13/09/20121 1 Physical and Hydraulic Properties of Soils Gravels, Sands Silts Clays Grain size: Coarse grained Can see individual grains by eye Fined grained Cannot see individual grains Fine grained Cannot see individual grains Characteristics: Cohesionless Nonplastic Granular Cohesionless Nonplastic Granular Cohesive Plastic Effect of water on engineering behavior: Relatively unimportant (exception: loose saturated granular materials and dynamic loadings) Important Important Effect of grain size distribution on engineering behavior: Important Relatively unimportant Relatively unimportant 2 13/09/2012 2 Role of Classification System in Geotechnical Engineering Practice Classification and Index Properties (w, e, ¸, GSD, LL, PI, etc.) Classification System (USCS, British, etc.) Engineering Properties (permeabilities, compressibility, shrink swell, shear strength, etc.) Engineering Purpose (highways, airfields, foundations, dams, etc.) 3 Liquid Limit, % 0 10 20 30 40 50 60 70 80 90 100 P l a s t i c i t y I n d e x , % 0 10 20 30 40 50 60 CL CH ML or OL Micaceous or diatomaceous fine sandy and silty soils; elastic silts; organic silts, clays, and silty clays OH or MH A - lin e U - lin e ML CL-ML Inorganic and organic silts and silty clays of low plasticity; rock flour; silty or clayey fine sands In organic clays of high plasticity Medium plastic inorganic clays Lowplastic inorganic clays; sandy and silty clays Silty clays; clayey silts and sands Characteristics Equal LL increasing PI Equal PI increasing LL Dry Strength Increases Decreases Toughness near PL Increases Decreases Permeability Decreases Increases Compressibility About the same Increases 4 13/09/2012 3 Strength Parameters Cohesive Soils • Total Stress Analysis Method • Undrained Shear Strength (Unconsolidated Undrained or Consolidated Undrained Triaxial Test, Field Vane Shear Test) • Effective Stress Method • Internal Friction Angle (Consolidated Drained or Consolidated Undrained Triaxial Test with pore water measurement) • Effective cohesion (Consolidated Drained or Consolidated Undrained Triaxial Test with pore water measurement) Cohesionless Soils • Internal friction angle (Direct Shear Test, estimated from field tests) 5 Compressibility and Consolidation Parameters Cohesive Soils • Elastic Deformation • Undrained Deformation Modulus, Eu • Poisson’s Ratio, v • Consolidation Settlement • Compression Index, Cc; Recompression Index, Cr • Initial Void Ratio, e o • Preconsolidation Pressure, Pc; OCR (Overconsolidation Ratio) Cohesionless Soils • Deformation Modulus, E • Poisson’s RatIo, v 6 13/09/2012 4 Risk Based Flow Chart for Site Characterization 7 In‐Situ Tests Reference/Manual: • ASTM D420 Site Characterization for Engineering Design and Construction Purposes • Manual on Subsurface Investigations • National Highway Institute • Publication No. FHWA NHI‐01‐031 • Federal Highway Administration • Washington, DC • Geotechnical Site Characterization • July 2001 • by Paul W. Mayne, Barry R. Christopher, and Jason DeJong 8 13/09/2012 5 In‐Situ Tests The objectives of any subsurface investigation are to determined the following: • Nature and sequence of the subsurface strata • Groundwater conditions • Physical and mechanical properties of the subsurface strata 9 In‐Situ Tests The applicability and usefulness of in‐situ tests (Lunne, Robertson & Powell, 2007): 10 13/09/2012 6 In‐Situ Test Methods 11 Cone Penetration Test (CPT) Advantages: • Fast and continuous profiling • Repeatable and reliable data (not operator‐dependent) • Economical and productive • Strong theoretical basis for interpretation Disadvantages: • High capital investment (for piezocone‐CPTU and seismic‐SCPT) • Requires skilled operators • No soil sample • Penetration can be restricted in gravel/cemented layers 12 13/09/2012 7 Standard Penetration Test (SPT) SPT is used as an indicator of relative density and stiffness of granular soils as well as an indicator of consistency of other ground. SPT is applied in a wide range of geotechnical application including shallow and deep foundation and the assessment of liquefaction potential Advantages: • Simple and rugged • Low cost • Obtain a sample • Can be performed in most soil types Disadvantages: • Disturbed sample (index tests only) • Crude number (N value) • Not applicable in soft clays and silts • High variability and uncertainty 13 Field Vane Shear Test (FVST) FVST is used to evaluate the undrained shear strength, s uv , of soft to stiff clays and