a t t h e acceleration of heave b y flooding is a feasible p r e . and swelling of partially saturated soils is n o t fully u n d e r s t o o d . t h e w a t e r was p u m p e d o u t . t h e f o u n d a t i o n placed on t h e wet soil. In view of t h e past experience and actual case studies. E x p a n s i o n of partially saturated clays will c o n t i n u e after c o m p l e t i o n of t h e s t r u c t u r e . d e p t h of p e n e t r a t i o n .162 FOUNDATIONS ON EXPANSIVE SOILS LIQUID LIMIT (%) 40 40 J 50 1 60 1 70 1 80 1 90 1 100 1 cc 10 0 I I I I I I Figure 104. soil swelling c o n t i n u e d . After 6 weeks of soaking. t h e m a x i m u m surface heave h a d t a k e n place.c o n s t r u c t i o n p r o c e d u r e for light s t r u c t u r e s . t h e r e f o r e . As t i m e passed. A . J. and swelling t o o k place t h r o u g h o u t t h e mass of t h e soil and n o t merely along a seepage p a t h . t h e w a t e r moved from the fissures i n t o the b l o c k y soil mass. F o r 5 m o n t h s thereafter. T h e m e t h o d of c o m p a c t i o n is generally limited b y available e q u i p m e n t . less t h a n 1. S o m e of t h e disadvantages of t h e p r e w e t t i n g m e t h o d are as follows: 1. t h e t i m e required for p r e w e t t i n g can be critical. This p r o c e d u r e can c o n t i n u e for as long as 10 years. 2.SOIL STABILIZATION 163 great a m o u n t of research will b e required before c o m p l e t e evaluation of the p r e w e t t i n g practice can be m a d e . While p r e w e t t i n g m a y prove t o be a possible m e t h o d of stabilizing t h e soil b e n e a t h t h e floor slab. It is highly q u e s t i o n a b l e if a u n i f o r m m o i s t u r e c o n t e n t can be o b t a i n e d in pre w e t t e d areas. T h e length of p o n d i n g t i m e required is usually a b o u t 1 t o 2 m o n t h s . and 4 . differential heaving can be critical even after a p r o l o n g e d period of p r e w e t t i n g . After t h e swelling has reached its m a x i m u m p o t e n t i a l . t h e bearing capacity of a stiff clay can be reduced t o a very low value. F r o m a c o n s t r u c t i o n s t a n d p o i n t . . Water can o n l y seep i n t o t h e stiff clay t h r o u g h fissures. F o r lightly loaded slabs. which p r o h i b i t s t h e use of c o n v e n t i o n a l footing f o u n d a t i o n s . u n i f o r m d i s t r i b u t i o n of m o i s t u r e c o n t e n t is n o t likely t o take place. A m o i s t u r e c o n d i t i o n of less t h a n s a t u r a t i o n is often a d e q u a t e t o inhibit objectionable uplift. 3 . Even this length of time m a y be objectionable as being t o o great. T h e c o m p a c t e d dry d e n s i t y . it is d o u b t f u l t h a t footing f o u n d a t i o n s can be placed o n p r e w e t t e d soil. 4 . it is d o u b t f u l t h a t this m e t h o d will be an i m p o r t a n t c o n s t r u c t i o n t e c h n i q u e for building f o u n d a t i o n s o n expansive soils. While p r e w e t t i n g m a y play an i m p o r t a n t role in t h e c o n s t r u c t i o n of slabs. m o i s t u r e migrates t o the lower moisture-deficient soil and induces further swelling. Such d e p t h is insufficient t o provide a balanced m o i s t u r e z o n e for t h e c o n s t r u c t i o n of i m p o r t a n t s t r u c t u r e s . T h e m o i s t u r e c o n t e n t . p a v e m e n t . or canal lining. T h e last t w o r e q u i r e m e n t s are n o t critical in actual c o n s t r u c t i o n . t h e m o i s t u r e c o n t e n t of the u n d e r s l a b soil seldom decreases. E x p e r i m e n t s indicate t h a t p o n d i n g w a t e r can effectively p e n e t r a t e t h e soil t o a d e p t h of 4 feet within a reasonable t i m e . In saturated c o n d i t i o n s . T h e m e t h o d of c o m p a c t i o n . COMPACTION C O N T R O L T h e a m o u n t of swelling t h a t occurs w h e n a structural fill is e x p o s e d t o additional m o i s t u r e d e p e n d s u p o n t h e following: 1. 2. Wet soil will i n d u c e swelling. t h e surcharge load is usually very small. T h e surcharge load.000 psf. and c o n s e q u e n t l y . 3 . Allowing t h e p o n d i n g w a t e r t o migrate i n t o the lower moisture-deficient soils. 5. E x p e r i e n c e indicates t h a t in a covered area. As a result. 164 Placement condition FOUNDATIONS ON EXPANSIVE SOILS As early as 1959. and Gibbs). Percentage of expansion for various placement conditions when under unit psi load. IVS / 1 V b / &.* S %/V *J%* V T MOISTURE CONTENT • PERCENT OF DRY WEIGHT Figure 105. as shown on figure 105. (After Holtz . Dawson [67] suggested that highly expansive soils be compacted to some minimum density rather than to a maximum density. Holtz and Gibbs [68] show the influence of density and moisture on the expansion of a compacted expansive clay. t h e r e will n o t be migration of m o i s t u r e t o t h e u n d e r l y i n g moisture-deficient soils and long w a i t i n g periods. Design L e o n a r d K r a y n s k i of W o o d w a r d . pulverize. ( S t a n d a r d A A S H O ) . and increases only w i t h t h e increase of initial dry d e n s i t y . T h e s h o r t c o m i n g s of p r e w e t t i n g m e t h o d s m e n t i o n e d in t h e preceding section can b e eliminated b y c o m p a c t i o n c o n t r o l . p e r cu. w i t h reference t o figure 2 8 and table 10. a t o t a l of 12 t o 15 samples will be a d e q u a t e t o define m o i s t u r e . 2 3 . Using t h e p l a c e m e n t c o n d i t i o n s .SOIL STABILIZATION 165 It can be seen t h a t expansive clays e x p a n d very little w h e n c o m p a c t e d at low densities and high m o i s t u r e b u t e x p a n d greatly w h e n c o m p a c t e d at high densities and low m o i s t u r e s . p e r cu.-lb. it is possible t o scarify. T h e cylinders are t o be c o m p a c t e d using three different efforts. 0 0 0 t o 5 . F o r i n s t a n c e . 0 0 0 psf and t h e swelling p o t e n t i a l decreases from 6. With m o d e r n c o n s t r u c t i o n t e c h n i q u e s . t h e swelling pressure decreases from 1 3 . (Modified A A S H O ) . T h e controlling e l e m e n t is density. F r o m t h e m e a s u r e d p e r c e n t e x p a n s i o n . T h u s . All of this can be accomplished w i t h o u t changing the m o i s t u r e c o n t e n t . ft. F r o m a s t u d y of t h e s e results. T h e process of r e c o m p a c t i n g swelling clays at m o i s t u r e c o n t e n t s slightly above t h e i r n a t u r a l m o i s t u r e c o n t e n t and at a low density should be an excellent a p p r o a c h . a 2-inch-diameter core m a y be e x t r a c t e d and t e s t e d in t h e c o n s o l i d o m e t e r for swell.7 t o 4.7 t o 4.-lb. 2 0 0 ft.2 p e r c e n t . A d e q u a t e m i x should be p r e p a r e d for three P r o c t o r cylinders at each m o i s t u r e c o n t e n t . Referring t o c h a p t e r 2. Figure 22 indicates t h a t t o decrease t h e swelling p o t e n t i a l from 6. ft. C o m p a c t i n g stiff clay at 4 t o 5 p e r c e n t above o p t i m u m is very difficult. an increase of m o i s t u r e c o n t e n t of a b o u t 5 p e r c e n t will be required. will be unnecessary.d e n s i t y curves as s h o w n on figure 106. and 5 6 . it was established t h a t t h e swelling pressure of clay is i n d e p e n d e n t of t h e surcharge pressure. T h e m a i n advantage of using this a p p r o a c h is t h a t t h e swelling p o t e n t i a l can be r e d u c e d w i t h o u t t h e adverse effects caused b y i n t r o d u c i n g excessive m o i s t u r e i n t o t h e soil. 2. t h e r e f o r e .-lb.2 p e r c e n t . t h e swell was negligible for any degree of c o m p a c t i o n . 4 0 0 ft. T h e m a i n reason m o i s t u r e c o n t e n t is i m p o r t a n t is t h a t m o i s t u r e c o n t e n t can generally result in low density fill. b y decreasing t h e dry density of a typical expansive clay from 109 t o 100 pcf. ft. Excess w a t e r will n o t be present in t h e soil. and r e c o m p a c t t h e n a t u r a l soil effectively w i t h o u t substantially increasing the c o n s t r u c t i o n costs. n o t t h a t high m o i s t u r e c o n t e n t will r e d u c e swelling. t h e average swell u n d e r a surcharge load of 144 psf is p r e d i c t e d t o be 5 . F r o m each c o m p a c t e d s a m p l e . T h e samples are subjected t o 144-psf surcharge pressure. degree of s a t u r a t i o n . such as 1 2 . p e r cu. which is 10-1/2 p e r c e n t .. Clyde & Associates suggests t h e following design p r o c e d u r e on compaction control: 1. A reasonably good bearing capacity can be assigned t o t h e low density soil. t h e n submerged in w a t e r and allowed t o swell. a m o i s t u r e c o n t e n t of 19 t o 2 3 p e r c e n t and a d r y d e n s i t y ranging from 9 6 t o 102 pcf were selected as design specifications. 0 0 0 ft. 3 . curves of equal swell were p l o t t e d as s h o w n on figure 107. thickness of s t r a t u m . initial m o i s t u r e c o n t e n t . prior t o c o n s t r u c t i o n . Gizienski and Lee [ 6 9 ] show t h a t w h e n their test soil was c o m p a c t e d at a b o u t 4-1/2 p e r c e n t above o p t i m u m . Generally. For economic reasons. 1 to 5 feet of compacted material will be adequate with the range of 2 to 3 feet being the most commonly used. Preparation of specimens for earthwork specifications. A guideline has not been established as to the thickness . Experience indicates that if the subsoil consists of more than about 5 feet of granular soils (SC-SP). Such average and maximum swell is considered to be acceptable for the proposed type of construction. or the heaving of the lower expansive soils is so uniform that structural movement is not noticeable. This is not true in the case of man-made fill. SOIL REPLACEMEN T A simple and easy solution for slabs and footings founded on expansive soils is to replace the foundation soil with nonswelling soils.166 FOUNDATIONS ON EXPANSIVE SOILS Figure 106. underlain by highly expansive soils. there is no danger of foundation movement when the structure is placed on the granular soils. The required depth of compaction depends upon the degree of expansion and the magnitude of the imposed loads. the possibility of edge wetting exists. Therefore. It is concluded 1}hat either seepage water has never reached the expansive soils. (after Woodward-Clyde and Associates) percent with maximum swell potential of less than 8 percent. the extent of the selected fill must be limited to a maximum of 10 feet beyond the building line. The mechanics and the path of surface water seeping through the upper granular soils and into the expansive soils is not clear. 4. This t h i c k n e s s refers t o thickness of selected fill b e n e a t h t h e b o t t o m of t h e footings or b o t t o m of floor slabs. a l t h o u g h 5 feet is preferred. t h e d e p t h of r e p l a c e m e n t . T h e p e r t i n e n t r e q u i r e m e n t s c o n c e r n i n g soil r e p l a c e m e n t material. T h e following criteria have been used w i t h a certain degree of success: .SOIL STABILIZATION 167 0 5 10 Moisture Content 15 (%) 20 25 30 Figure 107. granular soils such as GW and SP. (after Woodward-Clyde and Associates) r e q u i r e m e n t for t h e selected fill. surface w a t e r can travel freely t h r o u g h t h e soil and cause w e t t i n g of t h e lower swelling soils. Type of material are t h e t y p e of r e p l a c e m e n t Obviously. All granular soils ranging from GW t o SC in t h e Unified Soil Classification System may! fulfill t h e n o n e x p a n s i v e soil r e q u i r e m e n t . and t h e e x t e n t of r e p l a c e m e n t . SC material w i t h a high percentage of plastic clay s o m e t i m e s will e x h i b i t swelling p o t e n t i a l . A m i n i m u m of 3 feet should always be insisted u p o n . for clean. However. In the o t h e r e x t r e m e . Determination of fill placement moisture and density. t h e first r e q u i r e m e n t for the r e p l a c e m e n t soil is t h a t it be n o n e x p a n s i v e . Disc h a r r o w s and p l o w s will be required t o break the clay i n t o reasonably sized clods. A n y selected fill will be satisfactory provided t h e material is n o n e x p a n s i v e . 2. 6 0 p e r c e n t of t h e swell in m a n y of t h e C o l o r a d o subgrade clays can o c c u r d o w n t o a 20-foot d e p t h . If necessary.168 Liquid limit. While b o t h t h e t h e o r e t i c a l a p p r o a c h and actual m e a s u r e m e n t concerning d e p t h of influence are urgently n e e d e d . the m o v e m e n t will be m o r e u n i f o r m . m o r e tolerable. Also. Depth of replacement T h e d e p t h of influence is a m o s t c o m p l i c a t e d q u e s t i o n t h a t m u s t be answered w h e n dealing w i t h soil t r e a t m e n t b e n e a t h the slabs o r footings. Uniform wetting t e n d s t o equalize heaving. 2 0 0 sieve 15 10 30 40 5-50 It is b e c o m i n g increasingly difficult t o locate materials. t h e r e q u i r e m e n t for imperviousness can be forfeited. say 10-by 10-by 3-feet. T h e r e is a definite gain in placing t h e s t r u c t u r e on a nonexpansive soil cushion. T h e o r e t i c a l l y . swell tests are t h e only positive m e t h o d of d e t e r m i n i n g t h e expansiveness of t h e material. Studies have s h o w n t h a t t h e swelling can t a k e place d o w n t o a d e p t h of as m u c h as 50 feet. b u t in practice it is difficult t o i n c o r p o r a t e granular soil w i t h stiff. t h e following should be p o i n t e d o u t : 1. . fulfilling the above r e q u i r e m e n t s . (such as t h a t used in Gizienski's e x p e r i m e n t ) u n d e r u n i f o r m s a t u r a t i o n c o n d i t i o n s . t h u s reducing t h e a m o u n t of i m p o r t e d fill required. and c o n s e q u e n t l y . in expansive soil areas such as M e t r o p o l i t a n Denver. such a m e t h o d is reasonable. Such an u n d e r t a k i n g will p r o b a b l y be as expensive as using t h e lime stabilization m e t h o d . percent G r e a t e r t h a n 50 30 50 Less t h a n 3 0 FOUNDATIONS ON EXPANSIVE SOILS Percent m i n u s N o . such tests should be c o n d u c t e d r a t h e r t h a n relying o n plasticity tests. When in d o u b t . T h e C o l o r a d o Highway D e p a r t m e n t established curves which show the relationship b e t w e e n t o t a l swell and t h e d e p t h below t h e surface of t h e subgrade [ 7 1 ] . Even if t h e d e e p seated soils swell. Theoretically. T o w h a t d e p t h should t h e n a t u r a l soil be r e c o m p a c t e d ? H o w m a n y feet of overexcavation will be required? H o w m a n y cubic yards of nonexpansive soil will have t o be i m p o r t e d ? These q u e s t i o n s c a n n o t b e intelligently answered until t h e a m o u n t of m o v e m e n t t h a t will o c c u r b e n e a t h t h e slabs o r footings can be assessed. T h e p o t e n t i a l vertical rise of a soil mass. Also. Gizienski and Lee [701 evaluated t h e theoretically c o m p u t e d uplift derived from l a b o r a t o r y test d a t a and t h e actual m e a s u r e m e n t t a k e n from a small scale field test. T h e y found t h a t t h e actual heave in t h e field was only one-third of t h a t estimated from t h e results of l a b o r a t o r y tests. can be less t h a n t h a t of t h e same mass subject t o local w e t t i n g only. t h e a m o u n t of uplift can b e evaluated from t h e d a t a derived from swell tests and pressure d i s t r i b u t i o n m e t h o d s . A great deal of emphasis has been given t o t h e possibility of blending granular soil w i t h the on-site swelling soils. dry expansive clays. 4 . Also. d e t r i m e n t a l heaving will o c c u r regardless of thickness of the selected fill. F o r s u p p o r t i n g footings. soil r e p l a c e m e n t is t h e best m e t h o d t o use in obtaining a stabilized f o u n d a t i o n soil. t h e larger t h e area of r e p l a c e m e n t . With this a r r a n g e m e n t . T h e t y p e of material used for backfill should be t h e same as used for t h e underslab selected fill. a high degree of c o m p a c t i o n o n expansive soils is n o t desirable. If t h e subgrade o r o p e n excavation b e c o m e s w e t t e d excessively before the p l a c e m e n t of the fill. the trapped w a t e r will cause heaving. such surcharge load can be i m p o r t a n t in preventing p o t e n t i a l heave. T h e n a t u r a l soil is scarified and r e c o m p a c t e d as described u n d e r " C o m p a c t i o n C o n t r o l " for a thickness of a b o u t 2 feet. m u c h larger t h a n in t h e artificial c o n d i t i o n . or such a scheme should n o t be a d o p t e d . t h e possibility of surface water entering t h e f o u n d a t i o n soil is greatly r e d u c e d . c o n s e q u e n t l y . Figure 108 shows t h e suggested e x t e n t of r e p l a c e m e n t for b o t h b a s e m e n t and n o n b a s e m e n t c o n d i t i o n s . S u c h capability c a n n o t be o b t a i n e d b y t h e p r e w e t t i n g m e t h o d . T h e c o m b i n e d thickness of 4 feet should be a d e q u a t e to c o n t r o l heaving. 6. t h u s enabling t h e material t o s u p p o r t either heavily loaded slabs or footings. In an artificial fill s i t u a t i o n . . F o r m o d e r a t e l y swelling soil. a degree of c o m p a c t i o n of 9 5 t o 100 p e r c e n t should T h e m a i n reason t h a t an artificially selected fill cushion is less effective t h a n a n a t u r a l granular soil b l a n k e t is t h a t in natural c o n d i t i o n s . T h e r e f o r e . it is always possible for surface w a t e r t o seep i n t o t h e deep-seated expansive soil at the p e r i m e t e r of the fill. T h e soils engineer should have the o p p o r t u n i t y of supervising t h e p l a c e m e n t of fill. It is possible t o c o m p a c t t h e replaced n o n e x p a n s i v e soil t o a high degree of c o m p a c t i o n . 9 0 p e r c e n t of s t a n d a r d be achieved. t h e m o r e effective t h e fill. a n d . a u n i f o r m pressure of a b o u t 6 0 0 psf is applied t o t h e surface of expansive soils. t h e load carrying capacity is limited. t h e b l a n k e t e x t e n d s over a large area. It should be n o t e d t h a t w i t h 4 feet of fill plus t h e weight of c o n c r e t e . T h e swelling p o t e n t i a l of t h e soil b e n e a t h the fill is very i m p o r t a n t as d e n s i t y and m o i s t u r e c o n d i t i o n s change at various l o c a t i o n s . T h e d e p t h of selected fill should never be less t h a n 3 6 inches and preferably 4 8 inches. T h e failure of t h e soil r e p l a c e m e n t m e t h o d generally occurs during c o n s t r u c t i o n . Extent of replacement P r o c t o r density should be a d e q u a t e . t h e n a n o t h e r 2 feet of selected c o m p a c t e d fill placed. In such case. Evaluation With present t e c h n o l o g y o n expansive soils. T h e following are t h e evaluations of soil r e p l a c e m e n t method: 1. 5. w i t h t h e c o m p a c t i o n c o n t r o l m e t h o d . T h e degree of c o m p a c t i o n of the selected fill d e p e n d s u p o n t h e t y p e of s u p p o r t i n g s t r u c t u r e . F o r s u p p o r t i n g slabs.SOIL STABILIZATION 169 3 . T h e thickness of the imported fill can be r e d u c e d if a c o m b i n a t i o n of t h e soil r e c o m p a c t i o n and soil r e p l a c e m e n t m e t h o d s is used. 7 Min. 4 . spreader. Slip j o i n t s m u s t be provided for all . 3 . N o special c o n s t r u c t i o n e q u i p m e n t . -Drilled Pier Building Lines NON BASEMENT CONDITION f— Drilled Pier Building Lines DEEP BASEMENT CONDITION Figure 108. T h e granular soil cushion also serves as an effective barrier against t h e rise of g r o u n d w a t e r o r perched water. With t h e e x c e p t i o n of a s t r u c t u r a l floor slab (suspended floor). it is strongly suggested t h a t floating slab c o n s t r u c t i o n be used. T h e cost of soil r e p l a c e m e n t is relatively inexpensive w h e n c o m p a r e d t o chemically treating t h e soil. o r mixer will be required.170 FOUNDATIONS ON EXPANSIVE SOILS -Ground surface Overexcavated and replaced with nonexpansive f i l l . 2 . 5. such as disc h a r r o w . T o guard against u n e x p e c t e d c o n d i t i o n s which might cause heaving. Suggested extent of fill replacement. soil r e p l a c e m e n t provides t h e safest a p p r o a c h t o slab-on-ground c o n s t r u c t i o n . T h e c o n s t r u c t i o n can be carried o u t w i t h o u t delay as is e n c o u n t e r e d in t h e p r e w e t t i n g m e t h o d . In o n e process. Application T h e a m o u n t of lime required t o stabilize t h e expansive soils ranges from 2 t o 8 p e r c e n t b y weight. Surface drainage a r o u n d t h e building m u s t b e p r o p e r l y m a i n t a i n e d so there is n o o p p o r t u n i t y for w a t e r t o e n t e r t h e expansive soils b e n e a t h t h e selected fill. t h e Chinese have used lime as a stabilizing agent in f o u n d a t i o n soils. 0 0 0 t o n s of lime. F o r centuries. t h e r e is a r e d u c t i o n in clay c o n t e n t and a c o r r e s p o n d i n g increase in t h e p e r c e n t a g e o f coarse particles. . Most of t h e lime stabilization projects were carried o u t b y t h e highway d e p a r t m e n t s of various states. T h e net result is a low base-exchange capacity for t h e particle w i t h a resulting lower v o l u m e change potential. T h e r e a c t i o n results in r e d u c t i o n of shrinkage and swell and i m p r o v e d w o r k a b i l i t y . a change of soil t e x t u r e t h r o u g h flocculation of t h e clay particles takes place w h e n lime is m i x e d w i t h clays. LIME S T A B I L I Z A T I O N T h e use of lime t o stabilize subgrade soil has been k n o w n t o engineers all over t h e world for a long time. additional non-exchanged calcium ions m a y be adsorbed so t h a t t h e t o t a l ion density increases. t h e use of t h e lime stabilization m e t h o d has been steadily increasing. T h e recently c o m p l e t e d Dallas-Fort W o r t h Regional A i r p o r t [74] claims t o have u n d e r t a k e n t h e w o r l d ' s largest lime stabilization project. F o r i n s t a n c e . a base exchange occurs w i t h t h e strong calcium ions of lime replacing t h e weaker ions such as s o d i u m o n t h e surface of t h e clay particle [12]. T h e clays are underlain b y shale of t h e Eagle Ford Formation. As t h e c o n c e n t r a t i o n of lime is increased. 6. its swelling p o t e n t i a l . c o n s u m i n g a b o u t 3 0 0 . A l t h o u g h t h e success of lime-treated subgrade is q u e s t i o n a b l e in m a n y instances. h e n c e . T h e subsoil consists of 8 t o 16 feet of expansive clay w i t h a p o t e n t i a l vertical expansion equivalent t o 10 p e r c e n t of t h e layer thickness. W. H o l t z [73] found t h a t lime drastically reduces t h e plasticity i n d e x and drastically raises t h e shrinkage limit of m o n t m o r i l l o n i t i c clays. G. lime is a favorable agent t o r e d u c e the swelling p o t e n t i a l of f o u n d a t i o n soils. Also. Reaction It is generally recognized t h a t t h e a d d i t i o n of lime t o expansive clays will r e d u c e the plasticity of t h e soil and. In t h e o t h e r process. T h e chemical reaction occurring b e t w e e n lime and soil is q u i t e c o m p l e x . as s h o w n on figure 1 0 9 . t h e T e x a s State Highway D e p a r t m e n t used nearly 1/2 million t o n s of lime for stabilization in 1 9 6 9 . T h e stabilization a p p a r e n t l y occurs as t h e result of t w o processes. M o d e r n engineering rejected t h e use of lime—in preference t o cement—because t h e c e m e n t a t i o n reaction of lime requires m a n y m o n t h s and t h e gain in s t r e n g t h is m u c h smaller t h a n with c e m e n t . Since s t r e n g t h is n o t a r e q u i r e m e n t .SOIL STABILIZATION 171 slab-bearing p a r t i t i o n walls so there is n o c h a n c e of slab m o v e m e n t disturbing t h e structure. Lime was applied in slurry consisting of one part lime t o t w o p a r t s w a t e r b y weight. within t h e stabilized layer. S D . WEATHERED Data) Oota ) S 40 PERCENT LIME ADMIXTURE BY WEIGHT PERCENT LIME ADMIXTURE BY WEIGHT HOUSTON BLACK CLAY C. CLAY FT. Effect of lime on plastic characteristics of montmorillonitic clays.BR. ILLINOIS PIERRE (Fro m Rood s 8 Streets ) (Fro m Thompson ) 2 3 4 5 6 PERCENT LIME ADMIXTURE BY WEIGHT PERCENT LIME ADMIXTURE • BY WEIGHT Figure 109. a d r y lime c o n t e n t of 6 . (Fro m SD. McDowell ) 'SHALE' MONTGOMERY COUNTY MR.172 FOUNDATIONS ON EXPANSIVE SOILS PORTERVILLE (US BR.S. T h e stiff clay subgrade was b r o k e n d o w n with a disc h a r r o w t o m a x i m u m sized clods of 4 t o 6 inches. (U. (After Holtz). F o r stabilization. 6 t o 7 p e r c e n t of lime was required. T h e thickness of t h e t r e a t m e n t ranged from 9 inches for t a x i w a y s and r u n w a y s t o 18 inches for a p r o n s . T h e slurry was applied t o t h e subgrade at 4 0 t o 6 0 p o u n d s pressure using w a t e r t r u c k s . T h e application rate was sufficient t o p r o d u c e . THOMPSON. CLAY. and T u c s o n is n o t k n o w n . t h e n d e e p p l o w e d b y ripper-type e q u i p m e n t for an additional 2 feet. and 1 p e r c e n t received t h r e e . Davidson [ 7 6 ] stated in 1965 t h a t t h e results of l a b o r a t o r y studies s h o w t h a t lime d o e s diffuse i n t o a soil-water s y s t e m . A d d i t i o n a l i n f o r m a t i o n o n the swelling p o t e n t i a l or swelling pressure of t h e soils u n d e r t r e a t m e n t in J a c k s o n . b u t it is very possible t h a t t h e a m o u n t of swell in these areas is mild. Calexico. a stable slab can b e e x p e c t e d .SOIL STABILIZATION 173 p e r c e n t . T h e subgrade was overexcavated 2 feet. the rate of diffusion was very slow and given b y t h e e q u a t i o n : L = 0. Arizona [ 7 7 1 . a d e e p plowing t e c h n i q u e was used. T h e successful use of mixing lime in expansive soils for highway and airport c o n s t r u c t i o n is encouraging. Lime slurry will disperse from t h e injection p o i n t t h r o u g h r o o t holes. and t h e d e e p plowing o p e r a t i o n was c o n t i n u e d until a good m i x was o b t a i n e d . L. California. In i n t e r s t a t e highway c o n s t r u c t i o n in Florida. in. Mississippi. K. O k l a h o m a . t = time. After c o m p a c t i o n . In J a c k s o n . Mixing lime in f o u n d a t i o n soils t o r e d u c e swelling has n o t b e e n serious considered in t h e past. fissures in clay. T h e m e t h o d consists of pressure injecting lime-water slurry i n t o t h e soil t h r o u g h closely spaced drill holes as s h o w n on figure 110. and in T u c s o n . friable m i x t u r e . By overexcavating t h e site b o t h in d e p t h (3 t o 4 feet) and area and replacing t h e soil in c o m p a c t e d layers having a d e q u a t e lime t r e a t m e n t . and on 3-foot centers. T h e e x t e n t of such migration is p r o b a b l y limited. and desiccation cracks. and o t h e r states lime stabilization was used t o a large e x t e n t . This is especially t r u e in t h e case of large w a r e h o u s e s or school buildings w h e r e t h e floor covers a large area and a structural floor slab is n o t feasible d u e t o t h e high cost. T h e a m o u n t of lime used was a b o u t 3 p e r c e n t b y weight. E x p e r i e n c e w i t h this project indicated t h a t t h e lime t r e a t m e n t n o t o n l y t r a n s f o r m e d t h e soil t o a nonswelling. t h e 2 feet of soil which had been removed was replaced in 6-inch-thick layers. Q u e s t i o n s arise concerning t h e lime pressure-injection m e t h o d and t h e e x t e n t of lime migration i n t o t h e swelling soils. With t h e present d a y limited k n o w l e d g e of lime stabilization. Pressure injection T h e pressure injection m e t h o d of lime stabilization has b e e n used in J a c k s o n . t r e a t m e n t of underslab soils w i t h lime deserves m o r e a t t e n t i o n . In O k l a h o m a [ 7 5 1 . located adjacent t o t h e building. F o r t h e e x p e r i m e n t a l c o n d i t i o n s . w h e r e t h e soils b e n e a t h 2 0 0 houses were t r e a t e d . days 10 p e r c e n t of t h e t r e a t e d soils had t o b e r e t r e a t e d . it was r e p o r t e d t h a t an estimated treatments. in Calexico. T h e drilled holes were 5 feet d e e p . Expansive clays are generally stiff and practically impervious. Mississippi. Lime was t h e n a d d e d . It appears t h a t . footing f o u n d a t i o n s should n o t be placed o n treated expansive soils. b u t also i m p r o v e d the s t r u c t u r a l capacity of t h e t r e a t e d layer. and c o m p a c t e d . m i x e d w i t h lime. w i t h t h e k n o w l e d g e gained from airport and h i g h w a y c o n s t r u c t i o n using lime.081 t* Where: L = lime p e n e t r a t i o n d i s t a n c e . a l t h o u g h t h e d e p t h of t r e a t m e n t required and t h e results of t h e t r e a t m e n t on a long term basis has n o t b e e n evaluated. Lime stabilization . . Miss. THE PIPES ARE JETTED IN THE GROUND.pressure injection method. (Calexico. c o n c l u d e d t h a t the success of lime t r e a t m e n t is p r o b a b l y because of m o i s t u r e barrier effects r a t h e r t h a n because of any widespread changing of soil p r o p e r t i e s . OF IN SLURRY TO GALLON S 3 / 4 " DIAMETER INJECTION HOLES 3 FOOT CENTERS (TYPICAL) AT Figure 110. LIME AND WATER ARE MIXED IN A BLENDING TANK PRIOR INJECTION. & Jackson. T h e r e is t h e p o t e n t i a l danger of triggering an excessive a m o u n t of swelling in t h e d e e p seated soils.5 inches