WALLACE,K. B. (1973).Structural Ge’otechnique23, No. 2, 203-218. behaviour of residual soils of the continually wet Highlands of Papua New Guinea K. B. WALLACE* The Paper illustrates how high porosity, a strongly cemented structure and highly hydrated kandite clay minerals control the geotechnical properties of residual sandy to silty clay soils of the Papua New Guinea highlands. These soils are characterized by high field moisture contents, relatively high shear strengths and low sensitivity, and are highly compressible when the applied pressure in an oedometer test exceeds a certain critical pressure. Measured values of critical pressure and compression index are compared with those reported for a wide variety of soils, and a linear relationship between volumetric strain and porosity is established for porous soils. The natural inhomogeneity of residual soils is emphasized. Examination of relationships between geotechnical properties and porosity and moisture content shows that porosity or dry density is a more reasonable basis for description and grouping of these soils for engineering purposes. The behaviour of the soils is interpreted in terms of an idealized model of the soil structure. The soil is considered to consist of a coarse open skeleton of cemented rock mineral and aggregated clay particles surrounded by a viscous gel. At low applied stresses, when the cemented bonds in the soil skeleton are intact, it is the soil skeleton that determines structural behaviour. At high applied stresses the porous soil skeleton still has a strong influence on structural behaviour. Cette communication montre comment la porosite BlevBe, la structure fortement cimentCe et les min6raux argileux trbs hydra& du type kandite, rdgissent les propri&s gkotechniques de sols r&iduels de type sableux & silt argileux dans les hautes terres de Panouasie-Nouvelle GuinBe. Ces sols sent caract&&s par des teneurs en eau en place dlevkes, des rCsistances au cisaillement relativement 6levees et une faible sensibilit6, et sent t&s compressibles lorsque la pression appliqu6e depasse une certaine valeur critique lors de l’essai oedom&rique. On compare les valeurs mesurees pour la pression critique et l’indice de compression B celles qui ont dt6 publiees pour une grande vari& de sols, et on etablit une relation liSaire entre la dkformation volumetrique et la porosit6, pour les sols poreux. On insiste sur l’hCt&ogCn&t6 des sols r6siduels. L’examen des relations entre les propriCtCs geetechniques et la porosit6 et la teneur en eau montre que la porositk ou la densite B set forme une base raisonnable de description et de groupement de ces sols pour le genie civil. On interprkte le comportement de ces sols en fonction d’un modkle idkalisd de la structure du sol. On considkre que le sol se compose d’un squellette grossier ouvert de minkraux cimentks et d’aggrtgats de particules d’argile entourtes d’un gel visqueux. Aux faibles contraintes, les liaisons cimentkes dans le squelette du sol restent intactes et le squelette du sol determine le comportement de la structure. Aux contraintes tlevtes, le squelette poreux du sol a encore une grande influence sur le comportement de la structure. residual soils described in this Paper occur at well-drained sites in the Western and Southern Highlands Districts of Papua New Guinea. The Highlands Districts are situated close to the equator (63, 14O”E) and consist of undulating valleys (4000-6000 ft above sea level) separated by extremely rugged mountainous terrain. Although these valleys contain about 40% of the country’s population, they are mainly undeveloped with the population depending on a simple subsistence agriculture. The climate is continually wet with an average annual rainfall of about 100 inches, evenly distributed throughout the year. The mean monthly rainfall nearly always exceeds the estimated mean evapo-transpiration and rain may be expected on an average of between 70 and The * Research Fellow, Department of Engineering, James Cook University of North Queensland, Townsville, Australia. 1964). halloysite and allophane with gibbsite occurring in large proportions. 1971). 1) is formed from a vesicular basic lava flow with possibly some minor ash contamination from above. while the succeeding sections describe their geotechnical properties and go on to discuss the relationship between these properties and the soil structure. 1971) have confirmed this zonality and have highlighted one difference of engineering consequence. The zonal nature of these soils has been recognized by earlier pedological studies (Haantjens and Rutherford. The Author’s studies (Wallace. a Pleistocene volcano which is known to have covered the area with andesitic ash showers (Blake and Liiffler. basalt and andesitic ash. At 5000 ft elevation the temperature usually varies from 55°F at night to 75°F during the day and there is very little monthly fluctuation in temperature. which also gives details of field moisture content. Three of the soils were formed from intermediate to basic volcanics and one from limestone.15 compared with a value of . Examination of quarry exposures. has a silica/alumina ratio of 1. clay mineralogy and soil chemistry. Because of the relatively remote and undeveloped nature of the Highlands the field work consisted of sampling from a few representative sites which had been chosen after extensive terrain description and examination of road cuttings in the region. As all the sites were adjacent to Mt Giluwe. Pleistocene to recent volcanic eruptions have covered large areas of the Highlands with layers of andesitic ash. especially on limestone. Clay mineral analysis was done by X-ray diffraction and differential thermal analysis (Amdel. THE SOILS The results presented in this Paper are taken from a more general study of field occurrence and geotechnical properties of these soils (Wallace. 