silts. Both peak and remolded strengths can be measured and their ratio is termed soil sensitivity (St) Advantages: • Simple test and equipment • Long history of use in practice Disadvantages: • Limited application to soft to stiff clays and silts • Slow and time‐consuming • Raw s uv values need (empirical) correction • Can be affected by sand lenses and seams 14 13/09/2012 8 Pressuremeter Test (PMT) PMT can be used to evaluate the stress‐strain response of a wide range of soils and rock Advantages: • Strong theoretical basis for interpretation • Tests large volume of ground Disadvantages: • Complicated equipment and procedures • Requires skilled operator • Time consuming and expensive • Equipment can be easily damaged 15 Cone Penetration Test (CPT) 16 Begemann Type Cone with Friction Sleeve Various Cone Penetrometer including Electric Friction and Piezocone Types 13/09/2012 9 Cone Penetration Test (CPT) 17 2 ) 1 ( u a q q n c T ÷ + = a n = net area ratio determined from calibration of the cone in a triaxial chamber. For most 10cm 2 Penetrometer: 0.75 < a n ≤ 0.82 CPT Results 18 13/09/2012 10 CPT Interpretation 19 CPT Interpretation Soil Profiling and Classification • Non‐normalized Chart • Most commonly used soil behavior type (SBT) method (Robertson et al., 1986) • Use cone resistance and friction ratio • For sounding up to 20 meter in depth • Normalized Chart • CPT readings must be corrected for overburden stress especially for the very shallow and very deep soundings • Robertson et al., 1990 SBT charts 20 13/09/2012 11 CPT Interpretation Soil Behavior Type (Robertson et al. 1986) 21 CPT Interpretation Normalized Soil Behavior Type (Robertson et al. 1990) 22 13/09/2012 12 CPT Interpretation Soil Behavior Type Index, I c 23 CPT Interpretation Equivalent SPT N 60 24 Robertson, et al., 1983 13/09/2012 13 CPT Interpretation Equivalent SPT N 60 25 For non‐normalized CPT Chart (Robertson et al., 1986) CPT Interpretation 26 Geotechnical Parameters • Undrained Shear Strength • Soil Sensitivity, St kt v T u N q s o ÷ = N kt = 10 – 20 (15 as an average) Plasticity N kt Sensitivity N kt s kt v T remolded u u t f N q s s S 1 . o ÷ = = 13/09/2012 14 CPT Interpretation 27 Geotechnical Parameters • Overconsolidation Ratio | | . | \ | ÷ = = ' ' ' vo v T vo p q k OCR o o o o k = 0.2 – 0.5 (0.3 is recommended) For moderate to high risk project site‐specific correlation with laboratory OCR is highly recommended CPT Interpretation 28 Geotechnical Parameters • In‐situ Stress Ratio, Ko | | . | \ | ÷ = ' 1 . 0 vo v T o q K o o Kulhawy and Mayne (1990) For mechanically overconsolidated soils not for cemented soils Note: CPT is not a reliable method to determine Ko 13/09/2012 15 CPT Interpretation 29 Geotechnical Parameters • In‐situ Stress Ratio, Ko Andresen et al. (1979) CPT Interpretation 30 Geotechnical Parameters • Friction Angle of uncemented, unaged, moderately compressible, predominantly quartz sands (Robertson and Campanella, 1983) 13/09/2012 16 CPT Interpretation 31 Geotechnical Parameters • Stiffness and Modulus Recent fill with an age less than 10 years Natural sands with an age greater than 100 years CPT Application 32 13/09/2012 17 Standard Penetration Test (SPT) ASTM D1586 33 Standard Penetration Test (SPT) 34 13/09/2012 18 Standard Penetration Test (SPT) 35 Factors Affecting SPT Drilling and borehole technique • Method of borehole advancement (wash boring, coring, percussion drilling) • Method of borehole support (casing or bentonite slurry) • Size of borehole (55 150mm) SPT equipment • Hammer design • Rod size and type • Sampler design (with or without liner) Test Procedure • Seating drive (6 inches = 15 cm) • Method of measurement (Delivered energy is not linear when SPT >50) • Rate of hammer blows (30 – 40 blows/minute) 36 13/09/2012 19 SPT Hammer Types 37 Automatic tripping mechanism Recommended SPT Test Procedure 38 13/09/2012 20 Factors Affecting SPT Interpretation Energy delivered • Energy rod ratio (ER) varies between 20 – 100% • Normalized to ER of 60% Overburden stress • Penetration resistance increases with increasing overburden pressure • Normalized to equivalent value at an effective overburden pressure of 1 atm (1 tsf = 100 kPa) Ground conditions 39 measured f N E N 60 60 = Factors Affecting SPT Interpretation Ground Conditions • Granular soils • Horizontal stress and stress history • Grain characteristics • Age and cementation • Drainage • Cohesive soils • Stress history • Sensitivity • Soil Structure • Weak and