in 1 year. It is t h e conclusion from b o t h l a b o r a t o r y and field e x p e r i e n c e t h a t lime m i g r a t i o n i n t o expansive soils is e x t r e m e l y slow.). POINTED AT BOTTOM AND PERFORATED IN LOWER FOOT WITH 1/8" HOLES-. REPORTED INJECTION PRESSURES AT NOZZLE ARE IN THE RANGE OF 2 0 0 TO 4 0 0 PSI TYPICAL SLURRY PROPORTIONS • 5 0 SACKS HYDRATED LIME ( 5 0 LBS / S A C K ) TO 9 0 0 OF WATER. TYPICAL PLAN INJECT SLURRY USING TWO PIPE SYSTEM OUTER PIPE 3 / 4 INCH DIAMETER. Co ( 0 H ) 2 CONTENT LIME AVERAGES 9 5 % . Woodward-Clyde-Sherard & Associates [ 7 7 1 . Using this formula results in a p e n e t r a t i o n distance of 1. in their investigation. DIAMETER. Calif. INNER PIPE IS 1/4" IN. CONTINUE TO INJECT SLURRY IN EACH HOLE UNTIL SLURRY COMES OUT OF GROUND AROUND ΓΗΕ PIPE. T h e rate of m i g r a t i o n can p r o b a b l y be increased b y i n t r o d u c i n g large q u a n t i t i e s of w a t e r t o carry t h e lime slurry.174 FOUNDATIONS ON EXPANSIVE SOILS • t z - DRILLED HOLES OR PRESSURE INJECTION POINTS DRIL L OR TRENCH THROUGH CONCRETE . SOIL STABILIZATION 175 CHEMICAL STABILIZATION Besides the use of lime. calcium The hydration a l u m i n a t e h y d r a t e s . b u t t h e c e m e n t r e d u c e d t h e shrinkage of air-dried specimens a b o u t 25 t o 50 p e r c e n t m o r e t h a n did the lime. p o r t l a n d c e m e n t releases a large a m o u n t of lime. B o t h c e m e n t and lime have b e e n used in h i g h w a y c o n s t r u c t i o n for m o d i f y i n g t h e swelling p r o p e r t y of t h e subgrade soil. Fly ash is s o m e t i m e s a d d e d t o t h e soil-lime m i x t u r e t o increase p o z z o l a n i c reaction. O t h e r inorganic chemicals such as s o d i u m silicate. and h y d r a t e d lime. t h e use of lime c a n n o t provide a semi-rigid e l e m e n t b e n e a t h t h e slab. the cost of c e m e n t is considerably m o r e t h a n t h a t of lime. o t h e r stabilize expansive soils. Most of these chemicals are effective u n d e r l a b o r a t o r y c o n d i t i o n s . If t h e deep-seated soil e x p a n d s . calcium chloride. During h y d r a t i o n . t h u s reducing damage caused b y differential heaving. Cement stabilization p r o d u c t s of p o r t l a n d c e m e n t include calcium silicate h y d r a t e s . T h e resulting p r o d u c t c o m m o n l y k n o w n as soil-cement is familiar t o m o s t soil engineers. T h e effect of c e m e n t and of lime was a b o u t the same in reducing soil e x p a n s i o n . J o n e s [ 7 2 ] a d d e d 2 t o 6 p e r c e n t of p o r t l a n d c e m e n t t o t h e expansive Porterville clay of California w h i c h resulted in the p r o n o u n c e d r e d u c t i o n of v o l u m e change characteristics. can be used t o with and fly ash have b o t h b e e n used in the l a b o r a t o r y successful results. and p h o s p h o r i c acid have b e e n used t o stabilize expansive soil. calcium h y d r o x i d e . Of course. b o t h organic and inorganic. C e m e n t chemicals. T h e mixing and dispersing m e t h o d s for c e m e n t are nearly identical t o t h o s e for lime. Spangler and Patel [80] reported o n the l a b o r a t o r y t r e a t m e n t of an expansive Iowa g u m b o t i l w i t h p o r t l a n d c e m e n t . level floor is essential and t h e use of a s t r u c t u r a l floor slab is e c o n o m i c a l l y prohibitive. s o d i u m chloride. b u t their application in t h e field is very difficult. T h e a c t i o n of c e m e n t o n clay minerals is t o r e d u c e t h e liquid limit. T h e a d d i t i o n of 2 p e r c e n t and 4 p e r c e n t of p o r t l a n d c e m e n t considerably reduced t h e p o t e n t i a l v o l u m e change of t h e soil. plasticity i n d e x . and t o increase t h e shrinkage limit and shear strength [ 7 9 ] . t h e swelling effect t e n d s t o d i s t r i b u t e u n i f o r m l y . t h e i n c o r p o r a t i o n of p o r t l a n d c e m e n t in clay increases t h e s t r e n g t h of t h e m i x t u r e . T h e use of c e m e n t and lime t o stabilize underslab soil in buildings is seldom r e p o r t e d . T h e difficulties of u n i f o r m l y i n t r o d u c i n g p o r t l a n d c e m e n t i n t o very fine-grained soils are generally greater t h a n w i t h lime because it is less soluble. In a d d i t i o n t o t h e above actions. S u c h c o n s t r u c t i o n is particularly favorable for the t r e a t m e n t of a large w a r e h o u s e floor w h e r e a crack-free. . D u e t o lack of strength. T h e r e is n o s u p p o r t i n g evidence t h a t any of t h e chemicals has e c o n o m i c a l l y w o r t h w h i l e benefits [ 7 8 ] . and p o t e n t i a l v o l u m e change. With 2 t o 6 p e r c e n t c e m e n t i n c o r p o r a t e d in t h e clay. It is believed t h a t t h e base e x c h a n g e and c e m e n t i n g action of p o r t l a n d c e m e n t w i t h clay is similar t o t h a t of lime. T h e r e appears t o b e a great p o t e n t i a l for using c e m e n t t o modify the underslab soils. t h e resulting soil-cement m i x t u r e acts as a semi-rigid slab. and 7 0 7 was i n t r o d u c e d b y Soil T e c h n o l o g y C o r p o r a t i o n in Denver. Specially designed e q u i p m e n t as s h o w n o n figures 114 and 115 was used. E x p a n s i o n tests were performed on t h r e e r e m o l d e d specimens of Denver clay shale. T h e ability of t h e fluid t o p e r m e a t e in t h e impervious soil is encouraging. t h e holes could be advanced a t o t a l of 10 feet in less t h a n a half m i n u t e . and swelling of plastic soil samples. H o u s t o n . T h e m a c h i n e could hydraulically b o r e 1-1/2-inch-diameter holes. shrinkage. . coefficient of permeability of t h e p r o p r i e t a r y Fluid 7 0 7 t r e a t e d soil was d e t e r m i n e d t o be a p p r o x i m a t e l y 6 feet p e r year. T h e fluid was m i x e d w i t h swelling clays and tested in t h e l a b o r a t o r y for physical characteristics. One specimen was treated w i t h distilled w a t e r . T h e specimen t r e a t e d w i t h distilled w a t e r was highly expansive. 7 0 6 . Texas. the second w i t h p r o p r i e t a r y fluid 7 0 5 . O t t a w a . C o l o r a d o . C o l o r a d o . T h e specimen t r e a t e d w i t h fluid 7 0 5 did n o t e x p a n d . Organic c o m p o u n d s such as Arguard 2 H T or 4-Terf-Butylpyrocatechol have b e e n used w i t h a limited degree of success. Figures 1 1 1 . and 113 give t h e test results and t h e change of A t t e r b e r g limits from high plasticity t o nonplastic. either b y m i x i n g or b y slurry injection. supplied b y the O t t a w a Silica C o m p a n y . and p e r m e a b i l i t y . m u s t b e perfected before t h e scheme can b e considered in practice. T h e auger was advanced b y a pressure of 3 0 0 t o 5 0 0 psi. or b y hardening t h e soil w i t h resins. T h e y found t h a t w a t e r solutions of chemical a d m i x t u r e s of this t y p e decreased plasticity. T h e sand used was Silica Sand N a t u r a l Grain. swelling p o t e n t i a l . T h e p e r m e a b i l i t y tests were p e r f o r m e d o n specimens comprised of a m i x t u r e of 15 p e r c e n t clay and 85 p e r c e n t silica sand b y weight. and t h e third w i t h p r o p r i e t a r y fluid 7 0 6 . T h e specimens were saturated w i t h distilled w a t e r and t h e a m o u n t of e x p a n s i o n d e t e r m i n e d . Illinois. T h e specimen t r e a t e d w i t h fluid 7 0 6 was m o d e r a t e l y expansive. A surcharge load of 100 psf was applied t o each specimen.176 FOUNDATIONS ON EXPANSIVE SOILS A great deal of research and field s t u d y will b e required before c e m e n t stabilization can be economically applied. A n effective application m e t h o d . T h e material was found t o be impervious t o distilled w a t e r during t h e 34-day testing period. T h e first large-scale e x p e r i m e n t o n t h e use of t h e p r o p r i e t a r y fluid t o o k place in D e c e m b e r 1974 in Denver. T h u s . 112. Organic compound compounds stabilize expansive soils by waterproofing. T h e clay used was Aquagel supplied b y t h e Baroid Division of t h e N a t i o n a l Lead C o m p a n y . by retarding water Organic a d s o r p t i o n . Permeability test is i m p o r t a n t . A p r o p r i e t a r y liquid k n o w n as Fluid 7 0 5 . as in actual application the fluid m u s t be able t o migrate i n t o t h e soil. t h r e e at a t i m e . C o n s t a n t head p e r m e a b i l i t y tests were p e r f o r m e d o n t w o remolded specimens of clay and silica sand using distilled w a t e r in o n e test and t h e p r o p r i e t a r y Fluid 7 0 7 in t h e o t h e r . have s h o w n t h a t certain organic c o m p o u n d s which furnish large organic cations w h e n dissolved in w a t e r have considerable p r o m i s e as a d m i x t u r e s t o increase t h e stability of such soils. P r o p r i e t a r y fluid was i n t r o d u c e d i n t o t h e holes u n d e r a pressure of a b o u t 10 psi. The. in l a b o r a t o r y investigation of highly plastic Iowa subgrade soils. Davidson and Glab [ 8 1 ] . t o a d e p t h of m o r e t h a n 10 feet in stiff clay and claystone shale. T h e holes were spaced 3 6 inches apart and in highly impervious soil t h e spacing was r e d u c e d t o 18 inches. U n d i s t u r b e d soil samples were t a k e n before and after t r e a t m e n t t o d e t e r m i n e t h e effectiveness of t h e application.3 pcf Moisture Content = 1 4 . Swell-consolidation test results on remolded sample of Denver clay shale treated with distilled water only. 3 . T h e holes should have a m a x i m u m spacing of 12 inches. . T h e results were n o t as e x p e c t e d . 2. T h e auger should t h e n be advanced t o avoid t h e fissures. 8 % Expansion at 100 psf when wetted ο ο ι ο ο APPLIED PRESSURE (psf) ι ο ο ο Figure 111. B o t h t h e plasticity index and t h e swell p o t e n t i a l did n o t significantly r e d u c e . T h e fluid m u s t b e applied u n d e r a pressure of n o t less t h a n 2 5 0 psi. It is believed t h a t w i t h further s t u d y o n field application and m e c h a n i c a l i m p r o v e m e n t . t h e above m e t h o d will eventually find an i m p o r t a n t place in t h e realm of chemical stabilization. T h e t r e a t m e n t was i n t e n d e d t o e x t e n d for a d e p t h of at least 6 feet.SOIL STABILIZATION 177 PLACEMENT CONDITIONS' Dry Density = 76. Valuable experience was gained from t h e e x p e r i m e n t . 0 % Atterberg LimitsLiquid Limit = 6 8 % Plasticity Index = 17 % ^ 3 0 . It was i n t e n d e d t o reduce t h e plasticity i n d e x of the expansive clays from a b o u t 4 0 t o 10 p e r c e n t and t h e swelling p o t e n t i a l from m o d e r a t e swelling t o nonswelling. some of which follows: 1. Pressure gages should be provided t o indicate a pressure d r o p w h e n t h e fluid flows i n t o t h e seams and fissures in t h e clay. however. 4 pcf Moisture Content = 1 1 . Swell-consolidation test results on remolded sample of Denver clay shale treated with Fluid 705. 5 % Atterberg Limits = Non-plastic No Expansion when wetted 8 100 APPLIED PRESSURE (psf) 1000 Figure 112.178 FOUNDATIONS ON EXPANSIVE SOILS PLACEMENT CONDITIONS' Dry Density = 8 1 . . 6 pcf Moisture Content = 1 2 .7% Expansion at 1 0 0 p s f when wetted 2 Ο Ο ο ο ο < 100 APPLIED PRESSURE (psf) 1000 CO Figure 113. . 2 % Atterberg Limits = Non-plastic 2 Ο CO 2 X 3. Swell-consolidation test results on remolded sample of Denver clay shale treated with Fluid 706.SOIL STABILIZATION 179 PLACEMENT CONDITIONS' Dry Density = 7 8 . Equipment used for chemical injection.180 FOUNDATIONS ON EXPANSIVE SOILS Figure 114. Injection heads bored hydraulically. Figure 115. . "Recommended Practices for construction of Residential Concrete Floors on Expansive Soil" Portland Cement Association Vol. J. L. Texas A & M Press." Interim Report 1967. C. and Glab. "Controlling the Expansion of Desiccated Clays During Construction. Zurich. "Expansive Clay-Properties and Problems. J. Highway Research Board. 17. I." Moisture Equilibria and Moisture Changes in the Soils Beneath Covered Areas." Proceedings. August. No." International Research and Engineering Conference on Expansive Soils.. "Lime Stabilization: Deep Flow Style. Vol." Concluding Proceedings. 1949. June. T. D. L. H. 1970 Bara. J. F. FHA Contract H-799.. "Lime Shaft and Lime Tilled Stabilization of Subgrades in Colorado Highways... F. G. E. Gizienski.SOIL STABILIZATION 181 REFERENCES McDowell. and Hardy. Third International Conference on Soil Mechanics and Foundation Engineering. "Modern Practices Used in the Design of Foundations for Structures on Expansive Soils. "Comparison of Laboratory Swell Tests to Small Field Tests." Road and Streets. J." Second International Research Conference on Expansive Clay Soils. G. B. Holtz. "Influence of Vegetation on Soil Moisture Content and Resulting Soil Volume Changes. "Volume Change in Expansive Clay Soils and Control by Lime Treatment... J. Dawson.." Quarterly. "Review of Expansive Soils... Texas A and M Press. Los Angeles. October. Demirel.. "Acceleration of Heave of Structures on Expansive Clay." Woodward-Clyde-Sherard & Associates. "Soil Pulverization and Lime Migration in Soil-Lime Stabilization. F. J. and Patel. 4. G. 29. Blight. 4. Planning and Research Division. L. M. G. "Remedial Methods Applied to Houses Damaged by High Volume-Change Soils." Proceedings. Highway Research Board.. California. June 1974. London. Vol. International Research and Engineering Conference on Expansive Clay Soils. Holtz. 29. R. 1958. 54.." Proceedings of Workshop on Expansive Clays and Shale in Highway Design and Construction. K. 1967. L. Gromko. S. T.. and Lee. 1959. of Highways. Colorado School of Mines. Dept. 54. Gizienski. March 1969. E.. Spangler. England. "Lime Stabilization of Expansive Clays at the Dallas-Fort Worth Airport. 1965. 1969." Quarterly of Colorado School of Mines. 1969. "Stabilization of Expansive Clay with Hydrated Lime and with Portland Cement. 1953..." Texas Highway Department. Vol. Vol. Jones. W. E. Davidson. E. J. "Modification of a Gumbotil Soil by Lime and Portland Cement Admixtures. C . S." Highway Research Board. W. G. Davidson. II. "The Influence of Soil Mineralogical Composition on Cement Stabilization. Kelly. Ο... J.. and Lee. H. "Remedial Procedures Used in the Reduction of Detrimental Effect of Swelling Soils. 193. 1959. M. "An Organic Compound as a Stabilization Agent for Two Soil Aggregate Mixtures. . Felt. and Gibbs." Bulletin. Vol. Α. R. State of Colorado. R. and Wet. Croft. Vol." Journal of the Geotechnical Engineering Division." Proceedings Highway Research Board. J. No. 1965. Thompson. P. "Comparison of Laboratory Swell Tests to Small Scale Field Tests. W. 1949. J. No." Geotechnique." Proceedings of the Second International Research and Engineering Conference on Expansive Soils. Chapter 9 INVESTIGATION OF FOUNDATION MOVEMENT INTRODUCTION Investigating t h e cause of f o u n d a t i o n m o v e m e n t of an existing building and prescribing remedial measures requires careful field investigation. a p a r t m e n t buildings. 3 . Most of t h e cracked buildings were t h e result of f o u n d a t i o n m o v e m e n t caused b y swelling soils. Foundation information Effort should b e m a d e t o o b t a i n t h e existing i n f o r m a t i o n f o u n d a t i o n relative t o t h e soil. U n f o r t u n a t e l y . school buildings. w a r e h o u s e s . Design criteria. and p a v e m e n t s . Soil tests o n individual sites have b e o m e a r e q u i r e m e n t after 1 9 6 0 . and m a n y years of experience. n o e x a m i n a t i o n and testing can replace e x p e r i e n c e . HISTORY STUDY T h e first step in t h e investigation of a building is t o o b t a i n c o m p l e t e i n f o r m a t i o n pertaining t o t h e building.200 cases of cracked buildings in t h e States of C o l o r a d o and W y o m i n g . F r o m t h e soil test d a t a . T y p e of f o u n d a t i o n . F o r buildings erected before 1 9 6 0 . this is similar t o t h e t r e a t m e n t of a p a t i e n t . such i n f o r m a t i o n is o f t e n t i m e s absent and it is necessary t o u n c o v e r m u c h of t h e required i n f o r m a t i o n b y soil e x p l o r a t i o n . offices. Prescription and t r e a t m e n t will b e relatively simple once t h e cause of illness has b e e n d e t e r m i n e d . and 6. 2. . Moisture c o n t e n t of f o u n d a t i o n soils. T y p e of f o u n d a t i o n soils. t h e a u t h o r has had t h e o p p o r t u n i t y t o s t u d y m o r e t h a n 1. I n q u i r y of t h e p a t i e n t ' s m e d i c a l record. Swelling p o t e n t i a l of f o u n d a t i o n soils. religious s t r u c t u r e s . In some respects. In t h e past 2 0 years. Water table c o n d i t i o n . 4 . and e x p e r i e n c e can o n l y be o b t a i n e d b y trial and error. As in t h e case of a d o c t o r . exhaustive l a b o r a t o r y testing. 5. These cases include residences. such i n f o r m a t i o n is generally s k e t c h y . swimming pools. and a l a b o r a t o r y diagnosis will b e necessary t o diagnose t h e cause of t h e sickness. a physical e x a m i n a t i o n . it will b e possible to d e t e r m i n e t h e following: 1. This will also provide i n f o r m a t i o n o n t h e d e p t h of p e n e t r a t i o n into b e d r o c k . either because t h e drawing is lost. 4 . 3 . t h e subsoil i n f o r m a t i o n has only limited use. underslab gravel. When examining t h e e x t e r i o r of t h e building. Pier r e i n f o r c e m e n t . A c o m p l e t e record on seepage w a t e r should be o b t a i n e d . w h e t h e r the design is m a d e b y a registered professional engineer or b y t h e c o n t r a c t o r . 2. items such as w h e n t h e building was c o m p l e t e d . T h e length of t h e piers. and w h e n the first crack appeared. Movement data Effort should be m a d e t o o b t a i n chronological d a t a o n t h e building m o v e m e n t . t h e subsoil investigation is n o t c o n d u c t e d for a specific building b u t for a general area. e x p a n s i o n j o i n t . and t h e c o n d i t i o n of t h e backfill. It is also necessary t o e x a m i n e t h e qualifications of t h e designer. t h e setting of t h e a u t o m a t i c sprinkling s y s t e m . p r i m a r y i n f o r m a t i o n can b e o b t a i n e d in t h e b a s e m e n t area. Water m a r k s or effloresce o n t h e wall usually tell t h e s t o r y of seepage water. T h e above soil test d a t a can be invaluable t o w a r d finding t h e cause of s t r u c t u r e m o v e m e n t . T h e foundation plan will reveal if t h e r e c o m m e n d a t i o n s given in t h e soil r e p o r t have been followed. All i n f o r m a t i o n o b t a i n e d from t h e o w n e r should be carefully scrutinized for its validity. a c o m p l e t e i n f o r m a t i o n of the pier system will be a p p a r e n t . and o t h e r details. it will be necessary t o break t h e c o n c r e t e slab t o reach t h e f o u n d a t i o n . t h e first a p p e a r a n c e of w a t e r in t h e b a s e m e n t . w h e n t h e first o c c u p a n t m o v e d in. Most owners d e n y excessive irrigation of t h e lawn and flower b e d s . b u t m a n y times it is advisable t o e x a m i n e t h e pier t o ascertain a p r o b l e m such as uplift. Care should be exercised t o locate t h e building u n d e r investigation t o t h e nearest test h o l e . Subdrainage system. If t h e above i n f o r m a t i o n is available. In t h e case of a n o n b a s e m e n t building. In t h e case of b a s e m e n t c o n s t r u c t i o n . T h e dead load pressure exerted on t h e footings o r piers. This is similar t o t h e case w h e r e t h e c o m p l e t e m e d i c a l record of a p a t i e n t is at t h e disposal of t h e examining d o c t o r . for b o t h i n t e r i o r and e x t e r i o r piers. In t h e interior of t h e building. T h e size of footings or piers. The second step is t o check the f o u n d a t i o n plan. such i n f o r m a t i o n m a y n o t be available. If t h e above i n f o r m a t i o n is n o t available. d o w e l bars. t h e investigation will b e greatly simplified. excavation can be easily m a d e outside of t h e building adjacent t o t h e grade b e a m .184 FOUNDATIONS ON EXPANSIVE SOILS S o m e t i m e s . Logs k e p t b y t h e driller are s o m e t i m e s available. It w o u l d b e a difficult j o b t o expose t h e entire length of t h e pier. it will b e necessary t o e x p o s e t h e f o u n d a t i o n system b y excavation. w i t h careful i n t e r r o g a t i o n and k e e n observation. or t h e c o n c e r n e d p a r t y d o e s n o t w a n t t o p r o d u c e it. However. In such cases. h e t e n d s t o exaggerate his findings. If t h e o w n e r i n t e n d s t o sue t h e builder t o recover his damages. Again. In such case. it is helpful to d e t e r m i n e the lawn watering practice. the l o c a t i o n . and 5 . as well as t h e w a t e r table c o n d i t i o n . the actual s t o r y can be revealed. so t h a t it is possible t o d e t e r m i n e as closely as possible t h e subsoil c o n d i t i o n b e n e a t h t h e building. These are: 1. Also i m p o r t a n t is t h e p e r f o r m a n c e of utility lines. it is d a n g e r o u s t o allow t h e s u r r o u n d i n g soils t o b e c o m e w e t t e d before t h e application of dead load pressure. and t h e e x t e r i o r walls are in excellent c o n d i t i o n . and w h e t h e r seepage has t a k e n place after heavy precipitation. F o r drilled pier f o u n d a t i o n s . t h e b a s e m e n t s h o w e r drain was n o t c o n n e c t e d t o t h e sewer line. t h e p r o b l e m caused b y expansive soil is well k n o w n . However. w i n d o w s sticking. The . T h e building is f o u n d e d w i t h drilled piers. n o t o n l y t o t h e soil engineer. A t t h e first sign of cracking. it can u n l o c k m a n y m o v e m e n t puzzles. a l t h o u g h swelling cracks are generally w i d e at t h e t o p and n a r r o w at t h e b o t t o m . In t h e i n f o r m a t i o n gathering process. T h e r e are instances w h e r e t h e soils were flooded during c o n s t r u c t i o n . and heaving m o v e m e n t t o o k place even before t h e building was c o m p l e t e d . and only substantive evidence considered. and cracks appearing in t h e e x t e r i o r and interior walls and even in t h e ceiling. t h e b e n t o n i t e in t h e soil has pulled t h e building apart and o t h e r s should be dismissed as hearsay b y an experienced engineer. t h e a m o u n t of observed w a t e r . Crack pattern F o u n d a t i o n m o v e m e n t s are reflected as cracks. Cracks caused b y swelling soils have t h e same general p a t t e r n as s e t t l e m e n t cracks. In t h e R o c k y M o u n t a i n area. It is n o t always possible t o d e t e r m i n e t h e site c o n d i t i o n s during c o n s t r u c t i o n . T h e defect was n o t discovered until an o d o r was d e t e c t e d in t h e b a s e m e n t . b u t o f t e n t i m e s even t o t h e l a y m a n . all hearsay should b e screened.INVESTIGATION OF FOUNDATION MOVEMENT 185 of seepage. b u t e m p t i e d i n t o t h e underslab soils. t h e i m m e d i a t e r e a c t i o n is t h a t t h e p r o b l e m is caused b y swelling soils. for s t r u c t u r e s f o u n d e d o n expansive soils. Stories such as an u n d e r g r o u n d river r u n n i n g u n d e r t h e s t r u c t u r e . DISTRESS STUDY T h e first sign of f o u n d a t i o n m o v e m e n t . This is generally followed b y d o o r s b i n d i n g . Pier uplift can begin before the placing of t h e f o u n d a t i o n c o n c r e t e . investigation revealed t h a t t h e interior h o u s e sewer was never c o n n e c t e d to t h e street sewer. In a n o t h e r case. t h e building is sliding d o w n h i l l . F i g u r e 117 indicates a severe crack t h a t developed in a twin-tee s t r u c t u r a l slab. These cracks had n o t h i n g t o do with foundation movement. Has t h e r e b e e n p l u m b i n g difficulty experienced in t h e past years? Has t h e floor drain b e e n plugged? In o n e instance. in t h e m o s t severe s e t t l e m e n t cases. Investigation of partially c o m p l e t e d h o u s e revealed t h a t t h e b a s e m e n t was covered w i t h m o r e t h a n 2 feet of s n o w w h i c h t h e c o n t r a c t o r had failed t o remove before enclosing t h e s t r u c t u r e . T h e same crack p a t t e r n s can develop from s e t t l e m e n t . b u t if such i n f o r m a t i o n is secured b y an observant o w n e r . F o r years. is t h e cracking of t h e floor slab. t h e error remained u n d e t e c t e d until the crawl space was e n t e r e d and an excessive w e t t i n g c o n d i t i o n discovered. diagonal cracks are usually associated w i t h a series of h o r i z o n t a l cracks as s h o w n o n figure 116. It is n o t always t r u e t h a t f o u n d a t i o n m o v e m e n t of a specific p o r t i o n of a s t r u c t u r e is responsible for certain cracks appearing in t h e i m m e d i a t e vicinity of t h a t m o v e m e n t . crack pattern varies f r o m horizontal t o diagonal which is quite different f r o m heaving cracks. Typical settlement cracks. . Floor cracks due t o shrinkage of twin-tee topping. Figure 117.186 FOUNDATIONS ON EXPANSIVE SOILS Figure 116. almost all lateral separation is caused b y differential heaving. especially t h a t of a h o u s e . 3 . is c o m p l e x . 5. M o v e m e n t of one p o r t i o n of t h e building can cause cracks t o appear at t h e o p p o s i t e end of t h e building. Vertical cracks beneath the beam pocket caused by lifting of I-beam. . T h e following crack analysis can serve as a guide: 1. Such m o v e m e n t has a strong resemblance t o lateral m o v e m e n t . Vertical cracks below t h e I-beam in t h e b a s e m e n t c o n c r e t e wall can be caused b y the lifting of t h e I-beam. Hairline cracks appearing above interior d o o r s and closets could be caused b y plaster shrinkage or t i m b e r shrinkage are n o t necessarily f o u n d a t i o n m o v e m e n t . If such cracks a p p e a r only in t h e e x t e r i o r brick course b u t n o t on t h e interior d r y wall. It is always p r u d e n t to explain t h e cause of m o v e m e n t in a general sense and treat and s t u d y the m o v e m e n t as a unit. Diagonal cracks b e l o w e x t e r i o r w i n d o w s or above e x t e r i o r d o o r s generally indicate footing or drilled pier f o u n d a t i o n m o v e m e n t . Separation of t h e w i n d o w frame from t h e brick course as s h o w n o n figure 119 generally indicates differential heaving. the cracks can be caused b y e x t e r i o r p a t i o slab heaving. 2. 