4. which showed similarities in the morphology and clay mineralogy of soils on such dissimilar parent materials as limestone. as do Soils No. 1 and No. This warm. Soils from four well-drained locations are described in Table 1. only 3-6 ft of soil rests abruptly on limestone. plasticity index. 1971). 1952) and to Kanto Loam of Japan (Miki. continually wet climate is the major factor determining the unusual properties of soils in this region. B. The combination of volcanic ash and continually wet climate have produced soils which are. Table 1 shows that the predominant clay minerals were the highly hydrated kandite minerals. 1960). percentage sand (as measured by wet sieving undried soil on a 75 pm sieve).204 K. close to the surface. it is extremely difficult to determine the relative influence of volcanic ash at each site. Basic data on these residual soils of the Papua New Guinea Highlands is presented in the next section of this Paper. sand mineralogy and local geomorphology indicates that the allophanic sandy clay (Soils No. The silty clay. 1960). in many respects. it is thought that the careful selection of sampling sites has produced data that are characteristic of the nature of soils on the parent materials within the region. Soil 1. 2 and No. 4) has been only slightly contaminated by ash (Wallace. The low silica/alumina ratios in Table 1 illustrate the severe leaching of silica during rock weathering. similar to the New Zealand andesitic ash soils (Birrell. 1971). WALLACE 90% of all days. The presence of allophane can remain undetected by available tests on soils which contain both hydrated halloysite and gibbsite. Although the data are inadequate for detailed statistical analysis and correlation. Soil depth on volcanics is generally greater than 20-30 ft whereas in many areas. The limestone soil (Soil No. 3) has been strongly influenced by andesitic ash whereas the halloysitic silty clay (Soil No. siltstone. 1971) after free iron removal by a dithionite-citrate system buffered with sodium bicarbonate (Mehra and Jackson. liquid limit. olive-brown :. yellow-brown fissured silty clay Firm.No.6 pH = 5. moist.4 2: 3 pH = 5. sandy . 1 on Volcanics No.. 4 3’ depth at Site No. silty clay I- l- Field description Description of soils Soi~~~~dnce Table 1. 3 5’-6” depth at Site No.3 1.5 pH = 6. 90 130 85 130 Typical field moisture content (105”G). fissured.15 SiOz Al.2 14 % Organic matter PH 0.6 1. % ___110 160 145 135 _____ - - Liquid limtit -___ 35 85 70 55 Plasticity index - - 15 (mainly aggregated clay minerals) 45 (rock forming minerals) 35 (rock forming minerals) 15 forming minerals) I[rock Approximate per. moist. sandy Firm.clay Firm. yellow-brown slightly friable..65 1. moist. 2 5’-6” depth at Site No. 4 on Limestone No. olive-brown.tred. moist. 1 location Firm..ztge - Gibbsite predominant with moderate amount: i of hydrated halloysite Allophane and vermiculite Allophane and vermiculite Hydrated halloysite predominant with moderate quantities of gibbsite Principal clay minerals - 1 pH = 5. 3 on Volcanics No.03 . 2 on Volcanics 7’-6” depth at Site No. the maximum shear strength occurring at a large deformation. ft which are common for equivalent. GEOTECHNICAL PROPERTIES Field moisture content In contrast to their firm moist appearance. There was general agreement between the Torvane shear strength measurements and the cohesive component of shear strength as measured in drained shear box tests on soaked samples of similar soil. The soils are not saturated. From this it was concluded that moisture suction does not contribute to the comparatively high undrained shear strength. In a later section of this Paper the high undrained shear strength is attributed to cemented bonds between the soil particles. . The field moisture content of most samples is between 80 and 100% of the liquid limit. The undrained shear strength is much higher than the values of 300 lb/sq. are shown in Fig. Generally the Torvane shear strength of undisturbed samples tested at field moisture content ranged from 800 to 1200 Ib/sq. normally consolidated. in that at low applied pressure the soil dilated during shearing and there was a pronounced peak shear strength. 1. WALLACE 2. ft. The measured degree of saturation of undisturbed samples was generally between 85 and 95%. ft but other samples from the same location gave values of 1100 Ib/sq. varying between 35 and 55 lb/cu. sedimentary soils (Skempton.206 K.33 and 2. B. while soils with a silica/alumina ratio in excess of 2. Stress-deformation curves for drained shear box tests which were carried out at two different applied pressures on soaked. and there is no consistent pattern of variation of moisture content with depth or between sites. This is illustrated by Soil 3 (Table 1) which contains 45% of sand particles but has a moisture content of 130%. This is very much higher than the values of 20-70% moisture contents that are typical of a wide range of sedimentary clays of low to quite high plasticity. ft. Field dry density of the sub-soil is correspondingly low.00 were considered to be non-Iateritic. undisturbed samples of Soil 2.00 as lateritic soils. 1971).8 for the parent materials. When the consistency limits are plotted on a Casagrande chart all soils are well below the A-line and would therefore be classified as highly plastic silts. Many of the samples tested had a moisture content which was about 10% lower than the liquid limit. The strength of the allophanic sandy clay (Soils 2 and 3) was slightly greater than that of the halloysitic silty clay (Soil 1). 1957 and Osterman. Generally the moisture content of the present soils varies between 80-190%. 