weathered rocks • Spacing of joints • Weathering • Hard inclusions 40 13/09/2012 21 Factors Affecting SPT Interpretation 41 Illustration of Variation in SPT Energy Ratio SPT Energy Measurement SPT Correction Factors 42 13/09/2012 22 SPT Interpretation • Generally restricted to clean cohesionless soils where penetration takes place under drained conditions. However correlations have been developed for a wide range of ground 43 SPT Interpretation Relative Density Friction Angle Stiffness and Modulus Undrained Shear Strength Stress History Compressibility 44 13/09/2012 23 SPT Interpretation Relative Density 45 SPT Interpretation Friction Angle 46 De Mello (1971) Hatanaka & Uchida (1996) : 13/09/2012 24 SPT Interpretation Stiffness and Modulus 47 >1000 years of age <10 years of age For clean uncemented silica sands SPT Interpretation Stiffness and Modulus 48 q net /q ult Stroud (1989) For clean uncemented silica sands 13/09/2012 25 SPT Interpretation Undrained Shear Strength 49 Ladd et al. (1977): Use SPT vs Undrained Shear Strength for relatively stiff and insensitive clays SPT Interpretation Stress History (OCR) 50 Kulhawy and Mayne (1990) 13/09/2012 26 SPT Applications 51 Field Vane Shear Test ASTM D 2573 52 13/09/2012 27 FVST Equipment 53 FVST Blade Size and Dimension 54 13/09/2012 28 FVST Interpretation Undrained Shear Strength • Assumed uniform stress distribution of top and bottom and along the blades and a vane with a height‐to‐width ratio, H/D = 2 • T max = maximum torque 55 3 max 7 6 D T s uv t = FVST Interpretation Sensitivity • After the peak strength is obtained the vane is rotated quickly 10 complete revolutions and the test repeated to measure the remolded strength 56 remolded uv peak uv t s s S ( ) ( = 13/09/2012 29 FVST Interpretation Correction Factor • It is common to correct FVST value prior to the application in stability analysis of embankment on soft soils, bearing capacity and excavation in soft ground 57 uv R mobilized s µ t = 5 . 0 05 . 1 × × ÷ = PI b R µ (minute) failure to time factor rate a log 0075 . 0 015 . 0 = = + = f f t b t b Chandler, 1988 Pressuremeter Test (PMT) ASTM D 4719 58 13/09/2012 30 PMT Equipment and Procedure 59 PMT Interpretation Index Parameters • Limit Pressure, P L • Net Limit Pressure, P L * = P L – P o • At rest total stress 60 ( ) t coefficien pressure earth lateral pressure pore c hydrostati surface ground below depth density bulk = = = = + ÷ × = o o o K u z u K u z P ¸ ¸ 13/09/2012 31 PMT Interpretation • Pressuremeter Modulus – From elastic theory for radial expansion of a cylindrical cavity: 61 cavity in the pressure cavity the of volume the modulus shear the = = = A A × = P V G V P V G PMT Interpretation • Pressuremeter Modulus – Use volume V corresponding to the mid point of the linear portion of the volume‐pressure curve, V PM 62 Ratio s Poisson' ) 1 ( 2 ) 1 ( 2 : Modulus s Young' = A A × + = + = A A × = v v v V P V E G E V P V G PM PM PM PM PM PM 13/09/2012 32 PMT Applications • Foundation Design – End bearing capacity (base support) – Net bearing capacity 63 ( ) o L P P k q ÷ = * k is a correction factor which is dependent on soil type, depth of embedment, PL, and foundation shape PMT Applications 64 Values of k for Footings and Cast‐in‐place Pile Tips 13/09/2012 33 PMT Applications 65 Values of k for Footings and Cast‐in‐place Pile Tips PMT Applications 66 Values of k for Footings and Cast‐in‐place Pile Tips 13/09/2012 34 PMT Applications 67 Values of k for Driven Piles PMT Applications 68 Values of k for Driven Piles 13/09/2012 35 PMT Applications 69 A d h e s i o n , k P a Side Friction for Piles Laboratory Triaxial Tests Unconsolidated Undrained Triaxial Test Consolidated Drained Triaxial Test Consolidated Undrained Triaxial Test with Pore Water Measurement 70 13/09/2012 36 Unconsolidated Undrained Triaxial Test 71 Unconsolidated Undrained Triaxial Test 72 13/09/2012 37 Unconfined Compression Test 73 Consolidated Undrained Triaxial Test 74 Total and effective stress failure envelopes 13/09/2012 38 Stress Path of CU Triaxial Test for Normally Consolidated Clays 75 Stress Path of CU Triaxial Test for Overconsolidated Clays 76 13/09/2012 39 1. Guide to Cone Penetration Testing for Geotechnical Engineering, P.K. Robertson and K.L. Cabal (Robertson), January 2009 2. Guide to In‐Situ Testing, P.K. Robertson, January 2006 3. An Introduction to Geotechnical Engineering, Holtz and Kovacs (1981) 77 References Thank you 78
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