4 . resulting in tension cracks as s h o w n in figure 1 1 8 . Actually.INVESTIGATION OF FOUNDATION MOVEMENT 187 Figure 118. structural a r r a n g e m e n t of a building. b y lowering t h e screw j a c k and revealing the I-beam. When t h e o n e pipe c o l u m n f o u n d a t i o n heaves. d o o r s stick and closets c a n n o t b e o p e n e d . T h e situation can be c o r r e c t e d .188 FOUNDATIONS ON EXPANSIVE SOILS Figure 119. at least t e m p o r a r i l y . Pipe c o l u m n s are provided w i t h a screw jack at t h e t o p . T h e m o s t c o m m o n instance is t h e uplift of t h e I-beam caused b y t h e uplift of steel pipe c o l u m n s . t h e o t h e r pipe c o l u m n is usually rendered idle and can be shaken loose b y h a n d . Separation of window frame from brick course. t h e joist system in t h e u p p e r level is d i s t u r b e d . When t h e I-beam lifts. Stress build-up M o v e m e n t of interior structural m e m b e r s can result in stress build-up in t h e s t r u c t u r e . T h e I-beam in t h e b a s e m e n t is c o m m o n l y s u p p o r t e d b y t w o t o t h r e e steel pipe c o l u m n s . T h e d o o r s are t h e n able t o b e o p e n e d and closed freely again. T h e o w n e r generally planes t h e d o o r only t o find t h a t it fails t o open and close again after a period of time. . and this will t e m p o r a r i l y close an old crack. In m o s t cases. t h e soils i m m e d i a t e l y b e l o w the f o u n d a t i o n level generally have b e e n w e t t e d excessively. T h e physical characteristics of t h e soils o b t a i n e d from t h e adjacent and r e m o t e test holes can be compared. T h e m a g n i t u d e of increase ranged from 2 t o 8 p e r c e n t .200 cases investigated. drying and shrinkage of soils seldom or never cause cracking in a building. In fact." O w n e r s s o m e t i m e s r e p o r t t h a t t h e cracks in their buildings are subject t o o p e n i n g and closing and a t t e m p t t o correlate the m o v e m e n t t o seasonal climate change. Test holes should be drilled adjacent t o t h e building and sufficient samples should be t a k e n for t h e d e t e r m i n a t i o n of t h e swelling characteristics and m o i s t u r e c o n t e n t of t h e soil. samples o b t a i n e d from areas unaffected b y building c o n s t r u c t i o n should give i n f o r m a t i o n relative t o t h e soil behavior at t h e t i m e of c o n s t r u c t i o n . L a b o r a t o r y testing will invariably show a low swell p o t e n t i a l . in t h e course of nearly 1. Careful observation will indicate t h a t t h e t o t a l n u m b e r of cracks appearing in a building is c o n s t a n t l y increasing and seldom decreasing. If possible. T h e m o s i t u r e c o n t e n t as well as t h e d r y d e n s i t y of all soil samples should b e d e t e r m i n e d . Usually. T h e underslab m o i s t u r e c o n t e n t was high. A t least o n e test hole should be drilled r e m o t e from t h e s t r u c t u r e and in an area unaffected b y building c o n s t r u c t i o n . In a n y event. and t h e n subjecting it to w e t t i n g . t h e soil i m m e d i a t e l y b e n e a t h the furnace in t h e b a s e m e n t was e x a m i n e d . In o n e building. S o m e t i m e s . it is necessary t o d e t e r m i n e t h e subsoil c o n d i t i o n s and w a t e r table. N o shrinkage is likely to take place b e n e a t h t h e central p o r t i o n of a covered area. . However. A d m i t t e d l y . t h e m o i s t u r e c o n t e n t should be carefully c o m p a r e d w i t h t h e m o i s t u r e c o n t e n t of t h e soil prior t o building c o n s t r u c t i o n . t h e actual swelling characteristics of t h e soil can only be revealed by air drying t h e soil sample. the m o i s t u r e c o n t e n t b e n e a t h t h e building area had increased. t h e stress d i s t r i b u t i o n is altered. When a new crack appears. t h e first o r d e r of investigation of a cracked building is a survey t o d e t e r m i n e w h i c h p a r t of t h e building has m o v e d and t h e m a g n i t u d e of m o v e m e n t . This t h e n leads t o the t h e o r y t h a t t h e subsoil has u n d e r g o n e cycles of d r y i n g and w e t t i n g . resulting in e x p a n s i o n and shrinkage. is n o t necessarily t r u e . INVESTIGATION Subsoils T o definitely define t h e cause of f o u n d a t i o n m o v e m e n t and t o r e c o m m e n d remedial measures. It should b e n o t e d t h a t for a building having cracking. Survey T o m o s t s t r u c t u r a l engineers. T h e c o m m o n l y assumed t h e o r y t h a t t h e soils b e n e a t h a s t r u c t u r e are subject t o w e t t i n g and d r y i n g .INVESTIGATION OF FOUNDATION MOVEMENT 189 Stress build-up caused b y slab bearing p a r t i t i o n walls has b e e n discussed u n d e r "Slabs on Expansive Soils. t h e opening and closing of t h e cracks are caused b y t h e shifting of t h e l o c a t i o n of stress c o n c e n t r a t i o n . careful testing can reveal t h a t t h e material possesses a high swelling pressure. t y p e of r e i n f o r c e m e n t . T h e m o i s t u r e c o n t e n t of t h e soil (soil samples should b e t a k e n every 12 inches t o a d e p t h of at least 5 feet). it should never be used as a clue t o f o u n d a t i o n m o v e m e n t . C o n d i t i o n of t h e c o n c r e t e slab. t h e results will b e of d o u b t f u l value if a reliable b e n c h m a r k is n o t utilized. T h e pier should b e well reinforced. assuming t h a t these are level at t h e time of c o n s t r u c t i o n . Such u n d e r t a k i n g is costly and time c o n s u m i n g . By investigating t h e test pit. A m o v e m e n t survey is of little value if n o t c o m p a r e d w i t h a previous survey r e c o r d . Pits should be o p e n e d adjacent to the grade b e a m and n e x t t o t h e interior s u p p o r t s . O n l y in t h e course of an i m p o r t a n t s t r u c t u r e is such a survey w a r r a n t e d . C o n d i t i o n of t h e air space b e n e a t h t h e grade b e a m (drilled pier f o u n d a t i o n ) . t h e following can be revealed: 1. T h e survey m u s t be c o n d u c t e d w i t h c o n t r o l p o i n t s carefully selected. t h e investigation p r o c e d u r e can b e greatly simplified. A reliable b e n c h m a r k should consist of a c o n c r e t e pier drilled d e e p i n t o b e d r o c k in a zone w h e r e m o i s t u r e change will n o t take place. Also. 3 . If t h e building has a crawl space. By referring t o a reliable b e n c h m a r k and c o n d u c t i n g a survey at intervals of once a m o n t h . however. Wherever possible. of m o i s t u r e m u s t b e d e t e r m i n e d . T h e f o u n d a t i o n system. 5. A m o v e m e n t professionally. C o n d i t i o n of t h e t o p of pier. Therefore. t e l e p h o n e pole or m a n h o l e cover are subject t o m o v e m e n t in an expansive soil area. T h e c o m m o n practice of referring t h e m o v e m e n t of t h e building t o a brick course or to t h e t o p of t h e grade b e a m . CAUSE O F MOVEMENT When all t h e investigation and s t u d y outlined above have b e e n c o m p l e t e d .190 FOUNDATIONS ON EXPANSIVE SOILS a survey will assist in t h e d e t e r m i n a t i o n of t h e general trend of building m o v e m e n t . t h e source . and 6. can be totally misleading. n o m o v e m e n t will t a k e place in an expansive soil area unless t h e f o u n d a t i o n soil b e c o m e s w e t t e d excessively. t h e presence of m u s h r o o m s . T h e presence of underslab gravel. Bench m a r k s such as t h e t o p Of a fire h y d r a n t . 2. t h e m o v e m e n t of t h e building can t h u s b e m o n i t o r e d . t h e cause of m o v e m e n t of t h e building can t h e n be d e t e r m i n e d . 4 . Obviously. and readings w h i c h rely o n these b e n c h m a r k s can be entirely misleading. gradation of gravel and thickness of t h e gravel layer. at least o n e pier should be uncovered for its entire d e p t h and examined for possible tension cracks o r voids at t h e b o t t o m of t h e pier. survey of a cracked building is of little value unless c o n d u c t e d Test pits T h e only positive m e t h o d of d e t e r m i n i n g t h e subsoil c o n d i t i o n and c o n s t r u c t i o n details in t h e f o u n d a t i o n system is b y opening test pits. T h e usual s h o r t c o m i n g s of t h e given criteria are insufficient d e a d load pressure exerted on a pier or pad f o u n d a t i o n system. therefore." Despite t h e possible deficiency of a drilled pier or footing system. A c o m m o n design defect is t h e use of m o r e s u p p o r t s t h a n necessary b e n e a t h t h e I-beam. Details such as t h e separation of slabs from bearing walls. In o n e large a p a r t m e n t c o m p l e x . T h e r e are e x c e p t i o n s w h e r e t h e c o n t r a c t o r abuses every rule of good c o n s t r u c t i o n practice in t h e f o u n d a t i o n c o n s t r u c t i o n and e x p e c t s t o go u n d e t e c t e d because t h e covered f o u n d a t i o n excavation will n o t be q u e s t i o n e d after t h e building is c o m p l e t e d . S o m e c o n t r a c t o r s t a k e the m a t t e r entirely i n t o their o w n h a n d s and p r e w e t t h e f o u n d a t i o n . if t h e c o n t r a c t o r follows every detail specified b y his consultants—the structural and soil engineers—then his liability in t h e event of future building damage will b e greatly reduced. in the c o u r t of justice. U n f o r t u n a t e l y . n o differentiation can be m a d e b e t w e e n ignorance and i n t e n t i o n a l errors.INVESTIGATION OF FOUNDATION MOVEMENT 191 Foundation design F o u n d a t i o n designs m a d e before 1960 are based u p o n soil r e p o r t s w r i t t e n w i t h a limited k n o w l e d g e of swelling soil p r o b l e m s and their solutions. insufficient pier length. and lack of r e i n f o r c e m e n t . t h e use of a d o w e l b a r t o c o n n e c t p a t i o slabs w i t h grade b e a m s . F o r i n s t a n c e . drill oversized piers and e x p e c t a stable building. U n f o r t u n a t e l y . s o m e having missed t h e grade for . This p r o c e d u r e has caused a great deal of heaving p r o b l e m s as explained previously in c h a p t e r 6 u n d e r "Slabs o n Expansive Soils. it is n o t surprising t o find t h a t t h e criteria established in these early soil r e p o r t s are n o t sufficient t o cope w i t h t h e c o m p l e x i t y of t h e swelling p o t e n t i a l of t h e soil. in t h e c o u r t . these buildings m a y r e m a i n in good c o n d i t i o n for years and give an excuse to the builder to c o n t i n u e this undesirable practice. a p r o p e r l y engineered f o u n d a t i o n system suffers only m i n o r distress even u n d e r t h e m o s t adverse c o n d i t i o n s . C o n s e q u e n t l y . o r f o r t u n a t e l y . A remedial m e a s u r e w o u l d b e t o use a heavier b e a m section w i t h fewer s u p p o r t s . Obviously. Damage caused b y swelling soils is n o t recognized b y t h e c o u r t as an act of G o d . very low. Most c o n t r a c t o r s a t t e m p t t o d o a good j o b in their building. this appears as a glaring m i s t a k e on t h e p a r t of t h e c o n t r a c t o r for n o t following the design. In t h e c o u r t . T r a d i t i o n a l design is t o d o w e l t h e e x t e r i o r p a t i o slabs i n t o t h e grade b e a m s w i t h d o w e l bars. however. Construction Legally. the absence of slab r e i n f o r c e m e n t should n o t be i m p o r t a n t w i t h respect t o slab cracking. t h e c o n t r a c t o r is t h e first target of t h e o w n e r in a lawsuit filed negligence. p u d d l e t h e backfill. every effort will be m a d e t o prove t h e c o n t r a c t o r ' s negligence. t h e piers were drilled off c e n t e r from t h e grade b e a m s . t h e absence of void spaces in t h e slab bearing p a r t i t i o n walls and o t h e r s are t h e results of i n c o m p l e t e specifications and t h e ignorance of t h e c o n t r a c t o r o n t h e t e c h n i q u e of building in expansive soil areas r a t h e r t h a n purposely o m i t t i n g t h e details. Most buildings t h a t suffer severe d a m a g e are designed and c o n s t r u c t e d b y c o n t r a c t o r s w i t h o u t t h e benefit of a soil or structural engineer. reinforce the footings instead of the f o u n d a t i o n walls. These piers are subject t o uplift. Dead load pressure exerted o n t h e interior piers is. When Figure 120. t h e r e f o r e . a s u b d r a i n system is generally n o t specified in t h e design. Breakage of utility lines. P o o r surface drainage causing surface w a t e r to e n t e r t h r o u g h backfill i n t o t h e f o u n d a t i o n soils. A perched w a t e r table c o n d i t i o n c a n n o t be foreseen at t h e t i m e of c o n s t r u c t i o n and is usually n o t m e n t i o n e d in t h e soil r e p o r t . Piers w e r e b o t t o m e d o n t h e u p p e r soils instead of t h e b e d r o c k . 3 . 2. Timber supports used to correct the missed piers. resulting in c h i m n e y tilting.192 FOUNDATIONS ON EXPANSIVE SOILS b e a m s c o m p l e t e l y . . T h e responsibility of d a m a g e caused b y perched w a t e r is difficult to define. Rise of g r o u n d w a t e r or d e v e l o p m e n t of p e r c h e d water. Some of t h e c o n s t r u c t i o n defects are s h o w n on figure 120. Drainage Water entering f o u n d a t i o n soils can b e from o n e or m o r e of the following sources: 1. However. As discussed in c h a p t e r 7. t h e p r o b l e m is solved. When a soil engineer provides t h e f o u n d a t i o n design criteria. In t h e event of a c o m p l a i n t . Remedial c o n s t r u c t i o n differs in each case. a subdrain system should b e provided. the b u i l d e r can t h e n have recourse back t o t h e owner. m a n y investigators c o n t e n d t h a t b y correcting t h e surface drainage. just as in the case of t h e prescription s u b m i t t e d t o t h e p a t i e n t . P r e c a u t i o n s given o n drainage are only an added factor of safety. When investigating a cracked building. h e should design for s a t u r a t e d soil c o n d i t i o n s . It is u n u s u a l w h e n t w o p a t i e n t s receive t h e same prescription. His design criteria are actually based o n t h e worst possible c o n d i t i o n s . and shrubs and flower b e d s planted adjacent t o the building are all p o t e n t i a l causes of w e t t i n g of f o u n d a t i o n soils. Of this n u m b e r m o r e t h e n 50 p e r c e n t involved residential houses w h e r e t h e investigation m e r e l y consisted of visual inspection. i n a d e q u a t e slope a r o u n d the building. t h e role of drainage in t h e cracked building has b e e n exaggerated. In the l a b o r a t o r y . In the last 15 years. for t h e private dwelling t h e r e c o m m e n d e d remedial c o n s t r u c t i o n is only partially c o m p l e t e d and t h e p r o b l e m c o n t i n u e s . this is far from t h e real cause of f o u n d a t i o n m o v e m e n t . T o provide an effective subdrain system against perched w a t e r c o n d i t i o n involves considerable cost. After t h e remedial c o n s t r u c t i o n has b e e n c o m p l e t e d . it is seldom possible t o r e t u r n t h e building to its original c o n d i t i o n . It m a y t a k e as long as a year before t h e s t r u c t u r e is finally stabilized and cosmetic w o r k can be started. h e saturates t h e soil sample to d e t e r m i n e t h e m a x i m u m swell p o t e n t i a l . i m p r o p e r location of t h e sprinkling system. REMEDIAL MEASURES It is relatively easy t o r e c o m m e n d the necessary remedial c o n s t r u c t i o n for a cracked building w h e n t h e cause of f o u n d a t i o n m o v e m e n t has b e e n d e t e r m i n e d . U n f o r t u n a t e l y .INVESTIGATION OF FOUNDATION MOVEMENT 193 t h e r e is a possibility of p e r c h e d w a t e r . he selects the soil w i t h the highest swell p o t e n t i a l for testing. In t h e m e a n t i m e m i n o r cracks will c o n t i n u e t o o p e n . A n alert developer issues each h o m e o w n e r a m a n u a l on t h e care of drainage a r o u n d t h e h o u s e . Generally. Damage caused b y perched w a t e r should be in t h e category of " A n act of G o d " . t h e m o s t obvious defect is t h a t of i m p r o p e r drainage a r o u n d t h e building. the art of coping w i t h present-day expansive soil p r o b l e m s is far from c o m p l e t e and t h e soil engineer can only h o p e that his r e c o m m e n d a t i o n s are carried o u t in full. loose backfill. Usually. In t h e field. C o n s e q u e n t l y . Almost 9 0 p e r c e n t of t h e w a t e r t h a t enters f o u n d a t i o n soils is derived from surface w a t e r . it takes a long t i m e for t h e s t r u c t u r e to adjust t o t h e improved state. . Moisture c o n t e n t in f o u n d a t i o n soils can increase substantially t o allow heaving even if t h e drainage a r o u n d t h e h o u s e is in excellent c o n d i t i o n . m o r e t h a n o n e t h o u s a n d distressed s t r u c t u r e s have b e e n investigated. If civil action is instigated resulting from f o u n d a t i o n p r o b l e m s . While it is t r u e t h a t p o o r drainage i n t r o d u c e s w a t e r i n t o t h e f o u n d a t i o n soils and causes heaving. t h u s minimizing any possible future d a m a g e . the strongest defense t h e c o n t r a c t o r possesses against t h e o w n e r ' s suit is t h a t t h e o w n e r has n o t provided p r o p e r drainage a r o u n d t h e building. Provide positive drainage a r o u n d the building. Provide a subdrain a r o u n d t h e building in the interior or e x t e r i o r at least 2 4 inches b e l o w t h e lower floor slab. and o t h e r lightly loaded s t r u c t u r e s . 3 . Provide a u t o m a t i c s u m p p u m p s in the b a s e m e n t . U n d e r p i n t h e pad w i t h piers drilled i n t o b e d r o c k . 2. Reinforce existing f o u n d a t i o n walls w i t h new reinforced grade b e a m to tie the s t r u c t u r e as in b o x c o n s t r u c t i o n . Post-tension t h e f o u n d a t i o n t o provide structural stability. 4 . . 2. i n s t i t u t i o n s . Again. Decrease pad size t o increase dead load pressure. F o r subdrain: 1. 3 . t h e o w n e r s are h a p p y w i t h t h e results o b t a i n e d from remedial c o n s t r u c t i o n and the case is closed. Saw cut t h e slab along t h e f o u n d a t i o n wall to allow free slab m o v e m e n t . t h o u g h varying greatly in degree. F o r c o n t i n u o u s footing f o u n d a t i o n : 1. 2. O t h e r p a t i e n t s visit t h e d o c t o r at an early d a t e . 2. Adjust screw j a c k o n t o p of pipe c o l u m n t o relevel interior I-beam. 4 . 4 . swimming p o o l s . 2. T h e remedial measures r e c o m m e n d e d for such s t r u c t u r e s are generally followed. 3 . referring to t h e case of p a t i e n t s and d o c t o r s .194 FOUNDATIONS ON EXPANSIVE SOILS O t h e r cases involve school buildings. 5. s o m e p a t i e n t s are in terminal c o n d i t i o n and t r e a t m e n t can only prolong t h e life span. Provide a positive o u t l e t for t h e existing subdrain system. Remove all shrubs and flower beds which are planted adjacent to the h o u s e . including d o o r frames and staircase walls. Loosen soils a r o u n d t h e pier to reduce uplift pressure. have the disease accurately diagnosed. In a b o u t 75 p e r c e n t of these cases. Provide voids b e n e a t h t h e footings at calculated intervals t o increase t h e dead load pressure. 6. R e m o v e and r e c o m p a c t backfill. F o r exterior: 1. and t h e p r o b l e m is solved. Relocate all lawn sprinkling h e a d s t o a distance of at least 10 feet from t h e building. Cut t h e t o p of pier and adjust t h e elevation of t h e piers b y shims. For basement: 1. Eliminate t h e m u s h r o o m at t h e t o p of piers. Provide slip j o i n t s to all interior slab-bearing p a r t i t i o n walls. F o r individual pad f o u n d a t i o n : 1. In a broad sense. T h e r e m a i n d e r of t h e cases r e p o r t c o n t i n u e d m o v e m e n t . w a r e h o u s e s . 2. the c o m m o n l y used remedial measures are as follows: F o r drilled pier f o u n d a t i o n : 1. U n d e r p i n t h e s t r u c t u r e w i t h piers drilled i n t o b e d r o c k . religious buildings. Provide c o n c r e t e a p r o n a r o u n d t h e h o u s e . R e c o n s t r u c t void space b e n e a t h t h e grade beams. Provide a d e q u a t e o u t l e t for all d o w n s p o u t s . 3 . 3 . T h e m i d d l e level. This is t h e only p o r t i o n of t h e entire building w h e r e slab-on-ground c o n s t r u c t i o n is used. c o n s t r u c t i o n is in general u p t o s t a n d a r d . designated as Wing Β S o u t h . A n inspection in July 1971 indicated t h a t m o v e m e n t was still c o n t i n u i n g . 1 9 6 4 . cafeteria. In J u n e 1971 further repairs were m a d e to a n u m b e r of interior c o l u m n s .000 psf. Shortly after c o m p l e t i o n . Yet damage t o t h e building caused b y uplift. In J u l y 1966 various c o l u m n s were jacked u p t o level t h e building and steel c o l u m n s were inserted for s u p p o r t . This level has a lower floor and o n e s t o r y above t h e lower floor. T h e piers were designed for an end bearing pressure of 2 0 . It is founded w i t h piers drilled i n t o b e d r o c k . T h e piers were t o p e n e t r a t e t h e shale b e d r o c k b y at least 4 feet and only t h e skin friction in t h e b e d r o c k was t o be assumed. T h e piers were also designed for a m i n i m u m dead load pressure of 15. T h e school building consists of t h r e e levels. T h e pier design system was considered t o be s o u n d in view of t h e limited k n o w l e d g e of pier design in 1 9 6 0 . In March 1972 t h e a u t h o r was engaged t o m a k e a c o m p l e t e i n d e p e n d e n t investigation i n t o t h e cause of cracking and t o d e t e r m i n e t h e necessary remedial measures. was so severe t h a t evacuation of t h e building was considered for reasons of safety. In F e b r u a r y 1 9 7 0 . In N o v e m b e r . In July and D e c e m b e r 1970 repairs were m a d e o n several piers. designated as Wing Β N o r t h . T h e pier load is heavy. T h e g y m n a s i u m . and crawl space c o n s t r u c t i o n allows t h e s t r u c t u r e t o b e free of possible damaging effects of slab heaving. 0 0 0 psf and a skin friction value of 2 .Case I DISTRESS CAUSED BY PIER UPLIFT GENERAL T h e case s t u d y is t h a t of a school building (fig. before positive remedial measures were t a k e n . In J u n e . 1964. design is a d e q u a t e . 0 0 0 psf. T h e r e m a i n d e r of the building is crawl space t y p e c o n s t r u c t i o n . as a safety p r e c a u t i o n a precast panel over a d o o r w a y had t o be r e m o v e d . t h e piers were found t o be in good c o n d i t i o n b u t surface drainage had n o t b e e n p r o p e r l y provided for and w a t e r had p e n e t r a t e d b e n e a t h t h e s t r u c t u r e causing soil swelling. HISTORY T h e school was c o m p l e t e d in 1962. Wing A is located at t h e west side of t h e building . are b o t h o n e story high w i t h n o lower floor. T h e n o r t h e r n p o r t i o n of Wing D has a b a s e m e n t locker r o o m . T h e b e d r o c k consists of essentially claystone and s a n d s t o n e shale located at d e p t h s 4 to 23 feet below t h e g r o u n d surface. distress of t h e building was n o t i c e d . t h e c o n t r a c t o r was advised t o repair t h e existing d a m a g e . and t h e u p p e r level. T h e lower level is designated as Wing C. 121) and is typical of pier uplift. and m u s i c hall are all one s t o r y w i t h a high ceiling and designated as Wing D. Exterior view of school building under study.196 FOUNDATIONS ON EXPANSIVE SOILS WING Β WING C Figure 121. . T h e lower level c o n s t r u c t i o n is confined in Wing C. In Wing B. T h e remedial c o n s t r u c t i o n o n Wing C began in 1973 and was c o m p l e t e d in 1 9 7 4 .DISTRESS CAUSED BY PIER UPLIFT 197 and is occupied b y a library and a d m i n i s t r a t i o n building. . w a t e r seeped freely i n t o t h e crawl space t h r o u g h t h e backfill. In t h e crawl space u n d e r Wing C. b u t t h e r e was evidence t h a t t h e soil has been w e t t e d in t h e past d u e t o infiltration of surface w a t e r . m a n y c o n c r e t e pedestals w e r e removed and replaced w i t h steel H c o l u m n s . Plan of school building under study. evidence was found t h a t w a t e r has flowed in freely and has washed t h e soil in t h e crawl space forming channels. t h e g r o u n d surface was relatively d r y . A t t h e n o r t h w e s t corner. Figure 122. T h e various wings are s h o w n on figure 122. Figure 123 shows t h a t t h e c o n c r e t e pedestals were crushed b y pier uplifting force in t h e same m a n n e r as c o n c r e t e cylinders are crushed in t h e c o m p r e s s i o n testing m a c h i n e . b o t h n o r t h and s o u t h . Water has b e e n entering b e l o w t h e grade b e a m for a lengthy period of t i m e . T h e arts and crafts r o o m s also s h o w e d m a n y areas of extensive d a m a g e . Most of t h e remedial measures u n d e r t a k e n in t h e past 10 years were c e n t e r e d at Wing C. Along t h e n o r t h wall in t h e crawl space. T h e east wall of t h e w o r k s h o p revealed severe m o v e m e n t . 000 psf.198 FOUNDATIONS ON EXPANSIVE SOILS Figure 123. Uplifting pressure 30. Compression failure of pedestal placed above pier and beneath grade beam. . T h e ceiling had pulled away from t h e structural walls b y as m u c h as 3 inches at t h e n o r t h end of t h e n o r t h . Almost every interior p a r t i t i o n in these areas was cracked. 0 0 0 psf. Swell tests c o n d u c t e d o n samples t a k e n from test holes located at t h e s o u t h side and west side of t h e building indicate t h a t t h e e x p a n s i o n is a b o u t 1 t o 2 p e r c e n t and t h e swelling pressure is a b o u t 3 . t h e slab-on-ground p o r t i o n of t h e locker and boiler r o o m s showed practically n o foundation movement. 0 0 0 t o 4 . INVESTIGATION Swelling potential Six test holes and t w o test pits were excavated in 1972 at t h e locations s h o w n on figure 122 and u n d i s t u r b e d samples t a k e n from t h e test holes. the soil Figure 124. E x t e r i o r cracks were found in t h e grade b e a m s and brick courses. . particularly in Wing C. Since t h e test h o l e s were drilled adjacent to the building. it is likely t h a t d u e t o excessive w e t t i n g c o n d i t i o n .DISTRESS CAUSED BY PIER UPLIFT 199 Surprisingly. Hairline cracks were found in t h e reinforced c o n c r e t e b e a m s in the crawl spaces. n u m e r o u s cracks were found in t h e i n t e r i o r walls of Wings Β and C. Cracks and separation of brick from ceiling. particularly o n t h e n o r t h wall of Wing C.s o u t h corridor b e l o w Wing D and t h e rest of t h e building as s h o w n on figure 124. In t h e u p p e r level. 8 ft.200 FOUNDATIONS ON EXPANSIVE SOILS has already swelled to its maximum limit. ft. Figure 125. The upper clays swell about 6 percent with a swelling pressure as high as 25. = 30 in. The typical dry clays found near the crawl space. . Consequently. the following calculation will indicate the stress condition around the piers: Data: Pier diameter Pier circumference Pier end area Portion of pier in upper clay Portion of pier in bedrock Swelling pressure of upper clays = 15. exhibit high swell potential as shown on figure 125.000 psf = 4. = 4. = 7.9 sq. Swell tests performed on undisturbed samples taken from test holes drilled outside of the building area present a different condition. the swell tests cannot reveal the initial soil condition.000 psf.0 ft. The swell characteristic represents the actual condition of the subsoil at the time the building was constructed.0 ft. Assuming the swelling pressure of the upper clays is 15. after remolding and upon subsequent wetting. Typical swell test performed on remolded samples. = 8.000 psf. b u t suggest t h a t a significant increase in m o i s t u r e c o n t e n t has occurred in t h e soil n e x t t o t h e building and a r o u n d t h e piers. .8x4x2.DISTRESS CAUSED BY PIER UPLIFT 201 P o r t i o n of swelling pressure responsible t o uplift Skin friction then Total uplifting force : • 15. p e r c e n t 17.6 2 .000 = 6 2 . 4 . 2 5 0 : 140. 0 kips Net uplift force Moisture analysis T h r e e piers.8 χ 8 χ 2 . Samples t a k e n in 1972 b o t h adjacent t o t h e building and well away from t h e building were c o m p a r e d to d e t e r m i n e t h e n a t u r e of m o i s t u r e m o v e m e n t s .U p p e r Clays Year 1961 1972 1972 1972 1972 1972 Location In test holes In test holes In test pits Along pier T-40 Along pier S-40 R e m o t e from t h e building. Test hole 6 Avg.2 T h e d a t a are n o t definitive.1 21.1 28.15 = 2 . m o i s t u r e c o n t e n t . 2 5 0 psf = 2 . 4 = 7 8 . S-40. T-20.4 kips Total withholding force = (Total area of pier in b e d r o c k ) χ (Unit skin friction) = 7.1 25.9 15. T h e m o i s t u r e c o n t e n t d e t e r m i n e d from samples t a k e n in t h e test pits and test holes was c o m p a r e d w i t h t h e values o b t a i n e d o n samples t a k e n in 1 9 6 1 .8 28. 4 kips = 1 4 0 . and W-38 were excavated t o t h e i r full d e p t h and samples t a k e n t o show t h e variation in soil p r o p e r t i e s w i t h respect t o b o t h d e p t h and radial distance from t h e piers. 0 0 0 psf (Total area of pier exposed t o w e t t i n g ) χ (Unit uplift) = 7.000 x 0. T h e d a t a are summarized as follows: N o r t h W a l l . m o i s t u r e c o n t e n t samples were t a k e n adjacent t o t h e walls of piers S-40 and T-40 and also 3 feet away. it is possible t h a t surface w a t e r has entered along t h e face of . m o i s t u r e c o n t e n t .9 A significant increase in m o i s t u r e c o n t e n t has occurred for t h e west wall.U p p e r Clay Avg.7 Three ft.0 20. T o d e t e r m i n e if t h e m o i s t u r e was p e n e t r a t i n g along t h e walls of t h e piers or soaking d o w n u n i f o r m l y from t h e surface. away 20. East and West W a l l s .2 18.3 23. T h e b e d r o c k actually appeared t o be drier in 1972 than in 1 9 6 1 .3 T h e above d a t a shows t h a t t h e m o i s t u r e c o n t e n t of t h e lower b e d r o c k has remained fairly uniform in t h e past 10 years. p e r c e n t Pier U p p e r clay At wall of pier S-40 T-40 28. b u t t h e change along t h e east wall is negligible. Test hole 6 FOUNDATIONS ON EXPANSIVE SOILS Avg.3 18.7 23. F o r pier T-40.8 21.7 21. m o i s t u r e c o n t e n t . T h e average m o i s t u r e c o n t e n t was as follows: Avg.202 North Wall-Bedrock Year 1961 1972 1972 1972 1972 Location In test holes In test holes Along pier T-40 Along pier S-40 R e m o t e from t h e building. T h e b e d r o c k i m m e d i a t e l y adjacent t o the pier appears to have slightly increased in m o i s t u r e c o n t e n t . away 18. indicating t h a t it has n o t been substantially w e t t e d .4 19.9 25.3 T h e d a t a strongly suggest t h a t t h e m a i n m o i s t u r e m o v e m e n t is i m m e d i a t e l y along the surface of the piers. m o i s t u r e Wall West Year 1961 1972 East 1961 1972 Source F r o m test hole F r o m test hole F r o m test hole F r o m test hole c o n t e n t .3 21. percent 17.0 Bedrock At wall of pier 23.8 T h r e e ft. p e r c e n t 21. Pier uplift T h e increased m o i s t u r e c o n t e n t a r o u n d piers S-40 and T-40 suggest t h a t b o t h have been subjected t o uplift.4 23. a 3/8-inch-wide h o r i z o n t a l crack was found just above b e d r o c k . T h e uplifting pressure exerted on the pier d e p e n d s on t h e swelling pressure of the s u r r o u n d i n g soils. it was found t h a t at least five piers had a distinct shear failure p a t t e r n as s h o w n on figures 126 and 127. Figure 126. C o n s e q u e n t l y . Tension cracks developed in t h e pier clearly indicate t h a t t h e u p p e r soils have e x e r t e d uplifting pressure on t h e u p p e r p o r t i o n of t h e pier. t h e entire pier has lifted. w h e n all the piers in Wing C were exposed during t h e remedial c o n s t r u c t i o n . T h e uplifting force exerted on each pier m a y reach as high as 2 0 0 kips.DISTRESS CAUSED BY PIER UPLIFT 203 this pier and m a y have even reached near t h e b o t t o m of t h e pier. This force is sufficient t o crush t h e c o n c r e t e pedestal formed o n t o p of t h e pier. Since excavation of t h e pit a r o u n d t h e pier relieved all t h e uplift forces on t h e side of t h e pier in the clay. t h e pier should have gradually settled as t h e pit was excavated and it was theorized t h a t the crack m u s t have been o p e n b y m o r e than 3 / 8 inch prior t o excavation. . F o r pier S-40. and t h e p o r t i o n of the pier in b e d r o c k is w i t h h o l d i n g t h e pier. Also. Failure of pier by shear resulting from uplift. Wing D is separated from t h e r e m a i n d e r of the school building w i t h e x p a n s i o n j o i n t s .204 FOUNDATIONS ON EXPANSIVE SOILS Figure 127.s o u t h c o r r i d o r t h e entire system is separated. while at the n o r t h . CAUSE O F MOVEMENT In general.s o u t h corridor is n o t i c e d . T h e m o v e m e n t is m o r e severe at t h e n o r t h side u n d e r Wing C. T o t h e west of Wing D . This explains w h y severe m o v e m e n t along t h e n o r t h . t h e cause of m o v e m e n t of t h e building is d u e t o t h e uplifting of the piers. several o t h e r c o n s t r u c t i o n defects were found. In a d d i t i o n t o t h e uplifting of t h e piers. T w o piers in Wing C are b o t t o m e d o n t h e u p p e r clay instead of drilled i n t o b e d r o c k as s h o w n on figure . C o n s e q u e n t l y . Failure of pier by shear resulting from uplift. at t h e eastern p o r t i o n of t h e school building f o u n d a t i o n m o v e m e n t is distributed t h r o u g h o u t t h e system and is n o t conspicuous. the entire school building is c o n n e c t e d w i t h grade b e a m s . T h e r e was a m i n i m u m of air space. b u t it appears t h a t t h e air space was n o t p r o p e r l y c o n s t r u c t e d . Figure 128. along the n o r t h wall in Wing C. Since t h e u p p e r clays have a m a x i m u m soil bearing value of a b o u t 3 . T h e piers were 3 6 inches in d i a m e t e r and 3 t o 4 feet in length. Improperly placed pier. In any event.DISTRESS CAUSED BY PIER UPLIFT 205 128. t h e soil has exerted uplifting pressure on t h e grade b e a m t h a t can reach as high as 2 5 . Actual length only 4 feet and bearing on clay. T h e p a t t e r n of t h e cracks indicates t h a t t h e b e a m s s u p p o r t i n g t h e slabs were deflected. as s h o w n o n figure 129. T h e cracks in t h e p a r t i t i o n wall are typical distress d u e t o t h e deflection and plastic flow of t h e long-span c o n c r e t e floor b e a m s . T h e entire length of air space b e n e a t h t h e n o r t h wall in Wing C was carefully i n s p e c t e d . r a t h e r t h a n 12 inches in d i a m e t e r and drilled i n t o b e d r o c k as had b e e n designed. Either t h e air space was n o t formed t o t h e specified thickness or t h e uplifting of t h e soil has closed t h e air space. Pier length should be 20 feet and bearing on bedrock. . 