1959). Martin and Doyne (1927) described soils with a silica/alumina ratio between 1. undrained shear strength has been determined using a Torvane. ft. The reIatively high shear strength is also indicated by the existence of stable. Shear strength The immediate. Ruxton (1968) has concluded that silica/alumina ratio is a good index of the degree of rock weathering in free-draining weathering environments in humid regions. Soils with a silica/alumina ratio less than 1. these soils have extremely high field moisture contents. At higher applied pressure the sample was compressed during shearing and there was no sharp peak shear strength. nearly vertical road cuttings up to 30 feet high. Several samples of Soil 2 had Torvane shear strengths of 2500 lb/sq. In their chemical classification of lateritic soils. It can be seen that the behaviour of the soil was similar to that of an overconsolidated clay.33 were called laterite soils. This conclusion is supported by the results of chemical analysis of the present soils (Wallace. Residual values of the angle of internal friction obtained from drained shear box tests on soaked. ft / Soaked. having a sensitivity of about 2. 3 640 980 lb/sq. --o---l Ib/rq. ft 100 300 . No. these soils are relatively insensitive. Lohnes et al.STRUCTURAL BEHAVIOUR OF RESIDUAL SOILS OF THE CONTINUALLY WET wN= 15 Ib/rq. for soils of such high porosity. in. 1 indicate that. UN=5 Horizontal HIGHLANDS in inches Direct shear box test results for loads of 5 and 15 lb/sq. The strength of undisturbed samples was not appreciably altered by soaking. 1. This is very much less than the sensitivity of the cemented Canadian sedimentary clays (Sangrey. 1 No. Both Table 2 and Fig. on Soil No. Ib/sq. 2 Table 2 illustrates how the undrained shear strength is affected by soaking and remoulding. movemenr OF PAPUA NEW GUINEA 207 in. in. but soaking greatly reduced the strength of remoulded samples. ft / I Soaked. ft 600 1040 Remoulded / Unsoaked. Table 2. Torvane shear strength before and after remoulding and soaking Soil Test condition Undisturbed 1 Unsoaked. (1971) report low sensitivity for a range of Puerto Rican lateritic soils. 300 420 at field moisture content lb/sq. lb/sq. undisturbed samples of sandy clay were: Soil 2 +d = 38” Soil 3 fjd = 29” Comparison of the descriptions of the two soils given in Table 1 shows that the higher strength of Soil 2 may be attributed to the lesser degree of weathering as indicated by the lower field moisture content and higher silica/alumina ratio of Soil 2. 1972b) but not much lower than that of many residual soils. Fig. 1 No. pJ C’. This is a subjective assessment based on the shape of the e-logp curves. On removal of the load the soil does not swell very much. 2 and the values obtained from oedometer tests on each of the four soils are summarized in Table 3. Finally. 1963).3 0.04.07 0.06.3 Compression i 1.29 0. 1. 0. No.5 ton/sq. The remoulded compressibility is also very high but is lower than that of the undisturbed soil at pressures exceeding the critical pressure. Gradwell and Birrell (1954) report values of 1. 3 No. ton/sq. Values for residual soils in the South-eastern United States (Sowers. B. 2 No. basalt and sandstone in Southern Brazil are widely scattered between O-6 and 4. for the least disturbed samples. ft. 1.6. Values of these indices are given on Fig.01-2. The resultant rebound curve. 0. In this respect.1 to 2.07 0. Kanto Loam (Koizumi and Ito.3 3. The relative compressibility and rebound of soils may be described by the following indices : and C.7-1.7 ton/sq.9-2.93 0’79. which is mineralogically Table 3. the compression index of the soil in its natural state (that is the slope of the e-logp curve at pressures exceeding the critical pressure. ft 1. Table 3 also gives some indication of the relative disturbance of ‘undisturbed’ samples.05 0. This critical pressure may therefore be attributed to cemented bonds between the soil particles as is discussed in a later section of this Paper. Soils that show a sharp change in slope of the curve over a narrow range of pressure around the critical pressure are described as least disturbed. Compressibility characteristics Soil .8 0. was roughly parallel to the compression curve for low applied pressures. WALLACE Compressibility Typical compressibility characteristics for these soils are shown in Figure 2 which presents the e-logp curves for oedometer tests on undisturbed and remoulded specimens of Soil 2.19 / i 1 Relative disturbance index 0. 3.6 1.31. 4 ~ I Critical pressure. At low applied pressure the compressibility is low but when the applied pressure exceeds a certain ‘critical pressure’ (pc) the compressibility of the soil is very high. ft and this may be compared with values reported for other residual soils. the compression index of the soil after remoulding C. ft for a wide range of volcanic clays.10. It should be noted that in all cases the critical pressure was much greater than the present overburden pressure..208 K. The magnitude of the critical pressure measured on samples of the various soils ranged from about 1 to 34 ton/sq. ft. Vargas (1953) shows that the critical pressures for residual clays on gneiss. 0. This assessment was found to agree well with the relative difficulty of preparing oedometer specimens from 4-inch tube samples.5. the Japanese soil.5 ton/sq. the swelling index (the slope of the rebound curve).59 j - most least least most disturbed disturbed disturbed disturbed . 2.39-1. 1963) vary between 1 and 5.74. 0’93 1.06 0. the compressibility characteristics are similar to those of overconsolidated soils but there is no geomorphological evidence to suggest that any of the soils have ever experienced overburden pressures of similar magnitude to the critical pressure. The continually wet climate and the presence of highly hydrated allophane and halloysite suggest that desiccation has not caused overconsolidation. .5 ton/sq. . (2) . 2. critical pressure the compressibility of the undisturbed samples with initial void ratios greater than 2 can be described by the relationship C = ae. . .STRUCTURAL BEHAVIOUR OF RESIDUAL I. + Ap may be described by the approximate relationship &$-olog. . . . (1) where a=0*6 and b=0.0 pressure in ton. to p. These correlations soils with an initial void ratio in excess of I. When the applied pressure exceeds the ship between compression index and initial void ratio. . 3. H. fr Typical e-logp curves for oedometer test on Soil No. .0 (or a saturation moisture content greater than 40%) there is a reasonably linear relationship between compression index and initial void ratio. indicate that for ratio are compared with the present results on Fig. . there is a Large point to point variations in residual large variation in values for any particular soil type. . soil properties are discussed in a later section of this Paper.-b .‘sq. 4.0 WET HIGHLANDS OF PAPUA NEW 209 GUINEA 5’0 2.7. the measured values of compression index are plotted against initial void ratio for It can be seen that there is a good linear relationboth undisturbed and remoulded samples. . I. ft.0 I/ O! 02 SOILS OF THE CONTINUALLY 05 Applied Fig. 2 and geotechnically similar to the present soils. has critical pressures within the range 2-5. For comparison of the compressibility of various soils at loads in excess of the critical pressure. . Similar correlations have been obtained by others as a result of more extensive tests on a Some of these other correlations between compression index and initial void variety of soils.op~ . the settlement. of a layer of thickness. . Compressibility correlations On Fig. due to an increase in pressure from p. S. From these diverse results it can be seen that although there is general agreement on the order of magnitude of critical pressures for a wide variety of residual soils. . USA and Columbia 5 e. co Fig. 3. . (3) . (5) Kay and Krizek USA Italian volcanic ash soils 1969): soils of western (1971): sedimentary roils:of USA Compression index correlations with initial void ratio (This approximation appears to be reasonable when considered with respect to the relatively low compressibility below the critical pressure and the uncertainty in estimating the critical pressure. 1 2 Initial Void 3 Ratlo. = [(a+b)n. for e. (1961): 0 Fig. . . 4.) Combining equations (1) and (2) gives S ae. . WALLACE 0 I 3 2 4 5 6 lnltial void ratio.-b -=l+elog. . . Relationship between initial void ratio and compression index of undisturbed and remoulded soils (I) The soils discussed in this Paper (2) Sowers (1963): residual soils of south-eastern (3) Penta et al. (4) Arango (Lambe and Whitman./l +e.210 K.-b] logIop+ * * . B.opc H 0 PO+AP Substituting initial porosity. no. 4 suggests that individual values may vary by f 30% or more from these correlations. Frost (1967) has emphasized the need for appropriate preparation prior to testing these soils. It should be emphasized that the preceding approximations are not suitable for close prediction of soil properties. . This linearity is discussed further in a later section of this Paper which considers the consequences of high moisture contents. EFFECT OF DRYING Without exception these soils become non-plastic when air-dried or oven-dried and do not regain their plasticity when wetted again. OF PAPUA NEW GUINEA . samples were dried and then remoulded and compacted to their original dry density. .--PC . . This flexibility (which is the inverse of the stiffness of the soil skeleton) is defined in terms of porosity by equation (4). (3) and (4) are a reasonable basis for analysing the structural characteristics of the soil. 211 (4) where A and B are constants such that A = (a + b) and B = b/(u + b). For the present soils. However it is thought that equations (l). The scatter of results obtained by the various investigators named on Fig. A= l-3 and B=0. To check the effect of drying on permeability. He has also observed that as a result of airdrying. . The general effect of drying on these creep characteristics is discussed later in the Paper. .55. This aggregation of soil particles on drying is illustrated on Table 5 which shows that results of particle size analysis of natural and dried samples of Soils 1 and 3. a comparison of equations (3) and (4) shows that the compression of the soil is more linearly related to porosity than to void ratio. (c) disturbance during sampling greatly increased measured creep rates at applied pressures less than the critical pressure.STRUCTURAL BEHAVIOUR OF RESIDUAL SOILS OF THE CONTINUALLY = A(no-B) WET HIGHLANDS PO+4 log. For example. (b) at loads greater than the critical pressure the creep rate was reasonably independent of applied pressure increment whereas. The permeability measured after saturation of the compacted sample was then compared with that of the same soil remoulded from the natural state without drying. the soaked. the permeability is increased more than one hundredfold. N 0*0002-0~0005). . -0*006) was roughly ten times the creep rate at loads less than the critical pressure (C.. the creep rate increased roughly linearly with applied pressure increment. The results of these tests are shown on Table 4. with respect to interpreting the general structural behaviour of the soil. Creep Some measurements of the rate of creep of oedometer specimens (the oedometer rings being lubricated with silicone grease and Ap/p = 1) have yielded the following conclusions : (a) at loads greater than the critical pressure the creep rate of undisturbed samples (C. . A simple interpretation of equation (4) suggests that B represents an apparent minimum porosity for the particular group of soils to which the equation is applied and the parameter A is the flexibility of the soil skeleton. remoulded CBR of one soil from the region was increased from 2 to 60%. where it can be seen that as