0 0 0 psf. N o t all t h e distress manifest in t h e building was caused by f o u n d a t i o n m o v e m e n t . 0 0 0 psf. it is possible t h a t s e t t l e m e n t of these piers has t a k e n place. All t h e p a r t i t i o n walls in t h e classrooms show cracks. R e m n a n t s of c a r d b o a r d used for forming t h e air space were found. care should be t a k e n t o insure t h a t there will b e n o large m u s h r o o m s present on t o p of t h e . All void space b e n e a t h the grade b e a m should be re-formed t o insure t h a t there will be at least 4 inches of space b e t w e e n t h e soil and t h e grade b e a m . which has completely closed.206 FOUNDATIONS ON EXPANSIVE SOILS Figure 129. 2. In m a n y cases. R e m o v e all backfill a r o u n d t h e building and replace c o m p a c t e d t o at least 9 0 p e r c e n t standard P r o c t o r d e n s i t y at o p t i m u m m o i s t u r e c o n t e n t . T h e a d e q u a t e c o m p a c t i o n of t h e backfill soil is very i m p o r t a n t to insure t h a t any surface w a t e r will n o t p e n e t r a t e t h r o u g h t h e backfill and i n t o t h e f o u n d a t i o n soils. beneath the grade beam. T h e remedial measures consist of t h e following: 1. structural defects and f o u n d a t i o n defects take place in t h e same s t r u c t u r e . REMEDIAL CONSTRUCTION Since t h e cause of f o u n d a t i o n m o v e m e n t and t h e source of m o i s t u r e t h a t entered i n t o t h e f o u n d a t i o n soils have b e e n defined. It is i m p o r t a n t t o isolate structural defects from f o u n d a t i o n m o v e m e n t w h e n investigating a cracked building so t h a t t h e cause m a y be d e t e r m i n e d . Backfill along t h e n o r t h wall of Wing C should consist of n o n e x p a n s i v e soils instead of t h e original soil. Four-inch void. t h e remedial measures should consist essentially of relieving t h e uplifting pressure e x e r t e d o n t h e piers and preventing additional w a t e r from entering t h e f o u n d a t i o n soils. At t h e same t i m e . careful surveys w e r e c o n d u c t e d t o d e t e r m i n e t h e vertical m o v e m e n t . In 1 9 7 2 . 132). c. Drainage a r o u n d t h e building m u s t be i m p r o v e d . 3 . m o v e m e n t of t h e piers should be arrested. Carefully establish t h e elevation of all piers b e n e a t h t h e school building b y referring t o t h e established b e n c h m a r k at t h e n o r t h side of t h e building. R e m o v e t h e asphalt paving in t h e T h e above remedial measures will p r e v e n t further d a m a g e t o t h e school s t r u c t u r e d u e t o expansive soils. area. and t h e t o p of pier cut-and-shimmed w i t h steel plates. 131). T h e d e p t h of loosening or removing of soil should be at least 8 feet. it will b e necessary t o loosen or remove t h e soils above b e d r o c k from a r o u n d all piers. T o facilitate c o n s t r u c t i o n . T h e piers a r o u n d t h e e x t e r i o r of t h e school building have lifted. By preventing w a t e r from entering t h e crawl space area. With t h e air space p r o p e r l y f o r m e d . All grade b e a m s were e x a m i n e d for s t r u c t u r a l s t r e n g t h . Also provide an a d e q u a t e e x p a n s i o n j o i n t b e t w e e n t h e sidewalk and the grade b e a m . A s t r u c t u r a l engineer should be c o n s u l t e d t o d e t e r m i n e t h e a p p r o p r i a t e n e w elevation of t h e school building c o m m e n s u r a t e w i t h t h e initial c o n s t r u c t i o n . Steel rings were installed a r o u n d t h e t o p of t h o s e piers t h a t suffered shear failure. It is reasonable t o lower t h e e x t e r i o r piers and allow t h e interior piers t o m a i n t a i n their original position. t h e entire crawl space area b e n e a t h Wing C was lowered. and should consist of t h e following: a. 130). Such u n d e r t a k i n g will have t o b e p e r f o r m e d b y h a n d inside t h e crawl space. O t h e r remedial m e a s u r e s such as providing a d e q u a t e air space b e n e a t h t h e grade b e a m s . 3 . . In Wing C. During t h e leveling o p e r a t i o n . h o w e v e r . 4 . and relocating t h o s e piers having insufficient length were p e r f o r m e d u n d e r close supervision. as follows: 1. at t h e central p o r t i o n of t h e building t h e piers have m a i n t a i n e d t h e i r original position. After t h e above remedial measures are m a d e . t h e load of t h e building will t h e n b e e x e r t e d on the piers. I m p r o v e t h e drainage in t h e c o u r t y a r d c o u r t y a r d and replace w i t h c o n c r e t e . R e c o n s t r u c t the c o n c r e t e side Walk a r o u n d t h e building t o provide an a d e q u a t e slope. This n o t only allowed w o r k m e n t o m o v e freely in t h e w o r k area b u t also remove at least 4 feet of soil a r o u n d the piers (fig. Typical records are s h o w n o n figure 1 3 3 . Slope t h e g r o u n d surface a r o u n d t h e building away from t h e s t r u c t u r e t o allow p r o p e r drainage. T h e n t h e o p e r a t i o n of releveling started. b . installing s u m p p u m p s t o eliminate perched w a t e r . T h r e e o r four piers were releveled in o n e o p e r a t i o n . A t o t a l of 56 piers w e r e releveled in Wing C over a period of 4 m o n t h s .DISTRESS CAUSED BY PIER UPLIFT 207 piers. T h e releveling p r o c e d u r e can b e started. 2. Each pier was carefully e x a m i n e d for defects after t h e s u r r o u n d i n g soil was r e m o v e d . removing and r e c o m p a c t i n g backfill. remedial m e a s u r e s as r e c o m m e n d e d above were started. T h e entire crawl space was lighted and c o n v e y o r belts installed for e a r t h removal. T h e grade b e a m s w e r e raised w i t h high-capacity jacks (fig. T h e air space should b e formed adjacent t o t h e sides of t h e piers. Heavy steel girders w e r e i n t r o d u c e d t o s t r e n g t h e n t h e defected b e a m s (fig. the d e a d load pressure will b e fully exerted o n t h e piers and t h e u p p e r soils will n o t exert uplifting pressure o n t h e piers. T h e case study of this school is a typical e x a m p l e of failure d u e t o pier uplift. Pier elevation before remedial c o n s t r u c t i o n . F o u r sets of major readings w e r e t a k e n as follows: 1. Pier elevation after t h e releveling o f t h e piers. t h u s partially eliminating t h e uplifting pressure exerted on t h e face of the piers. Pier elevation after t h e removal of soils s u r r o u n d i n g t h e piers. 3 . 2. Loosening of soil around the pier to eliminate uplifting pressure. present k n o w l e d g e of a drilled pier system in expansive soils calls for . In a d d i t i o n t o t h e c o n s t r u c t i o n defects. Pier elevation after air space b e n e a t h t h e grade b e a m s was cleared and load of building c o n c e n t r a t e d o n t h e piers. and 4 . t h e effect of t h e various stages of remedial c o n s t r u c t i o n can be reflected b y t h e s e t t l e m e n t of t h e piers.208 FOUNDATIONS ON EXPANSIVE SOILS Figure 130. F r o m figure 1 3 3 . . Jacking the grade beam in the releveling operation. Steel ring placed around the defective pier and steel girder installed t o strengthen the grade beam. Figure 132.DISTRESS CAUSED BY PIER UPLIFT 209 Figure 131. . Pier settlement after various stages of remedial construction.210 FOUNDATIONS ON EXPANSIVE SOILS PIE R ELEVATIO N AFTE R LOOSENIN G SOIL S AROUND N AFTE R RELEVELIN G OPERATIO N PIE R ELEVATIO THE PIER S Figure 133. R e m e d i a l c o n s t r u c t i o n for this school building has b e e n confined t o Wing C. It is e x p e c t e d t h a t a stabilized c o n d i t i o n can be achieved in t h e building w i t h i n a year. t h e building is still undergoing s t r u c t u r a l a d j u s t m e n t . Minor cracks appeared in t h e b l o c k wall as t h e result of releveling adjustment. Such p r e c a u t i o n s could have resisted t h e uplift pressures. . After a period of 6 m o n t h s .DISTRESS CAUSED BY PIER UPLIFT 211 b o t h a d e q u a t e r e i n f o r c e m e n t of t h e pier t o resist tension and d e e p p e n e t r a t i o n i n t o b e d r o c k t o provide for anchorage. w i t h t h e finished b a s e m e n t at t h e s o u t h end.500 psf and a m i n i m u m dead load pressure of 1 5 . Distress T h e h o u s e was built in 1 9 6 1 .s t o r y p o r t i o n of t h e h o u s e is s h o w n o n figure 135. T h e s e p a r a t i o n of t h e crawl space from t h e t w o . T h e building is a residential h o u s e located in west Denver. T h e w i d t h of t h e separation measures as m u c h as 1-1/2 inches. T h e piers w e r e designed for a m a x i m u m end pressure of 1 5 . T h e p i c t u r e w i n d o w above t h e crawl space area is also separated from t h e wall b y as m u c h as 1 inch. E x t e r i o r d o o r s w e r e j a m m e d and t h e p a t i o slab was a p p r o x i m a t e l y 1 inch l o w e r t h a n its original position. (As claystone b e d r o c k was practically exposed in t h e excavation.Case II DISTRESS CAUSED BY IMPROPER PIER DESIGN AND CONSTRUCTION GENERAL This is a typical case of i m p r o p e r design and c o n s t r u c t i o n of a drilled pier f o u n d a t i o n system. It was also r e c o m m e n d e d exceed 4 feet). T h e e x t e n t of m o v e m e n t began increasing steadily. C o l o r a d o . A subdrainage system leading t o a s u m p p u m p was later installed in t h e crawl space area of t h e h o u s e . A pier f o u n d a t i o n was r e c o m m e n d e d . T h e subsoils consist of a b o u t 4 feet of stiff clays overlying claystone b e d r o c k . T h e w a t e r table was found at a d e p t h of 7 feet b e l o w t h e original g r o u n d surface. T h e m o s t severe m o v e m e n t t o o k place b e t w e e n t h e crawl space area and t h e living r o o m area. Cracks a p p e a r e d in t h e h o u s e 6 m o n t h s after o c c u p a n c y . crawl space u n d e r t h e living p o r t i o n and garage at t h e n o r t h end. Cracks also appeared at t h e rear of t h e h o u s e b e t w e e n t h e o n e and t w o . 134). a skin friction of 1.s t o r y p o r t i o n s . EXISTING CONDITION Design T h e residence is a split-level s t r u c t u r e facing east. t h e length of t h e piers d o e s n o t . t h a t t h e piers should be drilled at least 4 feet i n t o c l a y s t o n e . 0 0 0 psf. 0 0 0 psf. It is a brick veneer and w o o d frame s t r u c t u r e w i t h a trussed r o o f system and s u p p o r t e d o n piers (fig. A subsoil investigation was m a d e before c o n s t r u c t i o n . In t h e interior of t h e h o u s e . 2. 0 0 0 psf. and from the p a t t e r n of t h e cracks and t h e n a t u r e of the swelling of t h e f o u n d a t i o n soils. In general.214 FOUNDATIONS ON EXPANSIVE SOILS Figure 134. indicating the uplifting of t h e s u p p o r t u n d e r t h e I-beam. severe cracks were found near t h e staircase leading to the b a s e m e n t (fig. T h e t o t a l length of t h e p o r t i o n of grade b e a m w i t h o u t void-forming cardboard is a p p r o x i m a t e l y 8 feet. U n d i s t u r b e d h a n d drive samples were t a k e n in the crawl space area b e n e a t h t h e grade b e a m s . 136). indicating severe m o v e m e n t . O n e of t h e posts in t h e crawl space area was loose. Slabs were raised. Most of t h e d o o r s in the h o u s e were j a m m e d . Location of exterior cracks. CAUSE O F M O V E M E N T T h e cause of f o u n d a t i o n m o v e m e n t for this h o u s e can b e summarized as follows: 1. t h e e x t e n t of cracking in this h o u s e is considered to be very severe. Figure 137 indicates t h a t t h e swelling Typical test results are s h o w n o n figure pressure is a b o u t 1 6 . Tests indicated t h a t t h e w e a t h e r e d claystone possessed high swell p o t e n t i a l . . 137. T h e lower w e a t h e r e d claystone exerted direct uplifting pressure o n t h e grade b e a m in this p o r t i o n of t h e h o u s e . Cracks were also found above m o s t d o o r s and w i n d o w s . N o air space was found b e n e a t h t h e grade b e a m near t h e m a i n e n t r a n c e in t h e crawl space. 9 inches w i d e . T h e I-beam w h i c h s u p p o r t s t h e u p p e r floor appears t o have m o v e d . severe uplifting m o v e m e n t of t h e soil b e n e a t h t h e f o u n d a t i o n has t a k e n place. With 8-foot-long grade b e a m s . DISTRESS CAUSED BY IMPROPER PIER DESIGN 215 Figure 135. 0 0 0 lbs. (See previous figure) w i t h o u t air space. 3 . t h e soils exerted n o t o n l y uplift pressure a r o u n d t h e p e r i m e t e r of t h e pier b u t also acted directly o n t h e b o t t o m of t h e piers. 7 8 5 sq.s t o r y p o r t i o n of t h e h o u s e . This pressure is sufficient t o cause t h e severe m o v e m e n t b e t w e e n t h e one-story p o r t i o n and t h e t w o . = 0 . ft. 1 4 ft. T h e m a x i m u m possible uplift in this case is as follows: Data: Pier d i a m e t e r Pier circumference Pier end area = 1 2 in. . Since t h e piers are o n l y 4 feet in length. Separation of living room from the two-story portion. = 3 . t h e total uplifting pressure exerted o n t h e grade b e a m can reach as high as 9 6 . 216 FOUNDATIONS ON EXPANSIVE SOILS Figure 136.5 kips = 16. .000 X 0 . 7 8 5 = 12.0 ft. Separation of house due to differential expansion · P o r t i o n of pier in bedrock Swelling pressure in bedrock P o r t i o n of swelling pressure responsible t o uplift = 16.000 X 0.000 psf = 3.15 = 2 . 4 0 0 psf then Total uplift force F r o m pier end = 16. 14 = 2 2 . 4 0 0 X 3 X 3. \ 2 \ ( \ > \ \ \ Λ \ \ Swelling Pies re inr Of i irnil s \ : 16. 6 kips Total and Total withholding force (Dead load pressure) = 3 5 . F r o m pier wall = 2 .1 1.785 = 11.000 X 0. C o n s e q u e n t l y . . T h e interior piers have even less dead load t h a n t h e e x t e r i o r piers.8 kips It is obvious t h a t t h e dead load pressure exerted o n t h e pier is n o t sufficient t o prevent uplift. Typical sample of weathered claystone obtained from beneath grade beam.5 22.0 APPLIED PRESSURE kit 10 100 Figure 137. 1 kips = 15.9 pcf p«rctnt Moisturt Ο \ Εxp( ns ioi cue t< Mi Jll (1er con$1ant ι >res sur e t ig.DISTRESS CAUSED BY IMPROPER PIER DESIGN 217 Noturol Natural Dry Unit Weight Contint = = 107.000 \ 0. causing t h e I-beam t o m o v e . consideration should have b e e n given t o t h e effect of w e t t i n g o n t h e structural stability. 2. Readjust t h e I-beam t o level the u p p e r s t r u c t u r e . T h e piers. interior c u p b o a r d s . 12. a d j u s t m e n t of t h e piers will b e possible w i t h o u t again removing all backfill. should b e shimmed w i t h steel plates. 10. after a d j u s t m e n t . If necessary. and provide slip j o i n t s so t h a t further slab m o v e m e n t will n o t affect t h e u p p e r s t r u c t u r e . F r e e t h e b a s e m e n t floor slab a r o u n d t h e p e r i m e t e r of t h e grade b e a m s . 4 . 8. T h e cause of w e t t i n g of t h e soils b e n e a t h t h e f o u n d a t i o n is from a high w a t e r table and p o o r drainage a r o u n d t h e h o u s e . t h e walls i m p a r t direct uplifting pressure t o t h e I-beam w h i c h disturbs t h e upper structure. T h e backfill in t h e b a s e m e n t p o r t i o n of t h e h o u s e should b e provided w i t h d e e p wells a p p r o x i m a t e l y 3 feet in d i a m e t e r a r o u n d t h e five exposed piers so t h a t in t h e future. F r e e t h e piers from t h e grade b e a m . T h e t o p of the wells should be covered w i t h suitable material so t h a t surface w a t e r will n o t seep i n t o t h e wells. R e m o v e t h e rear p a t i o slab for t h e entire length so t h a t t h e slab will be free from the grade b e a m . Initially. R e m o v e all m u s h r o o m s above t h e piers and r e c o n s t r u c t t h e air space in t h e same m a n n e r as in t h e crawl space p o r t i o n of t h e h o u s e . T h e design and construction of t h e h o u s e cannot a c c o m m o d a t e t h e severe uplifting of t h e soils. t h e existing cracks will be partially closed. Initial f o u n d a t i o n investigation indicates t h a t t h e w a t e r table is n e a r t h e b a s e m e n t floor level. and w h e n t h e slabs heave. 5. 5. T h e cause of m o v e m e n t of this h o u s e is d u e t o t h e swelling of the soils beneach t h e grade beam and t h e uplifting of t h e piers. R e m o v e all slab-bearing s t r u c t u r e s . Excavate a r o u n d t h e b a s e m e n t p o r t i o n of t h e h o u s e t o e x p o s e t h e grade b e a m . REMEDIAL MEASURES T h e following remedial m e a s u r e s w e r e r e c o m m e n d e d : 1. 7. It is . 3 . Check t h e grade b e a m b e n e a t h t h e walk-out d o o r at t h e rear p o r t i o n of t h e h o u s e t o insure t h a t t h e grade b e a m is tied in as a unit.218 FOUNDATIONS ON EXPANSIVE SOILS t h e piers b e n e a t h the I-beam have lifted.s t o r y p o r t i o n of t h e h o u s e are slab-bearing. t h u s disturbing t h e entire u p p e r s t r u c t u r e . 6. T h e p a r t i t i o n walls in t h e t w o . b o o k c a s e s . E x t e n d t h e e x t e r i o r subdrainage system from t h e rear side of t h e h o u s e t o t h e s o u t h of the garage t o i n t e r c e p t all possible sources of free w a t e r from entering the h o u s e . Precise leveling should be m a d e using t h e central grade b e a m as a reference p o i n t t o relevel t h e entire h o u s e . n e w grade b e a m s should be c o n s t r u c t e d to span above the walk-out d o o r . such as t h e stairway. It is possible t o definitely establish t h e a m o u n t of a d j u s t m e n t w h i c h is required for each individual pier t o r e t u r n the h o u s e t o a level position. e x p e c t e d t h a t after this has b e e n d o n e . furnace. 4 . 9. and so forth. 11. T h e remedial measures w e r e c o m p l e t e l y carried o u t . m o i s t e n e d layers w i t h a m e c h a n i c a l t a m p e r . R e m o v e all shrubs and flower b e d s from a r o u n d t h e h o u s e and extend t h e d o w n s p o u t s . 14. s o m e of t h e m o r e severe cracks started t o close as s h o w n o n figure 138.DISTRESS CAUSED BY IMPROPER PIER DESIGN 219 13. R e c o m p a c t t h e backfill in thin. 15. Shortly after t h e h o u s e was releveled. Regrade t h e backfill around t h e h o u s e so t h a t surface w a t e r will drain away from the house. This specific case w e n t to t h e court and t h e c o n t r a c t o r was ordered to p a y t h e $ 1 1 . S o m e 6 Figure 138. 0 0 0 cost of remedial c o n s t r u c t i o n w h i c h a m o u n t e d to a b o u t 50 p e r c e n t of t h e cost of t h e h o u s e . Movement of the house before and after remedial correction. It t o o k m o r e t h a n 6 m o n t h s before t h e s t r u c t u r e was stablized. . t h e r e f o r e . . 1 3 9 ) . Figure 139. r e a d j u s t m e n t of t h e piers was n o t necessary.220 FOUNDATIONS ON EXPANSIVE SOILS years after t h e remedial c o n s t r u c t i o n . n o serious f o u n d a t i o n m o v e m e n t had t a k e n place in this h o u s e (fig. Condition of front of house in 1974. Aspen. Crescent.Case III DISTRESS CAUSED BY HEAVING OF FOOTING PAD AND FLOOR SLAB GENERAL This case s t u d y is typical of t h a t of u n d e r e s t i m a t i n g t h e swelling p o t e n t i a l of t h e soil. T e n s i o n cracks near t h e t o p of t h e c o n c r e t e c o l u m n s . and Birch. . More t h a n 2 years have elapsed since t h e remedial w o r k was c o m p l e t e d and the buildings remain in perfect c o n d i t i o n . Cracks first appeared in t h e building in 1963 and have c o n t i n u e d steadily since. t h e e x t e n t of cracking is m o r e severe at t h e eastern g r o u p of cottages t h a n at t h e western g r o u p . A n investigation i n t o t h e cause of cracking was m a d e b y a soils engineer in O c t o b e r 1 9 6 6 . and B u t t e r c u p . Each g r o u p is c o n n e c t e d t o t h e service buildings as indicated o n figures 140 t h r o u g h 1 4 3 . HISTORY T h e buildings u n d e r investigation are in a State-operated ward for h o u s i n g t h e severely retarded and consist of 6 cottages. DISTRESS In general. A t t h a t t i m e . Typical k i n d s of cracking w h i c h t o o k place at t h e various buildings are as follows: 1. 0 0 0 psf and a m i n i m u m dead load pressure of 1. T h e 3 cottages located at t h e western p o r t i o n of t h e site are identified as C h e r u b . remedial measures were only partially carried o u t . 3 t o t h e east and 3 t o t h e west. Individual f o u n d a t i o n p a d s have heaved and severe floor heave has t a k e n place. it was r e c o m m e n d e d t h a t drainage a r o u n d t h e e x t e r i o r of t h e buildings be i m p r o v e d . designed for a m a x i m u m soil pressure of 3 . T h e cottages were c o m p l e t e d in 1962.500 psf. T h e s t r u c t u r a l design of t h e buildings indicates t h a t t h e y were f o u n d e d o n individual pads designed for t h e pressures r e c o m m e n d e d . T h e fill was c o m p a c t e d t o 100 p e r c e n t standard P r o c t o r density u n d e r the footings and t o 95 p e r c e n t s t a n d a r d P r o c t o r d e n s i t y u n d e r t h e slabs. T h e footings and slabs are f o u n d e d partly on c o m p a c t e d fill and partly o n n a t u r a l soils. Because of cost. T h e original soil r e p o r t r e c o m m e n d e d that the buildings be founded w i t h spread footings o n a c o m b i n a t i o n of c o m p a c t e d fill and t h e in-place natural sandy clays. T h e eastern c o t t a g e s are t h e Starlight. 7 in.3 in. 4. 4. Slab bearing p a r t i t i o n walls were cracked and slightly b u c k l e d . 1. 2. T h e e n t r a n c e t o t h e service buildings had m o v e d and diagonal cracks were found in t h e brick veneer. 2.222 FOUNDATIONS ON EXPANSIVE SOILS Figure 140.2 in.3 in. 2. and 5. 0.9 in.1 in.0 in. View of cottages 2.4 in. Corridors w h i c h c o n n e c t t h e service buildings to the cottages were separated both h o r i z o n t a l l y and vertically. T h e e x t e n t of cracking in t h e Aspen and C h e r u b cottages was less severe t h a n in the o t h e r four cottages. Typical distresses are s h o w n o n figures 144 t h r o u g h 146. 0. 3. T h e following slab m o v e m e n t d a t a was o b t a i n e d : Buttercup Crescent Starlight Cherub Birch Aspen 2.4 in. A p o r t i o n of t h e floor slabs had moved w i t h respect to the grade b e a m s . 0.1 in. .1 in. B u t t e r c u p t o Service Crescent t o Service Starlight t o Service C h e r u b t o Service Birch t o Service Aspen t o Service 2. Diagonal cracks are general in t h e brick at t h e j u n c t i o n of t h e corridors w i t h t h e service buildings. 3.3 in. 4. 3 inch differential slab m o v e m e n t . Location of exploratory holes for the cottages. This m o v e m e n t can b e associated w i t h t h e different loading c o n d i t i o n s in t h e cottages w i t h respect to t h e corridors. it is likely t h a t t h e slab raised m o r e u n i f o r m l y t h a n in t h e o t h e r buildings. . T h e corridors c o n n e c t i n g t h e service buildings t o t h e various cottages indicate definite m o v e m e n t . while t h e B u t t e r c u p building shows only a 2. T h e above d a t a indicates t h a t t h e differential slab m o v e m e n t is t h e greatest at Crescent. Considering t h e a m o u n t of w e t t i n g of t h e slab in this building ( B u t t e r c u p ) .DISTRESS CAUSED BY HEAVING OF SLABS 223 Ο Figure 141. 15 of w h i c h were drilled adjacent t o t h e cracked buildings. t h e source of m o i s t u r e t h a t entered the subsoils. With t h e t w o phases of investigation. In April 1970 a decision was m a d e t o initiate a second phase of investigation. Plan of test hole and test pit location for west cottages. all possible factors t h a t could influence t h e effectiveness of t h e remedial c o n s t r u c t i o n w o u l d b e covered. Aspen and C h e r u b . as well as t h e possible remedial measures n e e d e d . n o w showed severe m o v e m e n t . In t h e first phase of t h e investigation. INVESTIGATION In August 1968 investigation i n t o t h e cause of cracking of t h e buildings.224 FOUNDATIONS ON EXPANSIVE SOILS Figure 142. and t h e possibility t h a t a neighboring w a t e r t a n k had an u n d e c t e c t e d leak t h a t provided t h e m o i s t u r e was initiated. . w h i c h were in relatively good c o n d i t i o n in 1 9 6 8 . including r e c o m m e n d e d remedial measures. T h e m o v e m e n t of t h e various buildings b e c a m e so severe t h a t it was necessary t o evacuate t h e p a t i e n t s from all six buildings. In t h e m e a n t i m e . T h e r e m a i n d e r of t h e test holes were drilled away from t h e cracked buildings at locations s h o w n o n figure 1 4 1 . t h e c o n d i t i o n of all buildings c o n t i n u e d t o d e t e r i o r a t e . F o r m o r e t h a n 1-1/2 years. n o corrective action was t a k e n . 2 4 e x p l o r a t o r y holes were drilled at the site. was s u b m i t t e d in S e p t e m b e r 1968. A c o m p l e t e r e p o r t . the concrete slab was core drilled in 12 locations in each building and hand augered in the core hole to a depth of 6 feet. In the second phase of the investigation. Undisturbed samples were obtained in each auger hole. Determination of the swelling potential and the swelling pressure of the soils directly beneath the exterior footings at each building. Determination of the swelling potential and the swelling pressure of the soils beneath the floor slabs at various depths. 6. The behavior of the soils involves many variables some of which cannot be determined with certainty. 4. The investigation was directed mainly toward the following items: 1. 2. 5. This investigation is based solely on the statistical average behavior of the soils rather .DISTRESS CAUSED BY HEAVING OF SLABS 225 Figure 143. Prediction of the future behavior of the soils and of the effectiveness of the proposed remedial measures. A total of 82 holes was drilled inside the building. Determination of the water table elevation in the area. The location of all test holes and test pits is shown on figures 141 through 143. Plan of test hole and test pit location for east cottages. Determination of the variation of moisture content in the soil beneath the floor slab in each building to a depth of approximately 6 feet. 3. Determination of the possible sources of moisture which entered the buildings. Cherub ward .West complex . Bedrock was found at d e p t h s ranging from 8 t o 29 feet. T h e characteristics of t h e various subsoil strata are described as follows: . t h a n t h e result of a single observation or test. and m a n y o t h e r tests were m a d e so t h a t p r o p e r conclusions could b e d r a w n along w i t h r e c o m m e n d a t i o n s for remedial measures. T h e evaluation of the behavior of t h e soils is m u c h m o r e complicated and difficult t h a n for o t h e r elastic engineering materials. over 3 0 0 tests on m o i s t u r e c o n t e n t . Upper rounds .226 FOUNDATIONS ON EXPANSIVE SOILS Figure 144. In this investigation. m o r e t h a n 150 tests o n swelling characteristics. Subsoil conditions Subsoil c o n d i t i o n s at t h e site consist essentially of 0 t o 8 feet of fill overlying soft t o stiff clays.Wall braced t o prevent falling in. etc. Interior wall pulling away from exterior wall. An X-ray diffraction analysis indicated t h a t t h e total clay mineral ( n o t including clay-size q u a r t z or calcite.3 t o 18.6 p e r c e n t .DISTRESS CAUSED BY HEAVING OF SLABS 227 Figure 145. 