a result of dehydration and aggregation of the clay particles on drying. below the critical pressure. . In these tests the dispersing agent was sodium hexametaphosphate. cm/s Soil dried. % 45 j 20 18 ~ 3 It is often suggested that this dramatic improvement of remoulded soil properties by drying could be used to considerable advantage in construction.: Table 5. ft No. which are only a very small part of the total volume of the soil particles and therefore may be more variable than the mineralogy or the soil texture (d) the occurrence of fissures which are often close to vertical: these fissures could be the remnants of cracks or joints in the parent material or they could have been caused by slumping or lateral unloading of the soil as the landscape is dissected by streams. . These point to point variations in soil properties were often as large as variations with depth at a given site or variations between sites. 1 No. remoulded and saturated Permeability.212 K. Ib/cu. % :z 1 Silt. % 47” Saud.I: Effect of drying on particle size Soil Tested after drying (105°C) Tested from natural state Sand. A similar lack of homogeneity has been observed for decomposed granite (Lumb. the continually wet climate which is responsible for formation of the unusual clay minerals usually persists throughout construction and limits the practicality of soil improvement by drying to the most compact sites such as airports or dam sites. the properties of soils sampled from a given depth in a given test pit varied considerably. lb/cu. % No. 1962). :: cm/s . % :: 1 Clay. However. % 1 Clay. WALLACE Table 4.. 1954).:. particularly when the soil is formed under well-drained conditions: (a) variations in grain size and mineralogy of the parent material (b) irregular chemical weathering and leaching of weathering products due to nonuniform seepage of ground water through the variable and fissured. Several samples of Soil 2 had undrained shear strengths of about twice the typical average value for this soil. such fissures will often determine the stability of road cuttings . and for the New Zealand volcanic ash clays (Gradwell and Birrell. 3 / 3: 1 Silt. The results of six measurements of the moisture content of parts of a two-inch lump of Soil 3 ranged from 108% (n=0*76) up to 146% (n=O*80). ft 5. This inhomogeneity is due to some or all of the following factors which are basic to the occurrence of residual soils. Effect of drying on permeability Soil Soil remoulded from natural state and saturated Dry density. 3 Permeability. Dry density. B. DISCUSSION Variability of residual soil properties As is typical of many residual soils.4 x lo-’ :. 1 1 No. weathered parent material (c) the dependence of the undisturbed properties on the nature of cemented bonds..7x 10-T 2. it is particularly important that site investigation reports should recognize residual soils as such and contain objective descriptions of their undisturbed strength (firm. This figure indicates that. Therefore. Because of the irrelevance of conventional classification systems with respect to residual soils (Wallace. as compressibility and probably also shear strength are linearly related to porosity. these soils appear to be unusually strong. The highly non-linear relationship between saturation moisture content and porosity for porous soils is reproduced in Fig. climate. stiff etc. and therefore compressibility and drained shear strength could be expected to have a similar primary dependence on soil structure. where applicable. it could be argued that the basic mechanism of large volume changes during compression is one of local shear. high moisture content may be due to an unusual fabric of high porosity. the relative compressibility is linearly dependent on porosity rather than on void ratio and moisture content. To many soils engineers who intuitively associate high field moisture content with low shear strength. high sensitivity and high compressibility. Alternatively. and for a wide variety of other porous soils. which of course represent proportionally high void ratios and higher than normal porosities. porosity while the previous relationship between compressibility and porosity suggests that the interlocking component of shear strength could also be more linearly related to porosity. soft. 1970) and the important influence of macroscopic structure. the strength and stiffness can still be surprisingly high. Much of the stiffness and strength of these soils may be attributed to the cemented structure which is described in the next section. fragmented etc. it can be concluded that it is not reasonable to expect that soils with high moisture contents will have proportionally low shear strength and compressibility. The following remarks are relevant to the latter type of soil. Is it reasonable to expect proporThis raises a simple question of practical importance. This conclusion indicates that porosity or dry density is a more reasonable basis for description and grouping of these extremely porous soils for engineering purposes.) and their macroscopic structure (friable. drainage. at loads high enough to break down this cemented structure. compared with variations of porosity with moisture content in the range of moisture contents between 20 and 70x. slope and. The location of these sites would be chosen according to changes in the basic soilforming factors: parent material. fissured. The drained shear strength of these porous soils is a complicated function of the cross sectional area of the soil skeleton and The cross sectional area will be linearly related to the degree of interlocking of soil particles. tionally high compressibility and low drained shear strength at high field moisture contents? High field moisture contents (that is moisture contents in excess of 40%) may be due to the presence of a highly active clay fraction or. as is the case for the present soils which contain clay minerals of known low activity. 5.STRUCTURAL (e) BEHAVIOUR OF RESIDUAL SOILS OF THE CONTINUALLY WET HIGHLANDS OF PAPUA NEW disturbance of the cemented soil structure during sampling and during laboratory specimens which will add to the variability of test results GUINEA trimming 213 of This variability of residual soil properties suggests that site investigations for extensive engineering works should concentrate on more intensive evaluation at fewer sampling sites. . Analysis of compressibility correlations in a preceding section has shown that for the present soils.). High moisture contents The most remarkable characteristic of these soils is their unusually high field moisture contents. an increase of moisture content from 100 to 150% represents a relatively small increase in porosity. however. This idealized structure of fine-grained residual soils is sketched in Fig. the presence of bonds in the completely residual limestone soil and successful dispersal of the clay fraction after ‘free iron’ removal suggest that the bonds are associated with iron and aluminium hydroxides precipitated during rock weathering.6 x c 8 8 a. The results of compression and shear tests described earlier show that the particles of the primary soil skeleton are cemented together at their contacts to form a continuous threedimensional structural framework. WALLACE 214 Fig. as the amount of iron oxides required . Measured free iron content of the soil gave no indication of strength or stiffness but such a correlation would not be expected.4 LCernented Bonds 0. 1959).K. B. A typical highly plastic sedimentary clay (LL = 60. 5 (left). Other residual soils will have a skeleton that consists partly of rock-forming mineral? and partly of aggregated weathering products. The proposed structure is different from that of many sedimentary sandy and silty clays. A sandy clay. which may be considered to consist of coarse particles dispersed throughout a clay matrix (Trollope and Chan. PL = 30) at a natural moisture content of 40% would require the addition of a volume of water equal to about 25% of its bulk volume to bring the soil to the liquid limit.2 0 20 50 70 150 100 Moisture Content I%1 Fig. The fabric of the limestone soil can be considered to be similar except that the skeleton is comprised of aggregated clay mineral particles rather than the original rock-forming minerals.0 0.=27) One consequence of the high moisture content in construction on the present soils is that they require comparatively small amounts of additional water to bring them to the liquid limit. Idealized residual soil structure I.8 0. 6. plagioclase and quartz) surrounded by a viscous gel of highly hydrated clay minerals and sesquioxides. 0. would reach the same consistency after the addition of less than 10% (by total volume) of water. Variation of porosity with moisture content of a saturated soil (G. Cemented soil structure The fabric or macrostructure of the volcanic soils (as viewed at X20 magnification) consists of a coarse open skeleton of rock-forming minerals (hornblende. similar to the present soils. 6 (below). The nature of this bond has not been definitely established but the absence of carbonates. at a natural moisture content of 12O’A (LL= 135). This illustrates how the surface of trafficked earthfill will be reduced to a quagmire by relatively light rainfall. the low organic content. STRUCTURAL BEHAVIOUR OF RESIDUAL SOILS OF THE CONTINUALLY WET HIGHLANDS OF PAPUA NEW GUINEA 215 to form the bonds could be much smaller than the measured total free iron contents of between one and ten per cent. (h) The creep rate at high pressure is roughly ten times greater than at low pressures and is reasonably independent of applied pressure increment. is the initial porosity. It was thought that if oedometer specimens could be prepared from soil which had been dried and then resaturated without disturbance. B is an apparent minimum porosity and A is the flexibility of the soil skeleton.) (a) Removal of the gel only slightly lowered the critical pressure from 3. resaturated oedometer specimens of Soil 2 produced the following results. (d) Removal of the gel reduced the creep rate at high pressures to about 20% of that of the natural soil.-B). General structural behaviour Before discussing the observed soil behaviour in terms of the soil structure the earlier conclusions concerning the structural behaviour of the undisturbed natural soil are summarized as follows. (f) The volumetric strain at high pressures in the oedometer test was directly proportional to _4(n. (g) The creep rate at low pressures is approximately linearly proportional to the applied pressure increment.. when the soil skeleton strength controls drained stress-strain characteristics. respectively. 1958 and Newill. Higher free iron contents would also be associated with more advanced weathering of the soil skeleton. (k) Drained shear strength is comparatively high. (j) The soil is relatively insensitive. (In the following discussion ‘low pressures’ and ‘high pressures’ refer to applied pressures which are less than or greater than the critical pressure. 1972b). Creep rates for the dried-resaturated soil at low pressures were similar to those at high pressures.8 ton/sq. Sangrey and Townsend (1969) have shown that the .. Sesquioxide cementing agents have also been indirectly detected in some African clays (Terzaghi. 