6 p e r c e n t and t h e plasticity i n d e x ranging from 2 6 . Major minerals were q u a r t z and calcite. 5 p e r c e n t . t h e u p p e r soils were soft and their stiffness increased w i t h d e p t h .T h e fill consists of t h e on-site soils and it was often difficult t o distinguish b e t w e e n t h e fill and t h e natural soil. Minor minerals . Corner of Cherub ward. T h e actual m o i s t u r e c o n t e n t of the in-place fill ranged from 10.1 p e r c e n t . C l a y . In general.) was p r o b a b l y n o t m o r e t h a n 5 p e r c e n t b y v o l u m e of t h e t o t a l sample. 8 t o 3 2 . F i l l .T h e clays at t h e site had fairly uniform characteristics. especially in t h e d e c a n t e d fractions. 0 t o 5 3 .7 t o 16. T h e soil could be classified as on t h e borderline b e t w e e n C L and CH w i t h t h e liquid limit ranging from 4 2 . T h e fill was placed u n d e r controlled c o n d i t i o n and t h e o p t i m u m m o i s t u r e c o n t e n t ranged from 16. T h e stiffness of the clay varied w i t h the m o i s t u r e c o n t e n t . Interior Aspen ward — Floor crack.7 t o 14. T h e u p p e r p o r t i o n of claystone was highly w e a t h e r e d .0 p e r c e n t . and colloid c o n t e n t (percent m i n u s 0. T h e claystone b e d r o c k had essentially the same physical characteristics as t h e u p p e r clay.001 m m ) less t h a n 22 p e r c e n t . 0 0 2 m m ) of t h e typical sample was less t h a n 35 p e r c e n t . T h e shrinkage limit of typical clays ranged from 9. ." Bedrock—Bedrock consisted of c l a y s t o n e and s a n d s t o n e . t h e f o u n d a t i o n soil u n d e r t h e various buildings falls i n t o t h e category of "highly expansive soils. T h e claystone b e d r o c k had a resemblance t o stiff clay and it was difficult t o distinguish b e t w e e n t h e claystone and the u p p e r clay.228 FOUNDATIONS ON EXPANSIVE SOILS Figure 146. were m o n t m o r i l l o n i t e (unusually b r o a d 14-angstrom lines) w i t h possibly a trace of kaolinite and mica. T h e above physical analysis of t h e clay soils indicated t h a t in accordance with the established m e t h o d s of classifying expansive soils. H y d r o m e t e r analysis indicated t h a t the clay fraction ( p e r c e n t m i n u s 0 . T h e m o i s t u r e c o n t e n t of all soil samples b e n e a t h b o t h t h e slab and t h e footings was o b t a i n e d . With variable m o i s t u r e c o n t e n t and d e n s i t y . Source of moisture N o v o l u m e change will take place in t h e expansive soils unless t h e r e is a change in t h e a m o u n t of m o i s t u r e in t h e soil. 2. T h e m o s t difficult aspect in investigating t h e source of w a t e r was in d e t e r m i n i n g w h e t h e r t h e underslab soils were w e t t e d b y the i n t r o d u c t i o n of surface w a t e r d u e t o p o o r e x t e r i o r drainage or b y leakage of t h e underslab utility system. However. 4 . and 3. it is possible t o e s t i m a t e the m a g n i t u d e of floor and f o u n d a t i o n heaving. Possible rising w a t e r table c o n d i t i o n d u e t o increase in subsurface w a t e r v o l u m e .DISTRESS CAUSED BY HEAVING OF SLABS 229 Stabilized free w a t e r in the area was found at d e p t h s of 5 t o 19 feet b e l o w t h e t o p of t h e floor level. Average m o i s t u r e c o n t e n t for t h e entire building at various d e p t h s . Average m o i s t u r e c o n t e n t at t h e central p o r t i o n of each building at various d e p t h s . T h e increase of m o i s t u r e c o n t e n t can b e caused b y various factors as follows: 1. F r o m t h e swell p o t e n t i a l . 3 . Leaks in t h e sewer s y s t e m . T h e conclusions are based o n the statistical average of soil behavior. at t h e site of t h e buildings. Method of approach After all t h e test d a t a had b e e n a c c u m u l a t e d . F o r c o n s t a n t d e n s i t y .S w e l l i n g p o t e n t i a l is an index t h a t indicates t h e degree of v o l u m e change of t h e soil after s a t u r a t i o n . Leaks in t h e d o m e s t i c w a t e r s y s t e m . Surface runoff including rain. and lawn sprinkler water. A n average m o i s t u r e c o n t e n t was d e t e r m i n e d . T h e swelling p o t e n t i a l of soils u n d e r each building was o b t a i n e d and a curve w h i c h graphically s u m m a r i z e d t h e results was p r o p o s e d b y p l o t t i n g t h e m o i s t u r e c o n t e n t versus swelling p o t e n t i a l . A curve was p r e p a r e d for each building (fig.S w e l l i n g pressure can b e defined as t h e pressure required t o k e e p t h e v o l u m e of t h e sample c o n s t a n t . Since each building is s u r r o u n d e d b y grade . Average M o i s t u r e . w h i c h provided i n f o r m a t i o n o n t h e following: 1. 2. t h e m e t h o d of a p p r o a c h used in solving t h e p r o b l e m was as follows: Swelling P o t e n t i a l . Figure 148 is a typical g r a p h of swelling pressure versus m o i s t u r e c o n t e n t for each building. Average m o i s t u r e consent in t h e p e r i m e t e r of t h e building at various d e p t h s . m e l t i n g s n o w . swelling p o t e n t i a l . and 5. Leaks in t h e u n d e r slab h e a t i n g system. swelling pressure should have a c o n s t a n t value. t h e w a t e r table was at least 10 feet b e l o w t h e floor level. Swelling P r e s s u r e . T h e r e c o m m e n d e d remedial measures for each building are essentially based o n the swelling pressure. 147). and m o i s t u r e d i s t r i b u t i o n . t h e swelling pressure varies as s h o w n on figure 148. then a leak in the underslab utility lines is suggested. Moisture and swelling potential relationship at Aspen. it is most likely that exterior surface water has entered the underslab soils through the void space beneath the grade beams. 2. then the migration of exterior surface runoff into the underslab soils is suggested. However. If the moisture content around the perimeter of the building at a depth of more than 3 feet below the floor slab is high. If the moisture content directly beneath the concrete slabs (within 24 inches below the top of the floor slab) is high. the following conclusions can be established: 1.230 FOUNDATIONS ON EXPANSIVE SOILS Figure 147. beams 3 feet deep. therefore. . surface water can enter the subsoils only at a depth of at least 3 feet below the top of the floor slab. 000 12 14 16 MOISTURE 18 CONTENT (%) 20 22 24 Figure 148. T h e average m o i s t u r e c o n t e n t at various d e p t h s for each building will give a clear i n d i c a t i o n as t o t h e source of m o i s t u r e t h a t has entered i n t o t h e buildings.000 £ 5. t h e n n o i n t r o d u c t i o n of surface runoff or leakage in t h e utility lines is suggested.000 ASPEN 10. TREATMENT Based on t h e above reasoning.000 • • • 1. Moisture and swelling pressure relationship at Aspen. . 3 . the p r o b l e m t h a t existed in each building can be established and remedial measures prescribed. t h e n b o t h t h e i n t r o d u c t i o n of surface runoff and leakage in the utility lines are suggested. If t h e m o i s t u r e c o n t e n t a r o u n d t h e p e r i m e t e r of t h e building and t h e m o i s t u r e c o n t e n t of t h e building's i n t e r i o r are b o t h low. 4 .DISTRESS CAUSED BY HEAVING OF SLABS 231 20. If t h e m o i s t u r e c o n t e n t a r o u n d the p e r i m e t e r of t h e building and t h e m o i s t u r e c o n t e n t of t h e building's i n t e r i o r are b o t h high. In Test Pits 101 and 102.2 percent. The average m o i s t u r e c o n t e n t b e n e a t h the interior footings was 14. T h e following remedial measures are r e c o m m e n d e d for this building: 1.232 FOUNDATIONS ON EXPANSIVE SOILS Treatment at Birch Birch is the east building of the west c o m p l e x . 2. 147) and an average swelling pressure of 6. T h e sewer line which r u n s u n d e r t h e building and branches i n t o the various b a t h r o o m s should be exposed and carefully checked for leakage. This resulted in t h e flooding of t h e underslab soils. T h e slab-bearing p a r t i t i o n walls in this building should be r e c o n s t r u c t e d in such a m a n n e r t h a t slab m o v e m e n t will n o t affect t h e stability of the s t r u c t u r e . At a d e p t h of 6 feet below t h e t o p of t h e slab. This corresp o n d s t o an average swelling p o t e n t i a l of 6.7 p e r c e n t . F u r t h e r testing indicated that t h e underslab heating system had leaked and t h e sewer line had b r o k e n . T h e m o i s t u r e d i s t r i b u t i o n indicates t h a t the lower soils are in a very d r y s t a t e .6 p e r c e n t . T h e p e r i m e t e r of t h e floor slab should be saw cut t o insure t h a t the slab is separated from t h e grade b e a m s and t h a t there will be free m o v e m e n t of t h e slab w i t h respect t o the grade b e a m s . swelling will take place.8 p e r c e n t and the highest m o i s t u r e c o n t e n t 22.200 psf (fig. 3. U n d e r p i n n i n g of t h e e x t e r i o r o r interior footings will n o t be necessary. t h e average m o i s t u r e c o n t e n t decreased t o 11. Moisture c o n t e n t s for t h e entire building at various d e p t h s are relatively u n i f o r m w i t h the lowest m o i s t u r e c o n t e n t 15. adjacent t o the grade b e a m .6 p e r c e n t . F o u n d a t i o n soils at this building site consist essentially of controlled c o m p a c t e d fill. 4.5 p e r c e n t and swelling pressure of 1 0 . w a t e r was flowing from u n d e r n e a t h t h e slab. Most of t h e c o m p a c t e d fill soil b e n e a t h t h e footings possessed only low swell p o t e n t i a l . Treatment at Aspen Aspen is t h e n o r t h building of t h e west c o m p l e x .000 psf. T h e a m o u n t of cut in t h e site grading ranged from 5 t o 20 feet. and if the soils b e c o m e excessively w e t t e d .5 p e r c e n t and average swelling pressure of 14. A vertical slip j o i n t should also be provided w h e r e the p a r t i t i o n walls c o n n e c t with the exterior walls or columns. A n o t h e r pressure test was c o n d u c t e d in April . T h r e e test pits were o p e n e d at the e x t e r i o r of the building. Remedial measures t o t h e footings are n o t r e c o m m e n d e d . This c o r r e s p o n d s t o an average swelling p o t e n t i a l of 3. The swelling p o t e n t i a l of t h e soils b e n e a t h t h e e x t e r i o r footings is high w i t h a p e r c e n t of swell of 5. (fig. 148). In 1 9 6 8 . a mechanical engineer found t h a t there was only slight leakage in the underslab h o t w a t e r heating system b y c o n d u c t i n g pressure tests. A s t u d y of t h e original soil r e p o r t indicates t h a t at t h e n o r t h side of this building t h e r e is a p p r o x i m a t e l y 1/2 foot of cut and at t h e s o u t h side there is a p p r o x i m a t e l y 7 feet of fill. T h e possibility of f o u n d a t i o n m o v e m e n t is relatively slim. T h e f o u n d a t i o n soils at this building consist entirely of t h e natural soils. 0 0 0 psf. F u r t h e r slab m o v e m e n t should n o t exceed 1/2 inch. T h e m o v e m e n t of the floor slab in this building h a d only begun and future severe floor m o v e m e n t will take place even t h o u g h the u n d e r s l a b h e a t i n g system is entirely d i s c o n n e c t e d . Since the m o i s t u r e c o n t e n t was . it is almost impossible t o u n d e r p i n t h e interior footings w i t h o u t demolishing t h e entire building. and t h e pressure d r o p p e d from 130 to 27 psi in 20 m i n u t e s . Slip j o i n t s in the p a r t i t i o n walls will prevent t h e d i s t u r b a n c e of the u p p e r s t r u c t u r e . This is low c o m p a r e d w i t h u n d e r s l a b m o i s t u r e c o n t e n t of t h e o t h e r buildings. Treatment at Cherub C h e r u b is t h e west building of t h e west c o m p l e x . If for some reason t h e u n d e r s l a b soils c a n n o t be r e m o v e d . T h e possibility of f o u n d a t i o n m o v e m e n t was r a t h e r r e m o t e . t h e r e is 1 foot of cut and at t h e s o u t h side there is a b o u t 7 feet of fill. It was obvious t h a t in the preceeding m o n t h s m o r e leakage had developed in t h e u n d e r s l a b heating system which a c c o u n t e d for t h e severe m o v e m e n t in this building. T h e piers should be designed for a m a x i m u m end pressure of 3 0 . 0 0 0 psf. At t h e n o r t h side. it was r e c o m m e n d e d t h a t t h e interior footings be decreased in area b y c u t t i n g off the c o n c r e t e pad and t h u s increasing t h e unit dead load pressure. This c a n n o t be prevented unless all p r o b l e m soils b e n e a t h the slab are removed. Bedrock is shallow at t h e west side of the building. Most of t h e interior footings are placed o n n a t u r a l soils. T h e existing p o c k e t s of high m o i s t u r e c o n t e n t soils caused b y leakage of the h e a t i n g system will migrate t o t h e drier phase of soils and cause d a m a g e . T h e r e f o r e . 0 0 0 psf for t h a t p o r t i o n of the pier in b e d r o c k . 3. t h e r e is every possibility t h a t the floor slab will raise as m u c h as 3 inches above t h e present level. however.8 p e r c e n t . This c o r r e s p o n d s t o a swelling p o t e n t i a l of 5. 2. granular soils c o m p a c t e d t o at least 9 0 p e r c e n t P r o c t o r density at o p t i m u m m o i s t u r e c o n t e n t . T h e following remedial measures were r e c o m m e n d e d for this building: 1. It was e s t i m a t e d t h a t t h e dead load pressure on each pad could be increased t o over 6 . T h e average m o i s t u r e c o n t e n t of the soils b e n e a t h t h e interior footings was a b o u t 14. or t w o piers can be drilled u n d e r each c o l u m n w i t h a grade b e a m spanning over t h e t w o piers. T h e floor slabs in this building should be entirely removed and the soils b e n e a t h the slab removed for a d e p t h of 3 feet. T h e f o u n d a t i o n soils b e n e a t h this building are m o s t l y fill. T h e piers should also be designed for a m i n i m u m dead load pressure of 2 0 . 0 0 0 psf and a skin friction of 3 . T h e average m o i s t u r e c o n t e n t of the soils b e n e a t h t h e e x t e r i o r footings was a b o u t 2 2 . T h e piers can be drilled in a slanted position. 0 0 0 psf b y reducing t h e area of the concrete pad. T h e soils directly b e n e a t h t h e floor slab h a d an average m o i s t u r e c o n t e n t of 13.5 p e r c e n t . These soils should b e discarded and replaced w i t h standard n o n e x p a n s i v e .DISTRESS CAUSED BY HEAVING OF SLABS 233 1970. T h e interior footings should also be u n d e r p i n n e d . 8 p e r c e n t which c o r r e s p o n d s t o a swelling p o t e n t i a l of less t h a n 1 p e r c e n t . b u t unsightly cracks in the p a r t i t i o n walls and the floor slab will t a k e place.0 p e r c e n t . U n d e r p i n t h e e x t e r i o r footings w i t h piers drilled i n t o b e d r o c k . impervious. O n J a n u a r y 2 8 . 6 p e r c e n t . and 3 . T h e floor slabs in this building should be entirely r e m o v e d and t h e soils b e n e a t h the slabs removed t o a d e p t h of 3 feet. T h e interior footings should be decreased in area b y cutting off t h e c o n c r e t e pad and t h u s increasing the u n i t dead load pressure. 1970. b o t h interior and exterior. T h e m o i s t u r e c o n t e n t a r o u n d t h e p e r i m e t e r of t h e building is high and further swelling of t h e soils b e n e a t h the footings is unlikely. 0 0 0 psf and a swelling p o t e n t i a l of less t h a n 1 p e r c e n t . T h e r e is a strong possibility t h a t the i n t e r i o r footings will have m o v e m e n t . T h e following remedial measures are r e c o m m e n d e d for this building: 1. Treatment at Buttercup B u t t e r c u p is t h e east building of t h e east c o m p l e x . T h e soils directly b e n e a t h t h e floor slabs had an average m o i s t u r e c o n t e n t of 12. T h e existing local high m o i s t u r e c o n t e n t in t h e soil will migrate t o t h e drier soil and cause floor d a m a g e . T h e tests indicated t h a t there was n o leakage in t h e system. T h e leakage of the underslab h e a t i n g system had t a k e n place only r e c e n t l y and t h e effect of t h e leakage had n o t been reflected in the m o i s t u r e c o n t e n t of the soils. were f o u n d e d o n s t r u c t u r a l fill. In 1 9 6 8 . This c o r r e s p o n d s t o a swelling pressure of 2 . t h e n the possibility of further floor m o v e m e n t will be remote. Judging from t h e m o i s t u r e c o n d i t i o n of the soils b e n e a t h t h e e x t e r i o r footings. 0 0 0 psf. T h e average m o i s t u r e c o n t e n t of t h e soils directly b e n e a t h the footings was 2 5 . T h e f o u n d a t i o n soils at this building site consist of 2 t o 5 feet of controlled c o m p a c t e d fill. granular soils c o m p a c t e d t o at least 9 0 p e r c e n t standard P r o c t o r density at o p t i m u m m o i s t u r e c o n t e n t . At a lower d e p t h . If this is d o n e . t h e natural soils were generally d r y w i t h t h e m o i s t u r e c o n t e n t ranging from 18 t o 20 p e r c e n t and the swelling pressure reaching as high as 9 . 2. the e x t e r i o r m o i s t u r e c o n t e n t was generally high.1 p e r c e n t which c o r r e s p o n d s t o a swelling p o t e n t i a l of m o r e t h a n 8 p e r c e n t . also. T h e m o v e m e n t of t h e floor slab of this building has o n l y begun and severe floor m o v e m e n t will o c c u r even t h o u g h the underslab heating system is entirely d i s c o n n e c t e d . This definitely indicated t h a t m o s t of t h e w e t t i n g of this building had b e e n caused b y the migration of surface w a t e r i n t o t h e f o u n d a t i o n soils. a b o u t 4 p e r c e n t higher t h a n t h e interior m o i s t u r e c o n t e n t . t h e chance of increase in m o i s t u r e c o n t e n t b e n e a t h t h e interior footings is high and there is a strong possibility t h a t t h e footing f o u n d a t i o n will have future m o v e m e n t . These removed soils should b e replaced w i t h n o n e x p a n s i v e . a mechanical engineer again c o n d u c t e d pressure tests o n the underslab h o t w a t e r heating s y s t e m . .234 FOUNDATIONS ON EXPANSIVE SOILS low. This indicates t h a t t h e r e is leakage in t h e underslab heating system. it was n o t necessary t o u n d e r p i n t h e e x t e r i o r footings. similar tests were m a d e in this building which indicated t h a t t h e pressure d r o p p e d from 9 0 t o 7 6 psi in 15 m i n u t e s and from 100 t o 52 psi in 75 m i n u t e s . It was suspected t h a t all footings. T h e m o i s t u r e c o n t e n t d i s t r i b u t i o n analysis indicates t h a t t h e m o i s t u r e c o n t e n t near footing level was higher t h a n for the soils directly b e n e a t h t h e floor. It is n o t necessary t o u n d e r p i n t h e e x t e r i o r footings. T h e possibility of footing f o u n d a t i o n m o v e m e n t was relatively r e m o t e . This c o r r e s p o n d s t o an average swelling p o t e n t i a l of less t h a n 1 p e r c e n t . After t h e backfill has been r e m o v e d . This definitely indicates that t h e r e is a large leak in the u n d e r s l a b h e a t i n g s y s t e m . the excavation should remain o p e n for a period of at least 2 weeks t o insure t h a t all w a t e r t r a p p e d u n d e r the floor slabs can b e effectively drained. In 1 9 6 8 . It was. T h e following facts were n o t i c e d : 1. Also. It is likely t h a t all footings. It was c o n c l u d e d t h a t there are very definitely leaks in t h e sewer lines of the east complex. It is n o t necessary t o replace t h e underslab soils and the chance and m a g n i t u d e of future slab m o v e m e n t is l o w . . T h e soils b e n e a t h t h e exterior footings had relatively u n i f o r m m o i s t u r e c o n t e n t w i t h a m i n i m u m m o i s t u r e c o n t e n t of 20. t h e results of which showed t h a t t h e pressure d r o p p e d from 3 2 t o 0 psi in 6 0 seconds. a pressure test was m a d e on the underslab h e a t i n g s y s t e m . t h e r e f o r e . This indicates t h a t t h e a m o u n t of w a t e r t h a t seeped i n t o t h e soils from t h e e x t e r i o r of t h e building was relatively l o w . 1 p e r c e n t . Most of t h e w a t e r present in t h e underslab soils is derived from t h e leakage of t h e u n d e r s l a b heating system and possibly from leaking sewer lines.9 versus 21. T h e m o i s t u r e c o n t e n t of t h e soils at t h e p e r i m e t e r of t h e building was only slightly higher t h a n t h e m o i s t u r e c o n t e n t at t h e central p o r t i o n of t h e building. b o t h e x t e r i o r and interior.1 percent and maximum moisture content of 26. t h e r e is 2 feet of c u t and at t h e s o u t h side there is 4 feet of fill. D u e t o t h e excessive leakage of t h e underslab h e a t i n g s y s t e m .2 p e r c e n t which is considered t o be low. At t h e n o r t h side. T h e soils directly b e n e a t h t h e slabs had an average m o i s t u r e c o n t e n t of 2 1 . t h e r e f o r e . 0 p e r c e n t . 2. T h e r e was n o large difference b e t w e e n t h e m o i s t u r e c o n t e n t in t h e c e n t e r p o r t i o n and t h e m o i s t u r e c o n t e n t a r o u n d t h e p e r i m e t e r of t h e building (19. All backfill a r o u n d this building should b e removed t o e x p o s e t h e f o u n d a t i o n s y s t e m . T h e a m o u n t of w a t e r t r a p p e d in the underslab soils m u s t b e near s a t u r a t i o n which a c c o u n t s for the steady flow of w a t e r from the underslab soils i n t o Test Pit 110. a large p o r t i o n of t h e soils had reached a state of s a t u r a t i o n . T h e possible behavior of the floor slabs at this building can be evaluated b y studying the m o i s t u r e d i s t r i b u t i o n at various d e p t h s .2 percent). T h e possibility of foundation m o v e m e n t for t h e interior footings was also r e m o t e . t h e m i g r a t i o n of m o i s t u r e from interior t o e x t e r i o r is unlikely t o t a k e place. T h e f o u n d a t i o n soils at this site consist of b o t h cut and fill. n o t necessary t o u n d e r p i n o r m a k e remedial c o n s t r u c t i o n o n the i n t e r i o r footings. are founded o n t h e natural soils. Treatment at Starlight Starlight is t h e east building of t h e east c o m p l e x .DISTRESS CAUSED BY HEAVING OF SLABS 235 T h e average m o i s t u r e c o n t e n t b e n e a t h t h e interior footings was 2 2 . sewer tests t h a t were c o n d u c t e d indicate t h a t e x t e r i o r test pits had filled during the tests. Free w a t e r was found n o t only in the e x t e r i o r test pits b u t also in t h r e e test holes inside t h e building.9 p e r c e n t . This c o r r e s p o n d s t o an average swelling p o t e n t i a l of 1. 3 . a p p r o x i m a t e l y 6 feet b e l o w t h e t o p of t h e slab. It is likely t h a t all footings at this building are placed o n n a t u r a l soils. and this can cause d a m a g e . This c o r r e s p o n d s t o an average swelling p o t e n t i a l of 4. This definitely indicated t h a t t h e source of m o i s t u r e t h a t entered this building was from surface runoff. Since t h e footings are only lightly l o a d e d . Treatment at Crescent Crescent is t h e n o r t h building of t h e east c o m p l e x . It was obvious t h a t t h e p o t e n t i a l m o v e m e n t of t h e interior footings is high and in fact. c o r r e s p o n d i n g t o an average swelling p o t e n t i a l of 3. further m o v e m e n t of t h e footings is possible. 0 0 0 psf. some of t h e soils had a swelling p o t e n t i a l as high as 9 p e r c e n t and a swelling pressure as high as 3 0 .5 p e r c e n t .0 and t h e m o i s t u r e c o n t e n t at t h e central p o r t i o n of t h e building at t h e same d e p t h was p e r c e n t . Again in 1 9 6 8 .9 p e r c e n t . ranging from 0. which c o r r e s p o n d s t o a swelling p o t e n t i a l of 1. T h e p o t e n t i a l m o v e m e n t of t h e e x t e r i o r footings is at least 50 p e r c e n t .6 p e r c e n t . Free w a t e r was found in Test Pit 112. P o o r drainage a r o u n d t h e building was directly responsible for the w e t t i n g of the f o u n d a t i o n soils. and even t h o u g h t h e source of m o i s t u r e t h a t entered t h e building had b e e n cut off b y improving t h e drainage. special a t t e n t i o n should be directed t o improving t h e surface drainage c o n d i t i o n . 1 p e r c e n t . 0 0 0 psf. t h e average m o i s t u r e c o n t e n t decreased t o 14. 5 0 0 t o 1 2 .7 percent and t h e lowest 10. showed signs of t h e presence of free water. . either inside or outside of the building.5 p e r c e n t and an average swelling pressure of 6 . Moisture d i s t r i b u t i o n in t h e underslab soils was e x t r e m e l y erratic. at a d e p t h of 6 feet b e l o w the t o p of the floor slab. At t h e lower d e p t h . During this test.5 p e r c e n t .0 p e r c e n t and an average swelling pressure of 15. 0 0 0 psf. w i t h swelling pressure ranging from 2 .3 t o 3. w i t h t h e highest m o i s t u r e c o n t e n t being 22. N o n e of t h e test pits n o r test holes. This indicated t h a t leakage of the utility lines was n o t t h e main p r o b l e m at this building. Since t h e m a i n source of w a t e r w h i c h entered t h e building is from t h e surface runoff. 5 psi. At t h e west side t h e r e is 3 feet of cut and at t h e east side t h e r e is 1-1/2 feet of fill. T h e average m o i s t u r e c o n t e n t b e n e a t h t h e interior footings was 15.0 p e r c e n t . t h e pressure only d r o p p e d from 3 4 t o 3 3 . T h e f o u n d a t i o n soils at this site consist of p a r t fill and p a r t cut. t h e underslab h e a t i n g system was checked and only slight leakage was found. t h e r e is still the c h a n c e of m o i s t u r e migration from t h e wet area t o t h e dry area. T h e p e r i m e t e r m o i s t u r e c o n t e n t at a d e p t h of 3 feet was 21.1 percent 15. T h e p o t e n t i a l for further floor m o v e m e n t at this building is great.1 p e r c e n t . Soils at footing level in Test Pit 112 possessed only low swelling potential.4 p e r c e n t .236 FOUNDATIONS ON EXPANSIVE SOILS T h e swelling p o t e n t i a l b e n e a t h t h e footings was erratic. This c o r r e s p o n d s t o an average swelling p o t e n t i a l of 6.000 psf. T h e c o n d i t i o n of soils b e n e a t h t h e e x t e r i o r footings is represented b y Test Pits 112 and 1 1 3 . A s t u d y of t h e average m o i s t u r e c o n t e n t at t h e p e r i m e t e r of t h e slab indicated t h a t t h e average m o i s t u r e c o n t e n t was 2 1 . T h e soils directly b e n e a t h t h e floor slab had an average m o i s t u r e c o n t e n t of 16. T h e w a t e r seeped i n t o t h e pit from t h e underslab gravel and drained away in several d a y s . Moisture d i s t r i b u t i o n was erratic. T h e remedial m e a s u r e s given u n d e r " T r e a t m e n t at A s p e n " can be applied in their entirety h e r e . 2.5 percent. F o r t h e floor slabs. After removal. T h e r e is some chance of m o v e m e n t of t h e interior footings. T h e walk should b e at least 8 feet w i d e w i t h sufficient slope t o allow free drainage of w a t e r away from t h e cottages. a careful check should b e m a d e of t h e void space b e n e a t h t h e grade b e a m s t o assure t h a t it is p r o p e r l y formed.9 p e r c e n t and an average swelling pressure of 3 .8 p e r c e n t and a swelling pressure of 3 . A t a d e p t h of 6 feet b e l o w t h e t o p of t h e floor slab. some of t h e n a t u r a l soils possessed high swelling p o t e n t i a l . R e m o v e all backfill t o e x p o s e t h e f o u n d a t i o n system. A slope of 10 inches for t h e first 10 feet is r e c o m m e n d e d . 3 .2 percent.2 p e r c e n t . A s t u d y of t h e m o i s t u r e c o n t e n t of t h e underslab soils indicated t h a t t h e m o i s t u r e c o n t e n t directly b e n e a t h t h e floor slab was high and t h e p e r i m e t e r m o i s t u r e c o n t e n t was even higher t h a n t h e central m o i s t u r e c o n t e n t . With p r o p e r drainage. 0 0 0 psf. t h e possibility of f o u n d a t i o n m o v e m e n t is r e m o t e . Again in 1 9 6 8 . T h e swelling p o t e n t i a l at footing level was low b u t at lower d e p t h s . This c o r r e s p o n d s t o an average swelling p o t e n t i a l of 1. The average perimeter moisture content was on the order of 21. T h e remedial m e a s u r e s for this building are essentially t h e same as t h o s e r e c o m m e n d e d for Birch. T h e g r o u n d surface s u r r o u n d i n g t h e cottages should be sloped t o drain away from t h e cottages. t h e c h a n c e of further floor m o v e m e n t is n o t great. Most of t h e interior footings are placed o n n a t u r a l soils. Drainage improvement general In a d d i t i o n t o t h e remedial measures given for each building. T h e average m o i s t u r e c o n t e n t of t h e soils b e n e a t h t h e interior footings was 19. T h e m o i s t u r e c o n t e n t was relatively u n i f o r m w i t h i n 4 feet below t h e surface of t h e slab.4 p e r c e n t w i t h a swelling pressure of 6 .3 p e r c e n t and an average swelling pressure of 3 . F r o m t h e s t u d y of t h e soil b e h a v i o r b e n e a t h t h e e x t e r i o r footings. t h e swelling p o t e n t i a l reached as high as 2. It is n o t necessary t o u n d e r p i n t h e footings. This c o r r e s p o n d s t o an average swelling p o t e n t i a l of 2. T h e m o i s t u r e c o n t e n t decreased w i t h d e p t h . definite leakage was found in t h e underslab heating system. A c o n c r e t e walk should be provided a r o u n d t h e e x t e r i o r of each cottage. 5 0 0 psf. .3 p e r c e n t . T h e excavation should remain o p e n for a period of at least 1 week so t h a t all w a t e r w h i c h is t r a p p e d u n d e r t h e floor slab can be drained o u t . t h e r e c o m m e n d e d p r o c e d u r e is a choice b e t w e e n removal of t h e soils b e n e a t h t h e slab as r e c o m m e n d e d for Aspen or saw-cutting t h e floor slabs and improving t h e p a r t i t i o n walls as r e c o m m e n d e d for Birch. it m a y be necessary t o install swales at various l o c a t i o n s o u t s i d e of t h e cottages t o lower t h e g r o u n d surface and allow free drainage. n o free w a t e r was found. Tests indicated t h a t t h e pressure d r o p p e d from 35 t o 2 2 psi in 5 m i n u t e s .DISTRESS CAUSED BY HEAVING OF SLABS 237 In Test Pit 1 1 3 . 0 0 0 psf. 0 0 0 psf. An e x p a n s i o n j o i n t should b e provided b e t w e e n t h e walk and t h e grade b e a m . This indicated t h a t t h e underslab soils were w e t t e d b y leakage of t h e utility lines as well as b y surface runoff. This c o r r e s p o n d s t o an average swelling p o t e n t i a l of 1. If this slope in u n a t t a i n a b l e . as m u c h as 8. T h e walks should n o t be tied i n t o t h e grade b e a m s . t h e following t r e a t m e n t for improving t h e e x t e r i o r drainage was r e c o m m e n d e d : 1. T h e soils directly b e n e a t h t h e floor slab had an average m o i s t u r e c o n t e n t of 20. Installation of subdrains around the perimeter of the building. T h e lawn sprinkler heads should be located at least 10 feet from t h e f o u n d a t i o n walls of t h e buildings. t h e remedial c o n s t r u c t i o n has proven successful. T h e r o o f d o w n s p o u t s are n o t efficient. 5.238 FOUNDATIONS ON EXPANSIVE SOILS 4. m o s t of t h e damage could have b e e n avoided. Budget limitations did n o t allow t h e full remedial c o n s t r u c t i o n . was c o m p l e t e d in J u l y . It is entirely possible t h a t during heavy s t o r m s . It is u n f o r t u n a t e t h a t t h e severe swelling p o t e n t i a l of t h e subsoil had n o t b e e n recognized in t h e design stage. I m p r o v e m e n t is necessary in this area. Figure 149. . overflow. Had t h e buildings b e e n founded o n piers r a t h e r t h a n o n individual pads and had t h e underslab heating system b e e n eliminated. and n o t drain away t h r o u g h t h e d o w n s p o u t s . Considering t h e e x t e n t of original d a m a g e . All r e c o m m e n d a t i o n s were carried o u t e x c e p t t h e u n d e r p i n n i n g of t h e e x t e r i o r footings. Spray from buildings. 6. Drainage is n o t a b l y p o o r in front of t h e service buildings. at a cost of a half million dollars. t h e sprinkler heads should n o t be directed t o w a r d s t h e REMEDIAL CONSTRUCTION R e m e d i a l c o n s t r u c t i o n started in S e p t e m b e r 1 9 7 1 . These areas should be paved w i t h c o n c r e t e if positive surface drainage is n o t possible. T h e remedial c o n s t r u c t i o n . Figures 149 t h r o u g h 154 show certain c o n s t r u c t i o n p r o c e d u r e s . w a t e r could collect. 1 9 7 2 . Checking the void forming material beneath the grade beams. 239 .DISTRESS CAUSED BY HEAVING OF SLABS Figure 150. Repair of leaking plumbing.240 FOUNDATIONS ON EXPANSIVE SOILS Figure 151. . Note catch basins in the lawn area. Drainage around exterior of the building has been improved. .DISTRESS CAUSED BY HEAVING OF SLABS 241 Figure 152. Note: Further cracking has not occurred. Figure 154.242 FOUNDATIONS ON EXPANSIVE SOILS Figure 153. patched and repaired. Severely cracked brick wall. Concrete apron placed around the building w i t h properly constructed mastic joints. . slight f o u n d a t i o n m o v e m e n t will result in severe cracking. S t r u c t u r a l s t r e n g t h in t h e f o u n d a t i o n walls was lacking. t h e r e f o r e . HISTORY T h e h o u s e is located in Broomfield. N e i t h e r a soil investigation r e c o r d n o r s t r u c t u r a l design drawings were available. Also. C o l o r a d o . A n y slab m o v e m e n t will cause t h e p a r t i t i o n wall t o p u s h against t h e u p p e r floor joists and cause m o v e m e n t in t h e u p p e r levels. Such f o u n d a t i o n design creates a structural weakness in t h e m i d d l e p o r t i o n of t h e b a s e m e n t wall. a small c o m m u n i t y n o r t h of Denver. N o r e i n f o r c e m e n t was found in t h e c o n c r e t e f o u n d a t i o n wall o r in t h e footings. t h e r e is a s t r u c t u r a l d i s c o n t i n u i t y at t h e w a l k . W i t h o u t r e i n f o r c e m e n t in t h e c o n c r e t e . t h e b a s e m e n t f o u n d a t i o n wall is s t e p p e d d o w n from full b a s e m e n t height at t h e s o u t h end t o o n l y 2 4 inches at t h e n o r t h e n d . T h e h o u s e was c o n s t r u c t e d in 1 9 6 0 .o u t e n t r a n c e . it was n o t possible t o u n d e r p i n t h e building. This area is well k n o w n for its swelling soil p r o b l e m . Test pits were excavated t o e x a m i n e t h e f o u n d a t i o n system of t h e h o u s e . N o n e of t h e slab-bearing p a r t i t i o n walls have slip j o i n t s e i t h e r at t h e t o p or b o t t o m . In this instance t h e s t u d y involves a h o u s e t h a t was c o n s t r u c t e d w i t h o u t benefit of a d e q u a t e design. Use of post-tensioned steel o r pouring of a n e w f o u n d a t i o n wall appears t o b e t h e o n l y possible remedial c o n s t r u c t i o n . T o c o n f o r m w i t h t h e n a t u r a l g r o u n d c o n t o u r . b o t h o n t h e west and east sides in t h e brick walls above t h e b a s e m e n t . Exterior A t t h e e x t e r i o r of t h e h o u s e .Case IV DISTRESS CAUSED BY HEAVING OF CONTINUOUS FOOTINGS GENERAL This case s t u d y is typical of w h a t h a p p e n s w h e n c o n t i n u o u s footings are placed o n expansive soil w i t h o u t considering uplift forces. T h e footings are 2 0 inches wide and 8 inches in d e p t h . Most of t h e cracks appeared directly below and above . severe cracks were found. By so doing. Color a d o . at t h e n o r t h end of* t h e b a s e m e n t . This revealed t h a t t h e h o u s e is f o u n d e d w i t h c o n t i n u o u s spread footings o n t h e n a t u r a l soils. T h e h o u s e has a full b a s e m e n t and a t t a c h e d garage. w h e r e t h e grade b e a m is d i s r u p t e d . t h e h o u s e will be tied t o g e t h e r as a b o x and will be able t o w i t h s t a n d further differential movement. T h e space b e t w e e n t h e c o n c r e t e wall and brick course is filled w i t h cinder b l o c k . Cracks below window. exposing a p o r t i o n of t h e footings. as m u c h as three-quarters of an inch. . Cracks o p e n e d (fig. Basement interior T h e b a s e m e n t floor slab had b e e n r e m o v e d .244 FOUNDATIONS ON EXPANSIVE SOILS w i n d o w s in t h e m i d d l e p o r t i o n of t h e b a s e m e n t section of t h e h o u s e . at t h e east side of t h e h o u s e . A 4-inch gravel layer was placed b e n e a t h t h e b a s e m e n t slab. t h e b a s e m e n t floor slab was b a d l y cracked. Exterior drainage E x t e r i o r drainage c o n d i t i o n s of t h e h o u s e are p o o r . N a t u r a l drainage is from west t o east. 155). and e x t e n d e d t o t h e c o n c r e t e b e n e a t h t h e cinder b l o c k . Cracks were found in t h e footings as well as in t h e f o u n d a t i o n walls. leading t o a s u m p . 1 -_ T 1 _. Cracks were evident above w i n d o w s . Prior t o removal. N o severe cracks were found on t h e s o u t h side of t h e building. Upper floor T h e a m o u n t of cracking in t h e u p p e r floor is also severe as s h o w n o n figure 156.1 ]]·]ΐ|Μμ| zzrr • ι I I ™ _ I " 1 cri Figure 155. particularly at t h e n o r t h side of t h e h o u s e w h e r e t h e walk-out b a s e m e n t d o o r is located. At the n o r t h side of t h e building. and there is a strong t e n d e n c y for surface w a t e r t o e n t e r t h e f o u n d a t i o n soils from t h e west side. A t r e n c h was opened a r o u n d t h e b a s e m e n t p o r t i o n of t h e h o u s e in an a t t e m p t t o install drain tile a r o u n d t h e b a s e m e n t wall. severe cracks were also found below t h e w i n d o w s . and a few d o o r s were j a m m e d in t h e u p p e r level. SUBSOIL CONDITION T h e subsoil c o n d i t i o n s at t h e site consist essentially of slightly p o r o u s s a n d y clays at t h e s o u t h end. m o i s t c l a y s t o n e at t h e n o r t h end. and also t h e f o u n d a t i o n walls are n o t reinforced. N o free w a t e r was found in t h e test pits w h i c h had a d e p t h of 5 feet b e n e a t h t h e b o t t o m of t h e footings. Severe cracking above closet. T h e soils. and highly w e a t h e r e d . at present. indicated t h a t t h e u p p e r s a n d y clays at t h e s o u t h side of t h e building possessed o n l y low swelling p o t e n t i a l . CAUSE O F MOVEMENT T h e cause of m o v e m e n t of t h e h o u s e is d u e t o a c o m b i n a t i o n of uplifting m o v e m e n t of t h e f o u n d a t i o n soils and p o o r s t r u c t u r a l design as s u m m a r i z e d b e l o w : 1. p e r f o r m e d o n t h e u n d i s t u r b e d h a n d drive samples. Swell-consolidation tests. are in a very m o i s t c o n d i t i o n . while t h e w e a t h e r e d c l a y s t o n e at t h e n o r t h side of t h e building possessed m o d e r a t e t o high swelling p o t e n t i a l . U n d i s t u r b e d h a n d drive samples were t a k e n from t h e test pits. and t h e swelling p o t e n t i a l of t h e claystone should b e m u c h higher t h a n t h e tests indicate. T h e r e is a difference in d e p t h of t h e c o n c r e t e f o u n d a t i o n walls.DISTRESS CAUSED BY HEAVING OF CONTINUOUS FOOTINGS 245 Figure 156. It is a p p a r e n t t h a t t h e soils were in a m u c h drier c o n d i t i o n w h e n t h e h o u s e was c o n s t r u c t e d . T h e s t r u c t u r e of t h e h o u s e c a n n o t w i t h s t a n d even slight . C o n s e q u e n t l y . Judging from t h e high m o i s t u r e c o n t e n t of t h e f o u n d a t i o n soils. 3.246 FOUNDATIONS ON EXPANSIVE SOILS differential m o v e m e n t . T h e swelling of t h e w e a t h e r e d claystone requires o n l y slight m o i s t u r e increase. A t o n e t i m e . 2. T h e p u r p o s e of t h e post-tensioned cables is t o tie t h e entire s t r u c t u r e t o g e t h e r t o prevent u n e q u a l m o v e m e n t . t h e weight of t h e building will be c o n c e n t r a t e d at isolated locations and dead load pressure increased. It is estimated t h a t t h e a m o u n t of d e a d load pressure exerted o n t h e e x t e r i o r footings is a b o u t 6 0 0 psf. This is s u b s t a n t i a t e d b y t h e fact t h a t t h e f o u n d a t i o n wall. 0 0 0 psf. All shrubs and flower b e d s adjacent t o t h e building should b e removed and all r o o f d o w n s p o u t s should e x t e n d well b e y o n d t h e limit of all backfill. as well as t h e f o u n d a t i o n . T h e presence of free w a t e r is n o t necessarily t h e cause of these swelling c o n d i t i o n s . T h e new f o u n d a t i o n wall should have a d e p t h of a p p r o x i m a t e l y t h e full height of t h e b a s e m e n t . t o p and b o t t o m . T h e soils should b e removed intervals. T h e drainage a r o u n d t h e building should b e improved so t h a t w a t e r will drain away from t h e building. it is evident t h a t t h e entire area has b e e n u n d e r severe w e t t i n g c o n d i t i o n s . A n alternative remedial c o n s t r u c t i o n m e t h o d is installation of post-tensioned steel cables in b o t h t h e o u t s i d e and inside of t h e f o u n d a t i o n walls in t h e m a n n e r indicated in figure 159. have b o t h cracked severely. T h e new grade b e a m s should b e reinforced w i t h t w o 5/8-inch bars. t h e f o u n d a t i o n at t h e n o r t h end of t h e building has lifted. Air space b e n e a t h t h e footings should b e formed b y t h e use of void forming material. REMEDIAL MEASURES Since t h e cause of m o v e m e n t is d u e t o b o t h t h e structural weakness of t h e building and t h e swelling of the claystone beneath the footings. w a t e r seeped i n t o t h e b a s e m e n t at t h e west side t h r o u g h t h e seams b e t w e e n t h e c o n c r e t e and cinder b l o c k wall. t h e dead load pressure will b e increased t o a b o u t 3 . 2. This wall will tie t h e b a s e m e n t t o g e t h e r structurally and eliminate t h e existing structural weakness. T h e a m o u n t of swelling of t h e claystone at t h e n o r t h end of t h e building is a b o u t 5 times as m u c h as t h e a m o u n t of swell of t h e sandy clays at t h e s o u t h end of t h e building u n d e r t h e same m o i s t u r e c o n d i t i o n s . M e a s u r e m e n t confirms t h a t t h e r e is a difference in elevation b e t w e e n t h e n o r t h and s o u t h e n d s of t h e building b y as m u c h as 5 inches. 4 . Such w e t t i n g c o n d i t i o n s have caused t h e f o u n d a t i o n soils t o swell. 3 . the following remedial measures are recommended: 1. and should be tied in w i t h t h e existing f o u n d a t i o n wall in t h e m a n n e r s h o w n in figure 157. . In so doing. With voids formed b e n e a t h t h e footings. N o free w a t e r was found b e n e a t h t h e footings at t h e t i m e of inspection. Such dead load pressure is insufficient t o prevent t h e uplifting of t h e c l a y s t o n e b e n e a t h t h e footings. A n e w f o u n d a t i o n wall should b e p o u r e d a r o u n d t h e interior of t h e b a s e m e n t . eliminating t h e uplifting p r o b l e m . as s h o w n in figure from b e n e a t h t h e footings at a p p r o x i m a t e l y 10-foot 158. b u t will b r e a k any capillary w a t e r rise. However. 160). . t h e use of a subsurface drainage system will k e e p t h e w a t e r table b e l o w b a s e m e n t level. b u t it will t a k e a period of at least 6 m o n t h s b e f o r e equilibrium can b e established. T h e posttensioned cable system w a s used t o s t r e n g t h e n t h e f o u n d a t i o n (fig. T h e installation of a subsurface drainage system will n o t i m p r o v e t h e p r e s e n t situation. it is believed t h a t m o v e m e n t of t h e h o u s e will s t o p . T h e residence is in good c o n d i t i o n after t h e remedial c o n s t r u c t i o n .DISTRESS CAUSED BY HEAVING OF CONTINUOUS FOOTINGS 247 SECTION AA SECTION Β Β Figure 157. If t h e remedial m e a s u r e s presented above are p e r f o r m e d . T h e use of a gravel layer b e n e a t h t h e slab will n o t p r e v e n t cracking of t h e slab. Interior decorating and repairs should not begin u n t i l equilibrium has b e e n established. 5. Elevation pins should b e established a r o u n d t h e h o u s e and r e c o r d s k e p t as t o t h e a m o u n t of m o v e m e n t . Sketch of new grade beam for basement. R e m e d i a l c o n s t r u c t i o n was started shortly after t h e h o u s e was investigated. Pit I Removal of soils beneath the footings. .248 FOUNDATIONS ON EXPANSIVE SOILS • Figure 158. Existing foundation wall (concrete in good condition) Floor Slab 7^ SECTION A ·A Figure 159.DISTRESS CAUSED BY HEAVING OF CONTINUOUS FOOTINGS 249 PLAN Post Tension Cables installed maximum Γ -θ" below window. Remedial measures using post-tensioned steel cables. . Cable cased in grouted tubes.i.s. Applied tension on the order of 100 p. 250 FOUNDATIONS ON EXPANSIVE SOILS Figure 160. Post-tensioning the foundation wall. . n i n e t o w n h o u s e s are u n d e r investigation for f o u n d a t i o n m o v e m e n t . A r e p o r t outlining remedial c o n s t r u c t i o n was prepared b u t no corrective a c t i o n was t a k e n . 1 9 6 5 .Case V DISTRESS CAUSED BY RISE OF WATER TABLE GENERAL This case s t u d y involves 3 9 t w o . T h e t o w n h o u s e s are located in s o u t h e a s t Denver. t h a t had f o u n d a t i o n m o v e m e n t . Heaving of t h e floor slab can t r a n s m i t high swelling pressure t o t h e grade b e a m s . and in s o m e buildings severe m o v e m e n t of b o t h f o u n d a t i o n walls and floor slabs was n o t i c e d . A total of 2 5 1 u n i t s and a c l u b h o u s e were studied (fig. in this case. 3. In March. 0 0 0 psf and a m i n i m u m dead load pressure of 10. is ineffective. a consulting soil engineer was engaged t o m a k e a preliminary investigation i n t o t h e cause of cracking and m o v e m e n t in t h e various u n i t s . T h e following k n o w l e d g e was gained from this s t u d y : 1.s t o r y t o w n h o u s e s f o u n d e d o n a drilled pier system. C o l o r a d o . T h e m o v e m e n t of t h e drilled pier system is essentially caused b y a rise of ground w a t e r . 161). T h e w i d t h of t h e cracks ranged from hairline thickness t o as m u c h as 1 inch as s h o w n o n figures 162 and 1 6 3 . C o n s t r u c t i o n of t h e building c o m p l e x was started in S e p t e m b e r . 1 9 7 0 . 2. In areas w h e r e t h e r e is a strong possibility of rise of g r o u n d w a t e r . S h o r t l y after the c o m p l e t i o n of t h e various u n i t s . T h e s e cracks definitely indicated pier . T h e r e are four t o seven u n i t s in each building. HISTORY T h i r t y .000 Cracks were found in t h e brick course in a diagonal p a t t e r n from t h e t o p of t h e w i n d o w or d o o r o n t h e g r o u n d level t o t h e b o t t o m of t h e w i n d o w in t h e u p p e r level. T h e r e p o r t resulting from t h a t investigation r e c o m m e n d e d psf. T h e following typical distress was observed in m o s t buildings: Foundation walls t h a t t h e buildings b e founded w i t h piers drilled i n t o b e d r o c k designed for a m a x i m u m end pressure of 2 0 . m o v e m e n t of t h e buildings b e g a n . Most of t h e cracks had b e e n p a t c h e d and some had r e o p e n e d . t h e use of a drilled pier system should be carefully considered. Initial subsoil investigation for t h e site was m a d e in 1 9 6 5 . Chemical t r e a t m e n t of t h e u n d e r s l a b soil. C o n c r e t e floor slabs were covered with tile o r carpeting. In a d d i t i o n . Interior floor slabs Most of t h e b a s e m e n t areas were finished. C o n s e q u e n t l y . w h e n t h e slabs m o v e d u p w a r d . b u t also exerted uplift All buildings have u n i t s w i t h p a r t i t i o n s and in m o s t cases slip j o i n t s were n o t provided in t h e slab bearing p a r t i t i o n walls. cracks appeared pressure o n t h e f o u n d a t i o n walls. This is a typical slab crack w h e r e separation b e t w e e n t h e slab and f o u n d a t i o n wall was n o t p r o p e r l y c o n s t r u c t e d . Building location plan and ground-water contour. Such m o v e m e n t n o t o n l y caused t h e slab cracking. j a m m i n g t h e d o o r s and d i s t o r t i n g t h e floor system. T h e slabs bind o n t h e f o u n d a t i o n walls and are n o t free t o accept vertical m o v e m e n t . m o v e m e n t . . T h e slabs had heaved in m a n y buildings and t h e cracks generally followed a p a t t e r n parallel t o t h e f o u n d a t i o n walls. Partition walls parallel t o the f o u n d a t i o n walls. cracks developed in t h e u p p e r stories. As a result. t h e y exerted uplifting pressure o n t h e u p p e r s t r u c t u r e .252 FOUNDATIONS ON EXPANSIVE SOILS 95 70 GROUNDWATER CûNTO'Jf c Figure 161. C o n s e q u e n t l y . b o t h vertical and diagonal cracks were found in t h e b a s e m e n t f o u n d a t i o n walls. Typical cracking of the townhouse exterior walls. .DISTRESS CAUSED BY RISE OF WATER TABLE 253 Figure 162. 254 FOUNDATIONS ON EXPANSIVE SOILS Figure 163. Typical cracking of townhouse exterior walls indicating pier uplift. . t h e average m o i s t u r e c o n t e n t of t h e u p p e r soil w a s o n l y 14. T h e r e f o r e . however. 0 0 0 psf. T h e lower b e d r o c k consists basically of claystone and w e a t h e r e d claystone.5 t o 3 p e r c e n t w i t h t h e swelling pressure ranging from 0 t o 5 . t h e staircase walls in all cases rest directly o n t h e b a s e m e n t floor. t h e increase of m o i s t u r e c o n t e n t is n o t as obvious as t h e increase of m o i s t u r e c o n t e n t o f t h e u p p e r clays. Aprons C o n c r e t e a p r o n s c o n s t r u c t e d at t h e rear of t h e buildings were in m o s t cases cracked.DISTRESS CAUSED BY RISE OF WATER TABLE 255 In t h o s e t o w n h o u s e s w h i c h had b a s e m e n t areas t h a t w e r e n o t finished. A s t u d y of m o i s t u r e c o n t e n t of t h e c l a y s t o n e b e d r o c k also indicates an increase in t h e last few years. Tests indicated t h a t t h e application of chemicals o n t h e u n d e r s l a b soil did n o t have a p r o n o u n c e d effect in reducing t h e swelling p o t e n t i a l . After t h e new slab had b e e n placed. T h e r e is n o d o u b t t h a t t h e m o v e m e n t of t h e buildings can be a t t r i b u t e d t o a swelling soil p r o b l e m . 0 0 0 t o 2 5 . slab m o v e m e n t d o e s n o t c o n t r i b u t e t o t h e distress of t h e u p p e r stories. in several buildings mainly at t h e w e s t e r n p o r t i o n of t h e site. In general. A t t h a t t i m e . T h e swelling p o t e n t i a l of t h e u p p e r clays ranges from 0. This m o i s t u r e c o n t e n t is considerably higher t h a n t h e m o i s t u r e c o n t e n t r e p o r t e d b y t h e testing laboratories in August of 1 9 6 5 . t h e floor slab was removed and t h e soil b e n e a t h t h e slab injected w i t h stabilizing chemicals in an effort t o eliminate t h e swelling p o t e n t i a l of t h e lower soils.6 p e r c e n t . slabs again cracked. 8 p e r c e n t and t h e lowest of o n l y 11. slab m o v e m e n t can t r a n s m i t m o v e m e n t t o t h e u p p e r stories t h r o u g h t h e staircase p a r t i t i o n walls. T h e average swelling pressure is a b o u t 1 0 . 0 0 0 psf. 0 0 0 psf. T h e clays have an average m o i s t u r e c o n t e n t of 2 0 . m o v e m e n t of t h e floor slab was n o t checked and in m o s t cases. t h e cracks t h a t developed in this a p a r t m e n t c o m p l e x typify t h e cracks found in buildings f o u n d e d o n expansive soils. This can b e caused b y either t h e uplifting of t h e building relative t o t h e slab or t h e s e t t l e m e n t of t h e slab d u e t o i n a d e q u a t e backfill relative t o t h e building. SUBSOIL CONDITIONS Subsoil c o n d i t i o n s at t h e site consist of 7 t o 2 2 feet of stiff t o m e d i u m stiff clays overlying claystone b e d r o c k . 5 0 0 psf and t h e m o i s t u r e c o n t e n t s at various d e p t h s w e r e fairly u n i f o r m . Since t h e m o i s t u r e c o n t e n t of t h e c l a y s t o n e b e d r o c k is affected b y t h e p a t t e r n of seams and fissures of b e d r o c k . Slab treatment B e t w e e n F e b r u a r y and April of 1 9 6 6 . Gaps w e r e found b e t w e e n t h e slabs and t h e buildings. 3 p e r c e n t w i t h t h e highest m o i s t u r e c o n t e n t of 2 3 . S o m e s a n d s t o n e lenses were found in t h e claystone. indicating separation. T h e results o f t h e swell tests indicated t h a t t h e swelling pressure of t h e u n d e r s l a b soils ranged from 5 0 0 t o 2 .6 p e r c e n t . T h e swelling pressure of t h e lower claystone ranges from 5 . T h e a p r o n s were m u d j a c k e d in several buildings in an a t t e m p t t o correct t h e c o n d i t i o n . . t h e following w e r e derived: . 1970 w h e n this investigation was m a d e . t h e buildings suffering t h e m o s t severe d a m a g e are located in high water-table elevation areas. WATER TABLE When t h e soil and f o u n d a t i o n investigation was m a d e in A u g u s t . 2. A difference in t h e water-table elevation of 3 0 feet was observed b e t w e e n t h e n o r t h w e s t c o r n e r and t h e s o u t h e a s t c o r n e r of t h e p r o p e r t y . T h e g r o u n d surface was high in t h e west and low in t h e east. after area d e v e l o p m e n t . t h e swelling of t h e u p p e r clays is sufficient t o cause t h e floor slabs t o heave and crack. Figure 161 indicates an a p p r o x i m a t e c o n t o u r of equal elevation t o w a t e r t a b l e . t h e r e was a definite change of water-table c o n d i t i o n s . 3 . T h e water-table elevation is high at t h e w e s t e r n p o r t i o n of t h e site and low at t h e eastern p o r t i o n . however. Where t h e water-table elevation is low. Free w a t e r flows m o s t l y o n t o p of b e d r o c k and also flows in t h e fissures and seams of t h e b e d r o c k . Water was found at t h e t o p o r i m m e d i a t e l y b e l o w t h e surface of b e d r o c k . S o m e of t h e test holes were m o r e t h a n 2 0 feet d e e p . t h e grade b e a m s and pier system and t h e results of this investigation. As seen o n figure 1 6 1 . After carefully s t u d y i n g t h e c o n t o u r of equal elevation t o t h e w a t e r table and t h e general water-table c o n d i t i o n s in t h e area. T h e following c o n d i t i o n s w e r e observed: 1. Surface w a t e r d o e s n o t necessarily p e n e t r a t e directly from t h e g r o u n d surface i n t o t h e u n d e r l y i n g b e d r o c k b u t will flow from t h e high p o i n t t o t h e low p o i n t along t h e surface of bedrock. T h e w a t e r table follows fairly well w i t h t h e g r o u n d surface c o n t o u r . free w a t e r was found in almost every h o l e . Most of t h e f o u n d a t i o n wall m o v e m e n t is t h e d i r e c t result of t h e swelling of t h e lower c l a y s t o n e . T h e water-table c b n t o u r follows fairly well w i t h t h e b e d r o c k c o n t o u r .256 FOUNDATIONS ON EXPANSIVE SOILS It is c o n c l u d e d from t h e l a b o r a t o r y testing t h a t t h e u p p e r clays possess o n l y low swelling p o t e n t i a l and t h e lower c l a y s t o n e possess high swelling p o t e n t i a l . T h e general rise of t h e water-table is essentially caused b y a p e r c h e d w a t e r table in t h e developed residential area. relatively m i n o r d a m a g e t o t h e buildings was e x p e r i e n c e d . In J u l y . 1 9 6 5 . CAUSE O F MOVEMENT In reviewing t h e f o u n d a t i o n design. T h e rise of w a t e r table definitely has a bearing o n t h e f o u n d a t i o n m o v e m e n t of this a p a r t m e n t c o m p l e x . B e d r o c k in this area is shallow and c o m p o s e d essentially of claystone w h i c h is relatively impervious. n o free w a t e r was found in a n y of t h e 2 1 e x p l o r a t o r y holes. it is c o n c l u d e d t h a t in t h e last 5 years. With t h e piers spaced o n 11-foot centers. t h e entire lower b e d r o c k b e c a m e w e t t e d . T h e load carrying capacity of t h e piers was reviewed. T h e drill logs provided b y t h e driller indicated t h a t t h e required pier p e n e t r a t i o n had b e e n fulfilled in m o s t cases. each pier can b e subject t o a b o u t 22 kips of uplifting pressure. T h e r e c o m m e n d e d d e a d load pressure of 1 0 . S u c h defects decrease t h e d e a d load pressure e x e r t e d o n t h e piers. Pier uplifting T h e m o s t i m p o r t a n t reason for t h e m o v e m e n t of t h e f o u n d a t i o n walls is t h e uplifting of t h e piers. t h e c o n s t r u c t i o n was in a c c o r d a n c e w i t h t h e s t r u c t u r a l f o u n d a t i o n design. T h e piers should p e n e t r a t e t h e shale a m i n i m u m d e p t h of 5 feet. t h e design r e c o m m e n d a t i o n s are in a range o f s o u n d engineering p r a c t i c e . T h e r e should b e positive e x p a n s i o n j o i n t s b e t w e e n t h e slabs and t h e grade b e a m s t o allow free slab m o v e m e n t . t h e swelling pressure of t h e l o w e r b e d r o c k is a b o u t 1 0 . t h e skin friction value dissipated and r a t h e r t h a n holding t h e piers. m u s h r o o m s w e r e found o n t o p of t h e piers and t h e void forming m a t e r i a l b e n e a t h t h e grade b e a m s i m m e d i a t e l y adjacent t o t h e piers was a b s e n t . it was assumed t h a t t h e l o w e r b e d r o c k w o u l d n o t b e c o m e w e t t e d and t h e skin friction b e t w e e n t h e b e d r o c k and t h e piers w o u l d provide an additional factor of safety against pier uplifting. e x c e p t for t h e piers s u p p o r t i n g i n t e r i o r c o l u m n s and b e a m s . t h e effect is n o t sufficient t o cause t h e p r e s e n t distress of t h e various buildings. In e x t r e m e cases. T h e swelling pressure of t h e u n d e r s l a b soil is a b o u t 2 . In t h e original f o u n d a t i o n design. and w e r e found t o b e in a c c o r d a n c e w i t h t h e r e c o m m e n d a t i o n s . In m o s t cases. 