1968 and Sangrey. (b) Compressibility at high pressures was not affected by removal of the gel. 1961) and also in cemented Canadian clays (Quigley. Therefore it is expected that at low stresses. This is due to the large increase in permeability noted earlier in this Paper. ft.5 to 2. elastic soil skeleton. (e) At low pressures the soil compressibility is low and is similar to the rebound. there would be similarities between the behaviour of the two types of soil. (c) Consolidation of the dried-resaturated soil was almost instantaneous. where n. From the preceding statements it can be interpreted that the bonds are independent of the viscous gel and that at low pressure the bonds are intact and the structural behaviour is controlled by the stiff. Relative roles of cemented bonds and viscous gel When the present soils are dried the clay minerals aggregate so that the viscous gel is permanently destroyed. It is interesting to note that the rock-forming minerals present in the cemented Canadian clays are the same as those found in andesitic ash soils. these specimens would possess cemented bonds without the viscous gel and a comparison with the behaviour of normal undisturbed samples would reveal something of the relative roles of the bonds and gel. Consolidation tests on six undisturbed dried. (i) The undrained shear strength is comparatively high and is not affected by soaking. This reduction could be explained by the greater disturbance of the dried-resaturated specimens. From the literature it appears that many residual soils have the cemented structure idealized herein. these criteria would be modified by discontinuities at the design site. The dependence of the rate of creep of the intact soil on the applied pressure. together with the similarity of the creep of dried-resaturated soil above and below the critical pressure. even in a highly remoulded state. at low applied stresses and. The effect of the viscous gel in producing a much greater and apparently pressure-independent creep at high pressures could be attributed to a micropore mechanism (Barden. The primary dependence of compressibility on the porosity and critical pressure is well established for a variety of porous sedimentary and . at high applied stresses it is the porous soil skeleton which is most important in determining structural behaviour. Compression and shear tests have concentrated on the sandier volcanic soils for which it was easier to prepare good undisturbed samples but similar results have been obtained on fewer samples of volcanic silty clay and the more friable silty clay on limestone. This indicates that the viscous gel can drain freely through the porous skeleton and prevent any significant pore pressures on shear planes. However. From this discussion. suggests that at low pressures the creep mechanism is one of readjustment of stresses in the primary skeleton and fracture of the weaker bonds. This does not allow close definition of yield criteria such as those given by Sangrey (1972a) for finer grained.216 K. All the soils tested were well-drained lateritic soils. The strength of the cemented bonds in residual soils is quite variable. It is expected that point to point variability of soil properties will be high for most residual soils. it is concluded that the structural behaviour of the soil is consistent with the proposed model of the soil structure and that. B. when intact. WALLACE volumetric strain of intact Canadian clays is directly proportional to applied pressure. more homogeneous. increases with applied pressure and is to a large extent recoverable. even in a highly compressed or sheared state. CONCLUSION The conclusions arrived at in the preceding discussion are based on a limited number of tests on residual soils formed in a continually wet environment. General field observations of residual soils indicate that even if rough criteria could be established in the laboratory. the Author has observed several specimens in which the soil skeleton has tended to collapse to much lower porosities under repeated shear stresses. as is shown by the variation in critical pressures and undrained shear strengths. there is considerable interlocking of the coarse particles and that the proportion of the total load carried by the gel is small. High shear strength and a link between shear strength and degree of weathering are frequentlyreported for residual soils. cemented sedimentary soils. The lower compressibility of the present soils together with normal experimental variability precludes any more definite conclusions than that the volumetric strain of the present soils. to a lesser extent. The high value of apparent minimum porosity indicated by oedometer tests suggests that the porosity of the soil will be high. Similar progressive microfracturing has been studied in detail in investigations into the creep of brittle rock at low temperature and pressure (Scholz. 1970). 1968) and indicates that when the bonds are fractured the gel supports a portion of the load at contacts between the coarser particles. It is appropriate therefore to conclude by commenting on the breadth of application of the conclusions reached in this Paper. The low sensitivity and high drained shear strength indicate that. REFERENCES Australian Mineral Development Laboratories (1971). Primary and secondary consolidation of clay and peat. S. Mr Gavera Morea and Mr Thomas Tohiana during the field work is also acknowledged. Bucharest 493-500.E. Accumulated soils engineering experience with sedimentary soils has shown that correlations of the properties of the soils with simple parameters are extremely useful for determining relavant description and classification systems and for preliminary estimates of soil properties. J. and of civil engineering students Mr John Kavagu. Mr J. K. Proc. Frost. & Birrell. K. Soil Mech. R. 119-129. Conf. (1952).. Birrell. Unpublished Report MP2033/71.STRUCTURAL BEHAVIOUR OF RESIDUAL SOILS OF THE CONTINUALLY WET HIGHLANDS OF PAPUA NEW GUINEA 217 residual soils. J. Roland. (1968). Blake. Sot. Soil Sci. Sowers (1963) postulates a mechanism of fracture of bonds through unequal expansion of low plasticity residual soils when wetted. Soil Mech. One aspect of the behaviour of the present soils which is thought to be peculiar to a much narrower range of residual soils is the stability of the cemented bonds on soaking or drying. ACKNOWLEDGEMENTS The work that has been described in this Paper was completed while the Author was a member of the staff of the Papua New Guinea Institute of Technology. G. Mr Des Clancy. 