0 0 0 psf. t h e j o i n t b e t w e e n t h e slabs and t h e grade b e a m s is n o t effective in m o s t cases. free floating slabs should b e provided. 0 0 0 psf w i t h a few cases reaching as high as 2 5 .DISTRESS CAUSED BY RISE OF WATER TABLE 257 Foundation design T h e f o u n d a t i o n design criteria calls for an end-bearing pressure for piers of 2 0 . h o w e v e r . b u t considering t h e subsoils and t h e w a t e r table c o n d i t i o n s at t h e t i m e t h e subsoil investigation was m a d e . including t h e d e a d load pressure r e q u i r e m e n t . In several places. T h e side of t h e pier excavated i n t o t h e shale should b e r o u g h e n e d t o provide resistance t o uplift. With t h e rise of t h e w a t e r t a b l e . 0 0 0 psf. t h e pressure t r a n s m i t t e d from t h e slab t o t h e f o u n d a t i o n wall can reach 2 . . Slab m o v e m e n t d u e t o expansive soils has t r a n s m i t t e d uplifting pressure from t h e slabs t o t h e f o u n d a t i o n walls. c o n s e q u e n t l y . 0 0 0 p o u n d s per linear foot. 0 0 0 psf. Slab construction T h e m a j o r p r o b l e m is in t h e area of slab c o n s t r u c t i o n . In principle. 0 0 0 psf and a m i n i m u m dead load pressure of 1 0 . Foundation construction F r o m t h e i n f o r m a t i o n o b t a i n e d b y excavating from t h e inside and from t h e o u t s i d e of t h e building exposing t h e grade b e a m s y s t e m . A t p r e s e n t . 0 0 0 psf is low. This practice is n o t necessarily limited t o t h e p r o p e r t y completed. Before t h e d e v e l o p m e n t was c o n s t r u c t e d .5 . t e n d s t o a c c u m u l a t e instead of r u n n i n g off t h e site. can c o n t r i b u t e t o t h e rise of w a t e r table. High water-table c o n d i t i o n s prevailed in several areas in t h e general vicinity after t h e d e v e l o p m e n t was . 6 7.7X2. in isolated areas. A gap of a p p r o x i m a t e l y 3 inches b e t w e e n t h e soil and t h e b o t t o m of t h e pier was found. pier N o . This c o n s t i t u t e s an i m p o r t a n t factor t o w a r d t h e rise of g r o u n d water. In o n e test pit. investigated b u t pertains t o t h e entire general area.62X 1 0 . It is conceivable t h a t a large a m o u n t of lawn irrigation w a t e r will travel t h r o u g h t h e u p p e r soils and b e c o m e t r a p p e d at t h e surface of t h e b e d r o c k . t h e u n b a l a n c e d pressure is in t h e range of 10 t o 3 0 kips. O n l y a p o r t i o n o f t h e p r e c i p i t a t i o n p e n e t r a t e d t h e g r o u n d . 1 5 U n b a l a n c e d pressure 27.6 t o 20.000 psf 7 feet F o r interior piers w h e r e t h e actual dead load pressure exerted o n t h e pier ranges from 2. 6 kips 10 inches 10. lawn irrigation in t h e area will generally create perched water-table c o n d i t i o n s .9 kips.2 0 .258 FOUNDATIONS ON EXPANSIVE SOILS t h e swelling of t h e b e d r o c k actually lifted t h e piers. either from rain or from melting snow. T h e source of surface w a t e r is from the following: Precipitation Precipitation. m o s t of t h e precipitation drained from t h e site as surface runoff. It is concluded t h a t t h e w e t t i n g of t h e b e d r o c k plus t h e pressure t r a n s m i t t e d from slab heaving t o t h e grade b e a m had lifted t h e piers. precipitation will p e n e t r a t e t h e soils t h r o u g h the loose backfill a r o u n d each building and d u e t o p o o r drainage c o n d i t i o n s . With building c o n s t r u c t i o n . This verified t h a t t h e pier actually pulled o u t of t h e g r o u n d and w o u l d have cracked h a d it n o t b e e n reinforced. SOURCE O F MOISTURE T h e rise of g r o u n d w a t e r in this d e v e l o p m e n t is m a i n l y derived from surface w a t e r . Lawn irrigation After a d e v e l o p m e n t is c o m p l e t e d . As an e x a m p l e . t h e entire length of t h e pier was e x p o s e d . 8 in Building 8 was checked b y calculation as follows: A c t u a l dead load pressure o n t h e pier Pier d i a m e t e r Average swelling pressure Pier p e n e t r a t i o n i n t o b e d r o c k Uplifting pressure d u e t o s a t u r a t i o n of b e d r o c k . 0 0 0 X 0 .1 kips 27.5 kips 2 0 . 161). Area II In this area. resulting from c o r r o d e d service saddles. changing w a t e r table and o t h e r local c o n d i t i o n s m a y affect t h e stability of t h e s t r u c t u r e . F o u n d a t i o n m o v e m e n t in this area is caused b y t h e uplifting of t h e piers. in Buildings 8 8 . S o m e of t h e floor slabs have b e e n replaced and t r e a t m e n t of t h e underslab soils was m a d e in several buildings. w h i c h shows an increase of 4 million gallons over a period of 10 m o n t h s . relatively m i n o r f o u n d a t i o n m o v e m e n t has t a k e n place. t h e t o w n h o u s e c o m p l e x was divided i n t o t h r e e areas (fig.1 million gallons of w a t e r will eventually flow o n t h e surface of t h e b e d r o c k and cause a general rise of t h e w a t e r t a b l e . however. This is because of t h e high swell p o t e n t i a l of t h e l o w e r soils and t h e close p r o x i m i t y of t h e w a t e r table. T h e piers still m a i n t a i n an anchorage effect and expansive soils have n o t acted u p o n t h e surface of t h e piers. was found in front of Buildings 8 0 . T h e water-table elevation is relatively d e e p and t h e b o t t o m s of t h e piers are above t h e present w a t e r table. EVALUATION O F BUILDING CONDITIONS F o r discussion p u r p o s e s . This increase in c o n s u m p t i o n can be partially explained b y pipe leakage. 9 0 and 9 2 . 3 million gallons.DISTRESS CAUSED BY RISE OF WATER TABLE 259 Pipe leakage Leakage of t h e w a t e r lines. 2. T h e r e f o r e . During this same period in 1 9 7 0 . T h e buildings in this area have n o t suffered severe f o u n d a t i o n m o v e m e n t w h i c h is p r o b a b l y d u e t o t h e following: 1. These 4. It is possible t h a t these buildings will n o t suffer severe f o u n d a t i o n m o v e m e n t in t h e f u t u r e . It is n o t k n o w n h o w long t h e leakage had t a k e n place. and 8 4 . Most of t h e floor slabs in this area have b e e n replaced and t h e soils b e n e a t h t h e floor slabs have b e e n t r e a t e d w i t h special chemicals. b u t m o n t h l y w a t e r c o n s u m p t i o n from J a n u a r y t o O c t o b e r of 1969 was 2 4 . and t h e evaluations are as follows: Area I In this area. T h e l o w e r b e d r o c k consists of a c o m b i n a t i o n of s a n d s t o n e and c l a y s t o n e w h i c h d o e s n o t possess a high swell p o t e n t i a l . 8 3 . it is a n t i c i p a t e d t h a t severe m o v e m e n t will t a k e place in t h e near future. . t h e piers are relatively stable. F o r instance. severe f o u n d a t i o n m o v e m e n t has t a k e n place. T h e water-table elevation is high in this area and t h e b e d r o c k has b e e n w e t t e d excessively. 2 million gallons. T h e f o u n d a t i o n walls as well as t h e floor slabs have cracked and heaved. t h e c o n s u m p t i o n was 2 8 . T h e shims should b e adjusted w i t h an engineering level i m m e d i a t e l y after c o m p l e t i o n o f t h e p a d s and should b e readjusted after a period of 6 m o n t h s . it will probably basement. Since t h e water-table c o n d i t i o n s c a n n o t b e changed. If effective e x p a n s i o n j o i n t s c a n n o t b e o b t a i n e d . T h e c o n d i t i o n of these buildings is relatively g o o d . 5 0 0 psf and as m u c h dead load pressure as possible. t h e n slip j o i n t s should b e provided t o insure free p a r t i t i o n wall m o v e m e n t . be more economical to perform the underpinning o p e r a t i o n inside the . When t h e b a s e m e n t is finished. REMEDIAL MEASURES R e m e d i a l measures d e p e n d o n t h e e x t e n t of t h e present d a m a g e and can be best described u n d e r t h e t h r e e areas m e n t i o n e d b e f o r e : Area I In this area. 3 . t h e d a m a g e is of such e x t e n t t h a t drastic m e a s u r e s should b e t a k e n as recommended: 1. for t h e buildings in this area. this gap can b e tiled t o prevent dirt entering t h e gap. 2. All piers should be cut free from t h e f o u n d a t i o n system so t h a t t h e entire building will n o t b e associated w i t h t h e lower b e d r o c k . This is necessary because t h e source of t h e p r o b l e m . Since t h e swelling pressure o f t h e u p p e r soils in this area is a b o u t 2 . 0 0 0 psf. is caused b y t h e e x p a n s i o n of t h e lower b e d r o c k . n o r have t h e r e b e e n a n y remedial m e a s u r e s t a k e n . T h e use of interior slab-bearing p a r t i t i o n walls should b e discouraged. If such is necessary. At t h e n o r t h e a s t p o r t i o n of t h e site. 4. t h e g r o u n d w a t e r is well below t h e b o t t o m of t h e piers. Since it is necessary in m o s t cases t o r e m o v e t h e floor slabs. Shims should b e provided o n t o p of each pad so t h a t t h e elevation of t h e building can b e adjusted. T h e floor slabs have n o t b e e n replaced. a gap of a p p r o x i m a t e l y o n e half inch should b e left all a r o u n d t h e slab t o insure t h a t t h e slabs will n o t bind against t h e bearing walls. t h e r e should b e little danger of f o u n d a t i o n m o v e m e n t d u e t o t h e swelling of t h e u p p e r soils. T h e slip j o i n t should apply t o all d o o r frames and staircase walls.260 FOUNDATIONS ON EXPANSIVE SOILS Area III This area is located at t h e s o u t h w e s t p o r t i o n of t h e site w h e r e g r o u n d surface is high and t h e w a t e r table elevation is d e e p . Individual p a d s should b e provided b e n e a t h t h e f o u n d a t i o n walls t o s u p p o r t t h e building. T h e pads should b e designed for a m a x i m u m soil pressure of 2 . T h e above described u n d e r p i n n i n g o p e r a t i o n can be e x e c u t e d either from inside t h e b a s e m e n t area o r from o u t s i d e . it is necessary t o prevent direct c o n t a c t of t h e f o u n d a t i o n system w i t h t h e b e d r o c k . It should b e emphasized t h a t sheetr o c k o n b o t h sides of t h e p a r t i t i o n wall should also b e provided w i t h slip j o i n t s . 5. All floor slabs should b e separated from t h e bearing walls w i t h a positive expansion j o i n t . If t h e slabs are binding against t h e grade b e a m s . t h e entire building should b e releveled using shims o n t o p of each pier. T h e buildings are in relatively g o o d c o n d i t i o n and unnecessary a l t e r a t i o n s in this area should b e avoided. 7. t h e n t h e slabs should b e r e m o v e d and replaced w i t h an effective j o i n t system.DISTRESS CAUSED BY RISE OF WATER TABLE 261 6. It will be necessary t o carefully check all sewage and w a t e r pipes t o insure t h a t n o leakage has t a k e n place. it m a y be necessary t o provide a subsurface drainage system a r o u n d t h e p e r i m e t e r of t h e b a s e m e n t area. T h e end bearing pressure of t h e piers should n o t greatly exceed 4 0 . 2. close observation of f o u n d a t i o n m o v e m e n t should b e m a i n t a i n e d . t h e following are r e c o m m e n d e d : 1. Air space b e n e a t h t h e grade b e a m s should b e carefully checked for effectiveness. 3 . 0 0 0 psf. it is difficult t o specify the r e c o m m e n d e d remedial p r o c e d u r e for each building. 0 0 0 psf. such remedial m e a s u r e s are necessary and unavoidable. Where existing d a m a g e is relatively severe. 0 0 0 psf for t h e p o r t i o n of pier in b e d r o c k . If it is found at a later d a t e t h a t t h e r e are signs of f o u n d a t i o n m o v e m e n t . 4 . b u t t o insure t h a t n o further f o u n d a t i o n m o v e m e n t will t a k e place and t o eliminate s o m e of t h e existing d a m a g e . 5. Careful i n s p e c t i o n of t h e c o n d i t i o n of t h e floor slabs should be m a d e . T h e above remedial m e a s u r e s for Area I are expensive and difficult t o carry o u t . Area II R e m e d i a l m e a s u r e s for t h e buildings in Area II d e p e n d greatly u p o n individual building c o n d i t i o n s . . b o t h t h e s t r u c t u r a l engineer and t h e soils engineer should be informed so t h a t t h e y m a y d e t e r m i n e if remedial m e a s u r e s are necessary. Interior slab bearing p a r t i t i o n walls and drainage s y s t e m s should b e t r e a t e d as described u n d e r Area I Area III N o remedial m e a s u r e s are necessary in this area. However. Since the investigation covers as m a n y as 2 5 1 u n i t s . T h e soils engineer should be at t h e site during t h e e x e c u t i o n of t h e remedial measures r e c o m m e n d e d b y t h e s t r u c t u r a l engineer and t h e soils engineer. In s o m e cases. In general. This should b e carefully designed and p l a n n e d b y a s t r u c t u r a l engineer. w i t h a skin friction of 4 . Every effort should b e m a d e t o avoid t h e transmission of pressure from t h e slabs t o t h e f o u n d a t i o n walls. All m u s h r o o m s above t h e piers should b e r e m o v e d . T h e dead load pressure exerted o n t h e piers should b e n o t less t h a n 2 0 . Increase t h e d e a d load pressure exerted o n t h e piers b y eliminating a n u m b e r of piers and increasing t h e span b e t w e e n t h e piers. Gravel reduces volume change because it replaces the more active soil fraction. (passing the No. Apparatus 3. Some of these are size and shape of the soil particles. 1 upon drying. Because of the difficulty in evaluating these individual factors. Total volume change for expansive soils is determined from expansion plus shrinkage values for different ranges of water content.2 Expansion test data may be used to estimate the extent and rate of uplift in subgrades beneath structures or in structures formed from soils. Annual Book of ASTM Standards. Permeant water is applied axially for determining the effect of saturation and permeability. The specimens prepared for this test may also be used to determine the vertical or volume shrinkage as the water content decreases. applied loadings. Figure 1 illustrates the fixed-ring type. (3) influence of wetting on volume change. Consolidometers most used in the United States are of the fixed-ring and floating-ring types. Either . (2) rate of volume change. 4 standard sieve size (^ξ in. density. the volume-change properties cannot be predicted to any degree of accuracy unless laboratory tests are performed.2 The laboratory tests described herein are primarily intended for the study of soils having no particles larger than the No. G. 1. 4 standard ASTM sieve 3). 4 fraction of soils containing gravel material (plus No.1 The expansion characteristics of a soil mass are influenced by a number of factors. 4). and (4) axial permeability of laterally confined soil under axial load or no-load during expansion. C o m m e n t s are solicited. 2 Consulting Civil Engineer. Colo. Denver. If the test is made on the minus No. The test is made to determine (1) magnitude of volume change under load or no-load conditions. Significance 2. P a r t 3 0 . 3. it is important to test samples from the sites being considered. provided that natural conditions and operating conditions are duplicated. some adjustment is required in any analysis. HOLTZ2 1. T h e m e t h o d is b a s e d o n t h e experience of the submitter. for W i r e C l o t h S i e v e s for T e s t i n g P u r p o s e s . 2.1 Consolidometer—Conventional laboratory consolidometers are used for the expansion test. 3 S e e A S T M S p e c i f i c a t i o n E l l . 2. When uplift problems are critical. water content. load history and mineralogical and chemical properties.1 This method explains how to make expansion tests on undisturbed or compacted clay soil samples that have no particle sizes greater than τ$ in. Saturation (no drainage) takes place axially.APPENDIX A SUGGESTED METHOD OF TEST FOR ONE-DIMENSIONAL EXPANSION A N D UPLIFT PRESSURE OF CLAY SOILS1 S U B M I T T E D B Y W.). Scope 1. and shrinkage tests may be used to estimate the volume changes which will occur in soils T h i s s u g g e s t e d m e t h o d h a s n o official s t a t u s i n t h e S o c i e t y b u t is p u b l i s h e d a s i n f o r m a t i o n o n l y . high specimens will have a height of 1. For expan- specimen to allow application of water. The apparatus must allow vertical movement of the top porous stone for fixed-ring consolidometers. component parts.264 FOUNDATIONS ON EXPANSIVE SOILS of these is suitable. the friction between the soil specimen and container is smaller than with the fixed-ring apparatus. On the other hand. Both types are available commercially. although other sizes are used. all specimen movement relative to the container is upward during expansion. Sizes of container rings most commonly used vary between 4j-in.250 in. as expansion takes place. In the fixed-ring container. the diameter should be not less than 2 in. 1—Fixed-Ring Consolidometer. or vertical movement for top and bottom porous stones for floating-ring consolidometers. Measure the diameter of the specimen container ring to 0. A ring gage machined to the height of the ring container to an accuracy of 0. Porous stones are required at the top and bottom of the /Dial gage holder Clamping ring and water container J-Connecting rod ^-Gasket | 'Base p l a t e . for specimens of small diamter. except that the depth must not be less than f in. The specimen containers for the fixed-ring consolidometer and the floating-ring consolidometer consist of brass or plastic rings. the fixed-ring apparatus is more suitable for saturation purposes and when permeability data are required.' ' F I G . . deep. thus. However. In a test using the floating-ring apparatus. the ring gage for lj-in..001 in. diameter by 1} in. while large depths cause excessive side friction. Lesser depths introduce errors caused by the magnitude of surface disturbance. and other sion tests the larger diameter consolidation rings are preferred as they restrain fhe soil action to a lesser degree. deep and 2j-in. and the depth not greater than three tenths of the diameter. diameter by J in. In the floating-ring container. movement of the soil sample is from the top and bottom away from the center during expansion.001 in. is required.. becomes fully effective when the beam is balanced. The jack operates a yoke which extends up through the scale platform and over the specimen container resting on the platform. In addition to the container. Apparatus such as described in ASTM Method D 2435. accuracy. be careful to minimize disturbance of the soil specimen and to assure an exact fit of the specimen to the consolidometer container. Wire saws or trimming lathes may be used if a uniform tight fit of the specimen to the container is obtained. When sufficient specimen has been prepared so that it protrudes through the container ring. Test for Moisture-Density Rela4 Annual Book of ASTM Standards. Part 11. Test for One-Dimensional Consolidation Properties of Soils. Using 5. and place the assembly on the srmple in alignment with the guide arms.5-lb Rammer and 12-in. or extrusion devices may be used in conformance with the use of alternative apparatus and samples.4 Device for Preparation of Remolded Specimens—Compacted soil specimens are prepared in the consolidometer ring container. Procedure-Expansion Test 4. Remove the cutting bit. trim the excess material with a knife close to the cutting edge of the bit. thus applying or releasing load to the soil specimen. This procedure is satisfactory provided that the sampling has been done without any sidewall disturbance and provided that the core specimen exactly fits the container. and extrude the core directly into the container. flat cut surface of the specimen. fasten the cutting bit to the ring container. The desired applied pressure. and sensitivity for the work being performed.2. trimming lathes.3 Device for Cutting Undisturbed tions of Soils. and turn the container over. 3.2 Loading Device—A suitable device for applying vertical load to the specimen is required. in depth and of the same diameter as the container.) In trimming the sample. 4 is satisfactory and may be used.APPENDIX A 265 3. of material from the sides of the consolidometer specimen. The loading device may be platform scales of 1000 to 3000-lb capacity mounted on a stand and equipped with a screw jack attached underneath the frame. 3.1 Another satisfactory loading device utilizes weights and a system of levers for handling several tests simultaneously. (Alternatively. 3. which is measured on the scale beam. A compaction hammer of the same type required in Method A of ASTM Method D 698. trim the specimen flush with the surface of the container ring. Drop. Place a glass plate on the smooth. With the cutting stand guiding the bit. The yoke is forced up or down by operating the jack. trim it flush with the surface of the container ring with a straightedge cutting tool. the apparatus consists of an extension collar about 4 in. obtain a core of a diameter exactly the same as the diameter of the consolidometer specimen container. leaving very little material for the bit to shave off as it is pressed gently downward. 4 4. and knives for trimming the soil. Place the undisturbed soil block or core on the cutting platform. and cover it with a second . Hydraulic-piston or bellowstype loading apparatus are also very satisfactory if they have adequate capacity. (Other suitable procedures to accommodate guides for wire saws. a cutting stand with bit guide. mens—Perform the tests on hand-cut cube samples or core samples o£ a size that will allow the cutting of approximately I in.1 Preparation of Undisturbed Speci- Specimens—This apparatus consists of a cutting bit of the same diameter as the ring container of the consolidometer. Determine the weight of the soil specimen. After filling and trimming is completed. place a rubber sleeve around the protruding porous plates and load plates to prevent evaporation. weigh the soil specimen. 4.2 Preparation of Remolded Speci- mens—Use about 2 lb of representative soil that has been properly moistened to the degree desired and processed free from lumps and from which particles or aggregations of particles retained by a ï^-in.4 Initial Height and Weight of Soil Specimen—Clean and weigh the specimen container ring and glass plates and weigh them to ±0. Take care to remove any air that may be entrapped in the system by slowly wetting the lower porous stone and draining the stone through the lower drain cock. Apply the small seating load of 0. fill the pan in which the consolidometer stands with water. place a ring gage in the specimen container with the same arrangement of porous plates and load plates to be used when testing the soil specimen. Remove the extension collar and trim the excess material flush with the container ring surface with a straightedge cutting tool.266 FOUNDATIONS ON EXPANSIVE SOILS glass plate to control evaporation until it is placed in the loading device.3 Calibration of Dial Gage for Height specimen. Place the assembly in the loading machine in the same position it will occupy during the test. 4) sieve have been excluded. and glass plates to ± 0 . permeability readings can be taken at any time during the test by filling the percolation tube standpipe to an initial reading and allow the water to percolate through the specimen. If the specimen is not to be saturated at the beginning of the test. and degree of saturation. Compact the specimen to the desired thickness by the compaction hammer. 4. 4. Measure the . After the apparatus has been assembled with the ring gage in place. (No. void ratio. the wet density is not within 1. Compact the specimen to the required wet bulk density after adding the required amount of water as follows : Place the extension collar on top of the container ring and fasten the bottom of the container ring to a baseplate. and wet the specimen. the initial density. 0 1 g. Mark the parts of the apparatus so that they can be matched in the same position for the test. greater than the thickness of the container ring.025 kgf/cm 2) on the soil —To saturate the specimen attach the percolation tube standpipe. If. repeat the preparation of the remolded specimen until the required accuracy is obtained.35 psi (or 0. Use this information to compute the initial volume of the specimen. The true water content of the specimen will be determined when the total dry weight of the specimen is obtained at the end of the test. as described below.01 g before the ring is filled. After saturation has been completed. fill it with water. after weighing and measuring the specimen and computing the wet density. water content. After the specimen is wetted.5 Saturation and Permeability Data Measurements—Prior to filling the container ring with the soil specimen.0 lb/ft 3 of that required.025 kgf/cm 2) to the specimen.35 psi (or 0. apply a load equivalent to a pressure of 0. Assemble the specimen container and place it in the loading device. Weigh the exact quantity of the processed sample to give the desired wet density when compacted to a thickness \ in. The dial reading at this time will be that for the exact height of the ring gage. 4. Remove the ring and specimen from the baseplate and cover the specimen surfaces with glass plates until the specimen is placed in the loading device. By comparing the dial reading at this time with the dial reading obtained with the ring gage in place. determine the exact height of the specimen. ring. loaded. From these data.6. 4. one procedure is to test only two specimens: (1) loadedand-expanded. However.025 kgf/cm 2) to Specimen No. From the water content. (The Remove the specimen from the ring container and weigh it immediately and again after drying to 105 C.) As the specimen begins to expand.1 General Comments—The expansion characteristics of an expansive-type soil vary with the loading history.35 psi (or 0. and (2) expanded-and- permeameter tube head should be sufficiently low so that the specimen is not lifted. calculate the volume of air and. 1. Fig.35 psi (or 0. increase the load as required to hold the specimen at its original height. so that it is necessary to perform a separate test or several specimens for each condition of loading at which exact expansion data are required. apply the seating load of 0. 4. assuming it to be the same as the volume of air following the determination of permeability. and secure initial dial gage readings. Then reduce the load to \ . calculate the water content and degree of saturation.APPENDIX A 267 amount of water flowing through the sample in a given time by the drop in head. 2—Load-Expansion Curves.5. Maintain all loads for 24 h. and secure initial dial readings.025 kgf/cm 2) to Specimen No.— Specimen wetted.3 Expanded and Loaded Test—To measure expansion characteristics where the soil is allowed to expand before loading. (Curve B) and Specimen No.6.35 psi (or 0. 2. 2. an estimate of expansion can be made for any load condition as shown by Curve C. apply the seating load of 0. . Then saturate the soil specimen as described in 4. and \ of the maximum load and finally to the seating load of 0. or longer if needed.2 Loaded and Expanded Test—To measure expansion characteristics where the soil specimen is saturated under full load and then allowed to expand. in which Specimen No. to obtain constant values of height # rSpecimt *n wetted I" to I \ \ \ \ ^A^et ted \ ·> ^ 0 10 20 — Load-psi.6 Expansion Test: 4. 2 was expanded by saturation with water and then loaded (Curve A). F i g .6. 4. and specific gravity of the specimen.025 kgf/cm 2) and measure the height with each load. dry bulk density. 1 was loaded and expanded by saturation with water. Use a greater number of loadings if greater detail in the test curve is required. . Load the specimen successively to | . Cut this specimen from the same undisturbed soil sample as the expansion specimens.1 or 4. and measure the initial volume and height as described in 4.) Transfer the displaced mercury into a graduated cylinder. if greater detail in the test curve is required. Place the specimen in the container ring. Allow the specimen to expand under the seating load for 4$ h or until expansion is complete. Pour the excess mercury into the original container and replace the glass cup in the evaporating dish.268 FOUNDATIONS ON EXPANSIVE SOILS Then saturate the specimen as described in 4. and obtain its volume by the mercury-displacement method. 5.5. 4 After the specimen has been air-dried. Use a greater number of loadings. or remolded to the same bulk density and water content conditions as the expansion specimens. Apply the seating load of 0. Reduce the load to that of the seating load.4 Individual Load-Expansion Test —When it is desired to perform separate expansion tests for other conditions of loading apply the seating load of 0. 5. place a glass cup with a smoothly ground top in an evaporating dish. allow the specimen in the ring to dry in air completely or at least to the water content corresponding to the shrinkage limit (ASTM Method D 427. If the shrinkage specimen is cracked into separate parts. and then read the dial gage. Then load the specimen to the desired loading. Follow the procedures specified in 4.) 