1. Clay mineral analysis of eight soil samplesfrom the New Guinea Highlands. P. 8th Znt. L. Volcanic and glacial landforms Geol. E. Analysis of uncertainty in settlement prediction. Engng 2. Adelaide. This is thought to be associated with soil formation under continually wet conditions and will not apply to soils formed under seasonal climates. D. Cong. Conf. (1954). H. New Zealand Jnl Sci. 30-34. Some physical properties of New Zealand volcanic ash soils. of the Catholic Mission. (1967). No. (1971). porous skeleton. 37-48. Barden. The Author is grateful to the Institute for its generous support of the project. & Krizek. The geotechnical tests were carried out in the Institute’s Soil Mechanics Laboratories with the assistance of Mr M. The Author believes that when the general characteristics of residual soils are better known much of their behaviour will be interpretable in terms of our much wider knowledge of the mechanics of sedimentary soils. Such generalizations can only come from wider discussion of the formation. K. S. l-24. Kay.. A. H. 1st S. . Rizzi and Mr J. 1605-1614. 82. Low sensitivity whenremoulding at field moisture content is associated with the coarse cemented soil structure but it is not known to what extent residual soils can be generally assumed to have a coarse. H. (1971). Ge’otechnique 18. Asian 43-53. Amer. (1964). on Mount Giluwe. 1st A. and New Guinea. Melbourne. Technol. B36. J. although it is considered that such a structure is associated with formation under well-drained conditions. Physical properties of certain volcanic clays. but are subject to typical variations of & 30% around the predicted values. N.N. nature and structural behaviour of residual soils from a variety of sources. Importance Bankok. Soil zonality and parent rock in a very wet tropical mountain region. and his staff. Proc. NO. Territory of Papua Bull. Geotech. Proc.Z.. while the Author has observed that residual soils in the seasonally wet Eastern Highlands of New Guinea are much more difficult to sample in an undisturbed state and disintegrate readily on soaking. &Rutherford. W. Haantjens. Gradwell. Mendi. of correct pretesting preparation of some tropical soils. M. 2. The valuable support of the Southern Highlands District Commissioner. Residual soil properties will be more variable but any simple working generalizations on structural behaviour will be valuable aids in soils engineering. and Liiffler. Gaya. R. D. Conf Soil Mech. No. Report No. 2. Lumb. F. (1969). D. Acta Polytechnica. Civ. J. (1968). Croce. Some engineering properties of residual clay soils occurring in Southern Brazil. 139-152. No. 2nd Cong. 1. Soil structure and the step-strain phenomenon. Can. (1958). Jnl5. J. 117. Engrs 9. Ruxton. (1971). Discussion on: The Planning and design of the new Hong Kong airport. (1927). Sci. Proc. (1962). Bull. & Chan. (1953). Penta. & ESU. G. Proc. Trollope. . Jnf Geoi. M. 3. B. Laterite and laterite soils in Sierra Leone. (1969). Design and performance of Sasamua Dam. Canada: Queen’s University at Kingston. K. T. Lohnes. 76. 39-62. London: Pergamon Press. (1971). Proc. 63. Geotechnical properties of selected Puerto Rican soils in relation to climate and parent rock. Skempton. 1. Port Moresby.218 K. 2617-2624. & Doyne. K. l-39. Miki. Clays Clay Miner. 4. M. Jnl9. H. Lambe. Eng. (1960). Research Department of Civil Engineering. Rock Mech. Can. D. Proc. C. 263. Geotechnique 22. F. Conlon. Y. 5th Znt. 175-177. K. 82. (1972a). A. Measures of the degree of chemical weathering of rocks. Brazil 1. Vargas. New York: Wiley. Asian Reg. Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Sangrey. 7. Soil Mech. Civil Engineering Department Bulletin No. Soil Mech.. 6. 3. 302-318. No. Znstn Civ. Sowers. On the causes of natural cementation in sensitive soils. 3rd Znt. & Jackson. Quigley. Proc. Naturally cemented sensitive soils. Engineering Section. (1970). (1961). No. (1960). Lae. (1970). G. A. 305-307. R. SM2. R. Jnl Agric. W. T. & Whitman. P. 17. B. 518-527. (1968). No. Osterman. 0. Geotechnical properties of volcanic ashes occurring in Japan. W. Geol. (1963). K. (1972b). Ontario. 37-48. Engineering properties of residual soils derived from igneous and metamorphic rocks. Conf. WALLACE Koizumi. M. F. 42nd ANZAAS tong. C. K. D. Notes on the shearing resistance of soft clays. Conf. D. Characteristics of three sensitive Canadian chzys. Newill. Proc. (1961). No. Soil mechanics 318-321.. No. & Townsend. Znstn Civ. 226-243.. R. Geotech. R. V. Znt. Systematic description of soils for road construction in Papua and New Guinea. F. (1963). Sept.. Residual soils of the continually wet Highlands ofPapua New Guinea. (1959). Am. Sot. A. B. Discussion on: Landslide on the Toulnustouc River. C. Soil Mech. 67-71. B. Quebec by R. Geotech. A laboratory investigation of two red clays from Kenya. Scholz. JnI Soil Mech. Paper No. 0. A. H. 2nd Pan Amer. 530-547. Engineering properties of volcanic soils. Stand. Amer. & Demeril. 2. Soil and foundution 3. Geotechnique 11. Sot. Wallace. (1959). Delhi 1-8. Proc. Engrs 86. P. Compressibility of a certain volcanic clay. 369-394. Proc. Zurich 1. D. 7th Nat. Sangrey. No. L. Conf. 117-119. P. Terzaghi. Mehra. 285-291. Sot. The properties of decomposed granite. J. Geotechnique 12. A. Sangrey. Martin. Wallace. (1957). Fish. Role of microfracturing in rock deformation. 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Report "Structural Behaviour of Residual Soils of the Continually Wet Highlands of Papua New Guinea"