5. and reweighing it.1 Expansion Test Data—Calculate h — hQ h0 the void ratio as follows: volume of voids volume of solids Determinations—To measure volume .2.35 psi (or 0. and measure the volume.2. saturate the specimen as described in 4.4.5.2 for making loadings and all measurements and determinations. Calculations 6. J. 6.35 psi (or 0. Test for Shrinkage Factors of Soils). drying the material in an oven to 105 C. Then immerse the air-dried soil specimen in the glass cup filled with mercury using the special glass plate over the glass cup to duplicate the initial mercury volume determination condition. 4.6. J and—t times the maximum load found in 4.1 Specimen Preparation—When measurements of shrinkage on drying are needed.2 Volume and Height Shrinkage shrinkage.2.025 kgf/cm 2).6. then remove the specimen from the ring and make the determination specified in 4. to determine the reconsolidation characteristics of the soil. (A paper strip wrapped around the specimen side and held by a rubber band is effective in holding the specimen intact during handling.2. and then remove the excess mercury by sliding a special glass plate with three prongs for holding the specimen in the mercury over the rim. 5.2 If the height of the air-dried specimen is desired. prepare an additional specimen as described in 4. measure the volume of each part. Determine the water content of the soil specimen by weighing unused portions of the original sample of which the specimen is a part. remove it from the ring container. (See Method D 427 for general scheme of test and equipment.1 To perform the mercury displacement measurement.6.025 kgf/cm 2) to the specimen and measure the initial height. Procedure—Shrinkage Test 5. Allow the height to become constant and measure.6. Measure the height of the expanded specimen. Fill the cup to overflowing with mercury.2. place the specimen and ring container in the loading machine. and add the individual volumes to obtain the total. and allow the specimen to expand under the applied load for 48 h. or until expansion is complete. . as a percentage of the original height. 6. difference in head between headwater and tailwater.3 Type of sample tested (remolded or undisturbed. the total volume change can also be determined for several conditions of loading.2 Description of the soil tested and size fraction of the total sample tested.1. 6. depth.1 Expansion Test—Include the following information on the soil specimens tested in the report : 8. 2. 8. describe the size and type. Ls = length of the specimen. 2. i\ — initial volume of specimen (height of specimen times area of ring container). 8.2 To calculate the total percentage change in height from saturated to airdry conditions. Hi = initial head. as follows : A.In — t H{ where: k — permeability rate. 2.APPENDIX A 269 where: e = void ratio.. Plotting Test Data 7. Reports 8. 7. 5i X . This value is used as an indicator of total expansion but is based on initial conditions of density and water content. and ha — height of air-dried specimen.2 Permeability Test Data—Calculate the permeability rate by means of the following basic formula for the variable head permeameter: k = Ap X U As X 12 1.1. in.1 To calculate the total percentage change in volume from "air-dry to saturated conditions. hand-cut. A ρ = area of standpipe furnishing the percolation head. in. add the percentage shrinkage in height Ahs to the percentage expansion Δ wThen the specimen is saturated under specific load conditions. years. location).3. in. as described in 6. h = height of the specimen.3..1. hi = initial height of the specimen. and ho = height of the solid material at zero void content Calculate the expansion.1. or other). and / = elasped time. Since expansion volume data are determined for several conditions of loading. 6.1 Identification of the sample (hole number. the volume shrinkage as a percentage of the initial volume as follows: Vi where : Δ 8 = volume shrinkage in percentage of initial volume. 6.1 Expansion Test—The test data may be plotted as shown on Fig. in. hi — initial height of specimen. Calculate the shrinkage in height as follows: Δ*. in. 8. = X 100 h2 — hi X 100 hi where : Δ = expansion in percentage of initial volume.. and hi = height of the specimen under a specific load condition. percent = vd = volume of air-dried specimen from mercury displacement method. Aa = area of the specimen. difference in head between headwater and tailwater.3 Shrinkage Test Data—Calculate where: AhB — height of shrinkage in percentage of initial height. as extruded core. Hf = final head. . ft/year. if undisturbed. and . add the percentage shrinkage in volume on air drying Δ 8 to the percentage expansion in volume on saturation Δ 6 . other information such as the total change in volume and total change in height. and type of loading equipment. 8.1.1.1.8 Load and time versus volume- change data in other forms if specifically requested.1. if desired. 8.10 Permeability data and any other data specifically requested.1 through 8. give the comparison to maximum density and optimum water content (see Methods D 698)).6 A plot load versus volume change curves as in Fig.1. Report the load conditions under which the volume change measurements were obtained.270 FOUNDATIONS ON EXPANSIVE SOILS 8. 1. . specimen size).1.7 A plot log of time versus deformation if desired. and saturation degree data. include data on the decrease in volume from the initial to air-dried condition and.9.1.1.1.2 Shrinkage Test—For the report on shrinkage.9 Final water content. A plot of void ratio versus log of pressure curve may be plotted if desired. 8. 8. 8. 8.1.5 Type of consolidometer (fixed or floating ring.4 Initial moisture and density conditions and degree of saturation (if remolded. Include also Items 8. bulk dry density.5 and 8. 8. 6. U p o n w e t t i n g .APPENDIX Β T h e following are s o m e q u e s t i o n s c o m m o n l y raised b y o w n e r s . e t c . length of pier. 5. R e i n f o r c e m e n t will span across u n e q u a l heaving and r e d u c e cracking. T h e d e p t h of c o n t i n u o u s footings is usually o n l y 8 t o 12 inches. T h e answers given are based m o s t l y o n e x p e r i e n c e r a t h e r t h a n o n the t h e o r e t i c a l a p p r o a c h . After t h e soil has reached its m a x i m u m swell p o t e n t i a l . When w e t t e d . 7. m o i s t u r e migrates t o t h e drier p o r t i o n of t h e soil and heaving action c o n t i n u e s . O n c e t h e f o u n d a t i o n soils have b e c o m e w e t t e d . 3. White streaks in t h e clay are calcium deposits and n o t b e n t o n i t e . t h e soil heaves. m u s t be carefully designed. It is false t h a t w h i t e streaks in clay are b e n t o n i t e . such as size of footing. thickness of slab. and architects concerning t h e solutions a n d p r e c a u t i o n s for s t r u c t u r e f o u n d e d on expansive soils. 4. builders. R e i n f o r c e m e n t should be placed in t h e f o u n d a t i o n walls. It is t r u e t h a t a subsoil investigation should b e c o n d u c t e d for each s t r u c t u r e t o b e b u i l t in an expansive soil area. It is t r u e t h a t heavy r e i n f o r c e m e n t in t h e f o u n d a t i o n wall will m i n i m i z e cracking. It is false t h a t expansive soil b e n e a t h t h e f o u n d a t i o n s will settle b a c k u p o n removal of source of w a t e r o r o n p r o l o n g e d drying. . t h e r e is n o w a y t o r e m o v e t h e m o i s t u r e . w i t h possible e x c e p t i o n of areas i m m e d i a t e l y b e n e a t h h e a t d u c t s or furnaces. It is false t h a t heaving a c t i o n will stabilize after a n u m b e r of years. 2. It is false t h a t r e i n f o r c e m e n t should b e placed in c o n t i n u o u s footings. t h e heavy calcium c o n t e n t in t h e soil can cause excessive s e t t l e m e n t b u t n o t swelling. It is t r u e t h a t f o u n d a t i o n design based u p o n subsoil investigation should be o b t a i n e d before construction. TRUE AND FALSE 1. M o v e m e n t of t h e s t r u c t u r e will c o n t i n u e . Such d e p t h is n o t sufficient t o r e n d e r an effective r e i n f o r c e m e n t . P r o p e r f o u n d a t i o n systems. b u t t h e m a g n i t u d e m a y vary. Using subsoil i n f o r m a t i o n t h a t was o b t a i n e d in t h e vicinity r a t h e r t h a n at t h e specific s t r u c t u r e l o c a t i o n can be b o t h misleading and d a n g e r o u s . Moisture c o n t e n t b e n e a t h a n y s t r u c t u r e seldom decreases. . it is possible t h a t a high w a t e r c o n d i t i o n can trigger swelling even o n high g r o u n d . Extensive research will be required. Present day k n o w l e d g e of chemical stabilization is in its infantile stage. while t h e r e is great danger of triggering t h e expansion of d e e p seated expansive soils after t h e building is completed. 14. D o w n s p o u t s are preferred t o t h e h i d d e n r o o f drain s y s t e m . . It is t r u e t h a t d o w n s p o u t s should be long e n o u g h t o drain w a t e r away from a building. G o o d surface drainage is a necessary r e q u i r e m e n t b u t b y itself is n o t sufficient t o prevent heaving and cracking. Defective d o w n s p o u t s can b e i m m e d i a t e l y d e t e c t e d while t h e built-in r o o f drain system m a y develop leakage t h a t goes u n d e t e c t e d for m a n y years. This a c c o u n t s for t h e surprisingly few cases r e p o r t e d o n t h e cracks of gasoline service s t a t i o n structures in expansive soils areas. t o be effective. 13. before chemical stabilization can be widely a d o p t e d . t h e drains should be placed at t h e p r o p e r d e p t h w i t h p r o p e r o u t l e t s . 1. Subdrains will prevent free w a t e r from entering t h e f o u n d a t i o n soils. It is t r u e t h a t a subdrainage system is always a desirable e l e m e n t in t h e f o u n d a t i o n system. Therefore. 15.0. A m o i s t u r e increase of as little as 1 p e r c e n t b y weight is sufficient t o cause heaving action. 9. Ponding will affect o n l y soils b e l o w ground surface t o a shallow d e p t h . It is false t o assume t h a t if a building is situated o n high g r o u n d . It is false t h a t chemical stabilization can provide an answer t o all expansive soil p r o b l e m s . G r o u n d w a t e r does n o t follow g r o u n d c o n t o u r . It is t r u e t h a t good surface drainage will reduce t h e risk of f o u n d a t i o n m o v e m e n t in an expansive soil area. However. It is true t h a t if all the g r o u n d surface s u r r o u n d i n g a building is paved. swelling p r o b l e m s can be controlled. t h e heaving p r o b l e m can be eliminated. 12. Extensive paving a r o u n d a building can effectively c o n t r o l t h e migration of surface w a t e r i n t o t h e f o u n d a t i o n soils. It is false t h a t b y p o n d i n g t h e soil before c o n s t r u c t i o n . 11. and a p e r c h e d w a t e r table usually follows t h e b e d r o c k c o n t o u r . Other factors such as a d e q u a t e structural design and proper c o n s t r u c t i o n t e c h n i q u e s are equally i m p o r t a n t . It is false t h a t free water is necessary t o cause swelling. especially in t h e area of field application. Installation of a subdrainage system a r o u n d a building m a y i n t e r c e p t free w a t e r b u t will n o t prevent t h e increase of m o i s t u r e c o n t e n t in t h e f o u n d a t i o n soils. there will be n o swelling problem.272 FOUNDATIONS ON EXPANSIVE SOILS 8. resulting in severe d a m a g e . Lawn sprinkling systems should be installed at least 10 feet away from t h e building. Swelling clay can exert uplifting pressure on t h e interlocking gravel particles causing the floor t o heave. After m a n y years. Whenever possible a suspended floor system should b e a d o p t e d . c o n s e q u e n t l y . It is false t h a t lateral expansion is t h e cause of separation of w i n d o w s and d o o r s from frames. However. Simulated l a b o r a t o r y tests can easily d e m o n s t r a t e t h a t p u d d l i n g will n o t increase t h e soil d e n s i t y . It is t r u e t h a t t h e use of expansive soil as backfill can exert swelling pressure o n the wall and cause cracking. It is false t h a t p u d d l i n g of backfill can achieve t h e desired c o m p a c t i o n . b u t o f t e n t i m e s this is n o t economically feasible. 19. w h e n e v e r possible. t h e a c c u m u l a t i o n of m o i s t u r e will eventually cause problems. Most lateral m o v e m e n t is caused b y differential heaving w h i c h creates an impression of pushing away or pulling a p a r t . m o i s t u r e c o n t e n t in t h e soil b e n e a t h t h e plastic will increase steadily. 21. It is false t h a t a drilled pier f o u n d a t i o n system provides t h e best answer t o s t r u c t u r e s f o u n d e d on expansive soils. It is false t h a t sand and gravel placed b e n e a t h the floor slab will r e d u c e the uplift pressure of e x p a n d i n g clay b y providing void space i n t o w h i c h t h e clay can flow as it e x p a n d s . 17.APPENDIX Β 273 16. 23. Plastic m e m b r a n e s will allow surface w a t e r t o leak t h r o u g h t h e seams and holes b u t will n o t allow e v a p o r a t i o n . w i t h nozzles directed away from t h e s t r u c t u r e . It is t r u e t h a t b u r i e d utility lines should be avoided. . Initial small leakage will trigger m o r e e x p a n s i o n causing greater leakage. Swelling soil can shear w a t e r and sewer lines and cause leakage. 18. 20. in an expansive soil area. Excessive p u d d l i n g can i n t r o d u c e a large a m o u n t of w a t e r i n t o the f o u n d a t i o n soils. such h o r i z o n t a l pressure seldom fully develops. water will n o t be able t o seep i n t o t h e f o u n d a t i o n soils. since m o s t backfill is n o t tightly c o m p a c t e d . It is t r u e t h a t a structural floor system or a s u s p e n d e d floor system is t h e only solution t o slabs-on-ground c o n s t r u c t i o n in expansive soil areas. T h u s t h e chance of saturating t h e backfill can b e reduced. 22. It is t r u e t h a t lawn sprinkling systems should n o t be installed adjacent t o t h e building. It is false t h a t b y providing plastic m e m b r a n e s a r o u n d t h e h o u s e w h i c h are t o p p e d w i t h gravel b e d d i n g . It is d o u b t f u l t h a t any clay will flow i n t o t h e voids. 24. H o r i z o n t a l swelling pressure is a p p r o x i m a t e l y equal in m a g n i t u d e t o t h e vertical swelling pressure. 25. Statistically. t h e r e are p r o b a b l y m o r e cracked buildings f o u n d e d on piers t h a n w i t h spread footings.274 FOUNDATIONS ON EXPANSIVE SOILS A drilled pier s y s t e m . there w o u l d never be a swelling problem. a drilled pier f o u n d a t i o n m a y n o t be effective. if intelligently designed and c o n s t r u c t e d . can solve m u c h of t h e swelling soil p r o b l e m . in areas w h e r e there is a possibility of rising g r o u n d water. However. It is t r u e t h a t b y placing t h e building o n a single pier. Who can afford t h a t ? . 159. 173 Air space.255 structure. 26. 10. 106. 3 9 . 162 shale. 29. 65.68 conbel. 96. 184. 171 Base exchange capacity. 5 . 66. 34 Capillary (cont. 2 1 . 147 Asphalt membranes. 26.187. 255 Climate condition. 3 Canal. 39. 175 Cement stabilization. 172. 64. 83.) fringe. 148 Atmospheric pressure. 1 . 205. 165. 90. 111. 175 partially saturated. 13. 6. 4 0 . 272 Chemical analysis. 16. 16 Cement. 90. 149. 20. 146. 11 Adsorbed water. 34 Atterberg limits.219. 163. 8. 13. 3 . 38 one-dimensional. 151. 27. 1 3 . 1 4 . 64.175.111. 6. 88 Consolidation test. 18. 17 Chemical injection. 151. 11. 1 6 9 effort. 26 fixed-ring. 33 Asphalt mat. 146 Blow count. 72 Bond. 127. 9 Activity Method. 33 Catalytically-blown asphalt.183 Colorado School of Mines.11 type of. 112. 19. 83 cleaning of. 159.101. 10. 77. 83 isolated-shaft.152. 244 wall. 151 impervious. 6 1 . 175. 16 Cation exchange capacity. 84 construction of. 17. 121. 3 3 . 63. 110. 1 6 5 . 193.194.146. 73. 207.20.149. 171 Basement.247 tube. 72. 11. 86 Colorado. 10. 2 Belled pier advantage of. 150 Cone penetration. 16. 136. 134 Bearing capacity. 93.232 degree of.21 Adsorbed cation.194. 228 Colloidal clay. 149.120 Cohesion. 3 5 Β Backfill. 73. 159 Chemical stabilization.176. 65. 10. 77 size. 1 Colorado State Highway Department. 147 Cation.112. 2 6 . 149. 136. 87 Box construction. 246 deep. 165 Bearpaw shale.104. 98. 3 5 .INDEX A Absorption. 19. 11. 4 3 Australia. 176 Attractive force. 255 Cinder block wall. 149. 243 Brick wall. 24. 149 method of.19.154 potential. 4 3 . 6 7 . 86 shaft of. 122. 9 .99. 27. 9. 33. 84 Bench mark. 68 cantilever. 207 Bentonite. 122 C Caisson. 62. 1. 104. 72 Consolidometer. 100.163 Capillary force. 104. 24 stiff. 160 Climate rating. 150 mineral. 84 disadvantage of. 26. 22.177. 124. 122 . 246 Apron. 33 moisture movement. 190.86.194. 228. 1 1 1 . 44. 71 Canada. 206 control. 90. 131. 67. 170 slab.169.121. 246 Clay fissured. 1 1 3 . 190.163 Compactor. 125. 24 rise.251 Building Research Advisory Board. 17 Compaction. 4 1 . 206. 5 6 . 90.235 Base exchange. 67. 147.11.113. 45 Control joint.271 Black cotton.21.37.170. 20 Claystone shale. 100. 26 platform.149. 3 3 . 6 2 .106. 16. 10. 18. 4 5 . 6 8 ring. 11. 176 Cation exchange.176 Airport. 84 system. 168 Colloid content. 5. 111. 195. 255 Artesian. 152.207. 33. 50. 19. 187. 219. 4 1 . 34. 35. 9 1 . 34 Evapotranspiration. 8. 162. 27. 160. 167. 273 Drilling. 66 percussion. 194. 185 Doorframe. 33 F Factor of safety. 74. 8. 22.227 Fissures. 27 recognition of. 52. 73. 63. 103. 1. 110. 145. 184. 3 Excavation. 255. 135. 271 Free draining gravel. 125. 256. 3. 104 settlement. 8 distribution of. 183. 100. 245. 1 physical properties of. 126. 4 1 . 17. 33.17 Direct measurement. 42. 158. 28. 83. 34. 93. 170 Flood plain. 109 Foundation deep. 47. 35 Discontinuity of structure. 41 Dowel bar. 104 movement. 83. 173 Design criteria. 8. 252 Foundation soil. 1 Depth of desiccation. 35 Dead load. 4 1 . 103. 64. 113. 36. 92. 29 Dye adsorption. 78.19. 233 Deep plow. 183. 173. 17.159. 29. 40.129.44. 16.193. 103. 186 Cracked building.40 Depth of penetration. 64. 156. 175 Footings. 29. 160. 64. 61. 72. 16 structure of. 64 rotary. 222. 78 Drying and shrinkage. 60 Evaporation. 55. 35. 104 .47. 49 Drilled pier. 155 heaving.84. 129. 246 Fly ash. 47. 124 D Darcy's law. 73 Denver formation. 184 shallow. 190. 183 wall. 124 Free swell.221 interrupted. 63. 46. 4 8 . 29 Federal Housing Administration. 124 thermal analysis. 173 individual. 246. 169. 154. 62 Flooding. 37. 159 Crack pattern.176 Denver Blue Shale. 11.271 Earth pressure.124. 105. 44. 131. 124 Curing time. 85. 9 5 . 25η Fatigue of swelling. 33. 1 0 3 . 28. 1 0 6 . 257. 182. 28. 73. 155. 27. 33.190 Crawl space. 86. 165. 185. 71 damage caused by. 29. 16. 104. 166. 124 Floor slab. 243 foundation. 110.207. 66 Dry density. 190. 28. 14 Double oedometer method. 20. 18. 1 Effective stress. 13. 28 Degree of saturation. 27. 18 Electrostatic force. 185. 1 0 9 . 237. 111 movement. 165 Denver. 151. 27. 42 Electron microscope resolution. 74. 61. 127. 72 Expansion joint. 193. 8. 135.40 Curling. 4 9 . 66 Differential drying. 161. 17 Ε Ecca shale. 189 Drying and wetting. 16. 39. 8.44 Free water. 246. 272 Furnace duct. 74. 193. 66 Floor load. 192. 109. 185 Floor level.229 Drain tile. 162. 113. 63 auger.190. 165 maximum. 104 Distress study. 35.45. 29. 71. 256 Floating slab. 86. 3 nature of. 121.207 Covered area. 64 information. 189 initial. 207. 221 wall. 81. 56. 104. 168. 248 pad. 45. 7 1 . 39. 74 Desiccate.145. 27. 13. 169 Existing structure. 259 plan. 75. 195 Environmental condition. 142. 159. 199. 150.213. 1 origin of. 260 Expansive soils. 21. 183 mat. 221 continuous. 127. 52.194.135 Double layer structure. 109.122 Fill. 252 Flower bed. 101.40. 271 type. 157. 63. 103. 95. 24. 16. 27.191 Drainage. 173 Degree of expansion.276 INDEX Court yard. 101. 151. 93. 26. 148.214 214 Curing. 66 system. 29. 13 End bearing capacity. 160. 29. 207. 21. 89. 89. 62. 5 Mexico City. 76. 88 Meteorological. 145. 101. 160 transfer. 174 Lime-treated subgrade. 149 Menard pressurementer. 90. 66. 175. 50. 28. 149. 168. 73 M Masonry construction. 170. 218. 10. 34 fluctuation. 151 Isolation of pier uplift. 27. 1. 175 Honeycomb floor system. 122. 146. 35 Hydrometer analysis. 4 4 L Laboratory testing. 56 Intercepting ditch. 145 asphalt. 163 distribution. 37. 171. 149 vertical. 148 plastic. 2 7 . 34. 8. 19 Liquid limit. 62. 116.19.154 Ground water. 111. 150 Montana. 13. 174 Lime stabilization. 171. 84. 189 . 1 8 . 13.107.INDEX 277 G Garage slab. 152. 110. 28. 49. 111. 125. 33 Gravity flow. 104 J J-void. 174 horizontal. 9 . 184. 154. 192. 63. 2 7 . 273 polyethylene. 171 Lime slurry. 101. 227 Load bearing walls. 148. 258. 145. 122. 8 Montmorillonite. 61 Method of compaction. 33. 163. 192 Lime. 92. 27. 113 design of. 257 H Heaving differential. 96.5 Indirect measurement. 127 Horizontal swelling pressure. 37. 152. 10. 151 Laramie formation. 161.227 Mud jack. 33. 39 Mineralogical identification.141 Grade beam and pier system. 3 3 . 169 Highway. 17. 72. 117 Matrix suction. 151.127. 37 Gravitational potential. 101 Lawn watering. 148. 5 Mineralogical composition. 171. 1 1 . 35. 21. profile.40. 75. 158. 135 Load test. 5 . 74.256 Index property. 72. 170 Gravitational migration. 39. 93. 40 Moisture barrier. 255 Mushroom. 3 .26. 35. 3 4 . 129. 24 Gravity.27 Impervious. 148. 73. 1 . 26 Membrane. 108 Granular soils. 125. 28 India. 152 Intercepting drains. 152. 17 Israel. 44. 126. 147. 13. 135. 165. 173. 4 9 . 206. 9. 85 Isomorphism. 228 I I-beam. 82 Moisture content. 162. 40. 26. 6 . 2 1 . 194. 85 Lateral wall movement. 194. 151. 63.21.104 Masonry wall. 77. 22 Initial moisture content. 95. 16 Model pier test.172. 76. 113. 122.236 movement. 145. 99. 18. 151 Ion. 190. 110.124. 35. 190. 188. 170. 18. 89. 111 behavior. 28. 2. 24. 37. 187. 89.218 Illite. 4 4 . 126 Κ Kaolinite. 96. 17. 4 1 . 148. 128 Lateral resistance. 136 Mat foundation. 202 deficient. 101 rise of. 256 Gaade Grade beams. 1 Lateral pressure. 218. 167. 127. 160 equilibria. 272 Lightly loaded structure. 187 movement. 151. 89. 171. 106 potential. 35. 192. 173 Linear shrinkage. 94. 77.175. 38. 34. 9. 135. 27.44. 35. 16. 9. 10. 146 migration. 3 . 171 Irrigation ditch. 136. 145. 149.251 Granite.3. 258 Landslide. 125. 3. 89. 150. 40 Mexico. 146. 9 Humidity. 9. 115. 1 . 29. 56 Repulsive force. 47. 272 Roof drain. 95. 129.132.159. 1 6 2 . 44.126 Rational design. 3 5 . 202.225 Organic compound. 122. 13. 254. 72 disturbe. 164 .210. 165. 65 Samples core. 62. 85. 50. 4 4 . 35.136. 1.182 spacing. 83. 78. 66 Sampling method. 71.184 failure of. 7 3 .43 Residential houses.162 Penetration resistance.156. 136. 34 Plasticity Index.171. 6 0 . 273 Pore pressure. 125. 85 Rational pier formula.258 Pressure injection. 189. 1 1 0 . 85.104. 111 Roof downspout. 22. 84. 187.107.127. 9 3 . 34. 42 Ο Optimum moisture content.42 Portland cement. 7 1 . 66 split spoon.258. 55. 89. 13. 33 Prewetting. 74. 252 Patio. 65. 19. 2 1 . 190. 104. 47. 37. 44. 77. 61 Pierre formation. 125 friction.278 INDEX Ν Negative moment. 125 Post-tension. 35.74. 89. 25. 176 Osmosis pressure. 66. 82.44. 176 Puddling. 152. 84. 26 Osmotic potential.122.194. 63 Piers. 193 Remolded sample. 3 4 . 75. 184. 206. 92 straight-shaft.104. 26 Normal stress. 66 Shelby tube. 192. 5 2 . 39. 238. 152. 88 Penetration test. 85. 21.184. 35. 157 Poisson's equation. 272. 94 drilled. 208 shaft.d. 29. 63. 66. 121. 27 Sandstone.189 Partition wall. 1 0 . 7 1 . 111. 160. 88 length of.193 Resistivity. 72. 193. 38. 13 Poisson-Boltzmann equation.147.25 Precipitation.146 Plastic membranes. 28. 77. 210 settlement. 66 Perched water. 10. 185. 122. 92. 2 3 . 2 1 . 92. 6 5 .227 Plumbing. 13 Ponding. 1 6 3 . 108. 38. 1 1 1 . 92.163. 77. 1 6 . 63. 8 3 . 4 4 . 1 9 5 anchorage. 257 Placement condition.159.175.99 size. 247. 183. 156 S Sampler California. 71. 194. 2 2 .218 Pavement. 7 1 . 28. 250 Potential volume change. 1 5 9 . 108 foundation system. 24 Overburden pressure. 64. 85.272 Peripheral drain. 1 8 . 155 Permeability. 4 1 .161. 44 R Raised floor system.205 reinforcement. 260 deep. 1 6 4 Plastic limit. 40 Sampling. 77. 249.192 PVC meter method. 37.150. 14. 112. 175 Portland Cement Association. 111. 93. 7 2 . 9 2 . 73. 4 7 .135. 74 diameter of. 1 5 0 . 156. 205. 63 Rocky Mountain Area. 93. 98 belled. 124. 43. 73. 118.256.190 Proctor density. 68 undisturbed (see Undisturbed) Sample thickness. 246. 92. 9 8 . 106. 191. 62. 221 Proprietary fluid. 83 design capacity of. 2 8 . 73. 21 Q Quartz.255 . 89 foundation. 73 Ρ Pad. 33. 98. 173 Pressure release. 68 representative. 3 4 . 29. 3 7 . 93. 45. 4 4 . 176 Physiography. 155.34. 121. 1 1 2 PVC rating. 1 1 2 Overburden soil. 2. 2 4 . 64. 35. 86. 53. 38. 3 9 .150. 4 4 . 88 Reinforced brickwork. 2 9 . 1 0 1 .151. 34. 16 Osmotic consolidometer. 29. 259 bearing capacity of. 104 Remedial measures. 38. 56. 3. 149.208 uplift. 71. 129. 1 6 9 . 85 system. 111 Negative pore pressure. 74.47. 74. 246.152. 132. 38 mechanics of. 86. 44 Specific surface. 135.229.260 Slope stability. 63.145 Τ Tensile stress. 55.170. 72.255 soil. 64 Suction test. 50. 1 9 . 121. 14. 82. 4 6 . 129 floating. 112. 39.122. 29 Sump pump.225.189 Seasonal moisture change. 127 interior floor.143. 64. 145. 199 Slip joint.136.223 patio. 112 test.120 Surcharge load.73 Test pit. 131. 2 2 .136. 16. 4 9 . 1 9 0 . 272 Subsoil condition. 75. 24. 134. 134 prestressed. 245 Texas.213 Sprinkling system. 128. 83. 3 7 . 134. 88. 129. 34.199. 127. 6. 121 structural. 2. 76. 56. 40 Soil replacement. 27. 247. 155.199.225. 244 Support index. 233. 6 3 .194. 72. 9. 35 Soil test. 73. 6. 169 depth of.260 Slab-on-ground. 166. 37. 28.195. 47. 169. 1 9 0 . 203. 18.192.273 Slab-bearing partition wall. 26.229 Surficial geology. 74.110. 6 6 . 125 reinforced. 4 3 . 162 Texas A & M. 2 1 . 218. 6. 34. 3 8 . 1 1 2 percent of. 1 8 5 Seismic survey. 33. 175.197. 151. 49. 90. 2 1 .67. 77. 26 Structure engineer. 160.130. 193 Staircase wall. 26.152. 73 Sidewalk. 121. 4 5 . 18. 83. 4 4 . 207 Shear ring. 16. 152. 6 Specific gravity. 61 Surface water. 189.171 . 9 7 .3. 6 1 . 1.260. 272 Subdrainage system. 8 6 . 52.169 Settlement.247. 27 Stress release. 4 7 . 104. 52 partial.200 total.170 Soil stabilization. 33. 129. 3 7 . 28.132.168 extent of. 1 7 1 .INDEX 279 Saturation.170. 26. 155 Shale bedrock. 159 Soil suction. 131. 61. 125.173 Sonotube. 203.193 Soil lattice. 39. 111. 149. 73 Shear strength. 189 Structural fill. 8 4 . 6 1 . 17 Soil profile. 2 1 . 255 Stiffened slab. 2 0 . 125 Stratum thickness. 86 Sheet rock 131. 26.203. 14. 1 2 7 South Africa.94. 114. 77. 1 5 1 . 39 Soil-water system. 124 limit. 62 Swell index. 38 Side shear. 38. 26. 112. 4 0 . 171. 82 Shear failure. 88 Shear stress. 26.44.120.163 Surcharge pressure. 11 Split level. 1 2 1 . 147. 113. 4 1 . 112.43 Seasonal cycle.232 Sewer pipe.151. 60. 106. 27 complete. 184 Soil-water equilibria. 38. 4 1 . 46. 20.115. 64. 272 Surface geology. 135 Subdrain. 73. 3 8 . 121. 125.146. 94. 122. 160. 101.229 pressure. 128 movement.194.207 Silt. 204. 82 potential.171. 3. 110. 6 2 . 127. 4 5 . 122 Test hole. 37.122. 257 Slabs exterior. 129. 146. 122. 156. 33. 124. 1 2 6 . 151 Soil engineers.152. 4 6 . 145. 213. 1 5 5 . 155.165. 4 0 . 72 Sewer line. 185. 86.192.184. 140. 9 4 . 1 8 4 . 64.252.170.191. 167. 5. 165 Stress history. 64. 49. 3 3 . 104. 1 8 9 crack. 131. 82. 1. 155. 157. 34 Siltstone.134. 1. 3. 9 5 . 234 Stud wall. 213. 34.43. 24.168. 194. 113. 129. 33.154. 109.228 mechanics of. 111.169.63 Selected fill. 77. 121. 52.146 Site investigation.43 definition of. 175 Shear strength reduction. 160.232. 19. 55. 1 6 5 Surface drainage. 18. 221 Temperature crack. 86. 61 Skin friction.194. 95 Tension crack. 126. 149 Seepage. 111. 6 4 . 153.161 Spain. 175. 64. 27. 3 7 . 121. 148. 194. 11. 111.132. 37 test. 112 Texas Highway Department. 26 Shearing resistance. 168 Swelling characteristic. 74. 2 5 . 27. 29. 2 2 . 222.110. 134 Shrinkage. 40. 89. 88. 8 1 . 175 Undisturbed sample.273 Water main. 35.157. 13 . 183. 1 1 1 . 106 Underpin.B. 194. 88 U Vane shear test. 24. 109. 131. 227 Volume change. 63. 194. 136. 151 Zone of wetting. 11.228.125. 34 Triaxial shear strength.98.42 Unconfined compressive strength. 104. 76. 20. 34. 185. 129. 218.233 Under-reamed pier.S. 6 . 83. 89. 40. 215. 131. 41.111 Unsaturated soils. 47 movement. 76. 173. 17 Thermal gradients. 6 3 . 86. 37. 149. 1 6 1 .203. 81. 94. 194. 34.97.280 INDEX Texture. 198. 33.256. 96. 6. 214. 128. 35 Unwetted zone. 190 Underslab soil. 17. 85. 42 Void-space.259 Water transfer.16. 243 Wall paneling. 27. 184. 48. 28. 6 7 . 61 Total heave. 38. 107. 112 Undrained shear test. 80. 4 7 . 192 V W Waffle slab. 39 Topographie feature. 83. 227 Ζ Zone of aeration. 168. 67.99. 33 Water vapor.252 U. 84. 89. 124. 14 Vapor barrier. 232. 83 Underslab gravel. 86 Van der Waals' force. 88 Van't Hoff equation.S. 131.62 Water line. 65. 122. 55.259. 216 Uplifting pressure.93 Uplift. 88. 39.273 Utility trench. 41 Transpiration. 65. 217 Wyoming. 37.R. 6 1 . 185. 41. 83. 77 Zone unaffected by wetting. 96. 72. 148. 9 Verticel.154 Vegetation. 33.108. 152. 47. 88 United States. 99. 4 3 . 75. Bureau of Reclamation. 161 U. 159 Thermal transfer. 155 Vermiculite. 34 Weathered claystone. 28. 34.257 coefficient of. 4 1 .125 Walkout door. 38. 154 Vapor transfer. 113. 135. 245 tolerable.122 Thermal-osmosis. 28. 175 Ultimate settlement. 2 1 . 214 Withholding force.215. method. 62 Topographie survey. 33. 2 1 . 3. 77 Water table. 82 differential. 75. 145 Vapor pressure. 128. 44. 145 Time element. 183 X X-ray diffraction. 72. 21 Utility lines.231. 126 Void-ratio. 83. 35. 3 9 . 47 Uplifting force. 189. 218. 140 Water level. 50.