Triaxial Testing of Soils

Poul Lade has a career in laboratory experimentation at university level to study and model the behaviour of soils. With his students, the author developed testing equipment, performed experiments and build constitutive models for the observed soil behaviour while a professor at University of California at Los Angeles (UCLA) (1972-1993), Johns Hopkins University (1993-1999), Aalborg University in Denmark (1999-2003), and Catholic University of America in Washington, D.C. (2003-2015). Many of the experimental techniques developed over this range of years are explained in the present book.

• Preface xiiiAbout the Author xvii1 Principles of Triaxial Testing 11.1 Purpose of triaxial tests 11.2 Concept of testing 11.3 The triaxial test 21.4 Advantages and limitations 31.5 Test stages – consolidation and shearing 41.5.1 Consolidation 51.5.2 Shearing 51.6 Types of tests 51.6.1 Simulation of field conditions 61.6.2 Selection of test type 122 Computations and Presentation of Test Results 132.1 Data reduction 132.1.1 Sign rule – 2D 132.1.2 Strains 132.1.3 Cross‐sectional area 232.1.4 Stresses 242.1.5 Corrections 252.1.6 The effective stress principle 252.1.7 Stress analysis in two dimensions – Mohr’s circle 252.1.8 Strain analysis in two dimensions – Mohr’s circle 272.2 Stress–strain diagrams 282.2.1 Basic diagrams 282.2.2 Modulus evaluation 372.2.3 Derived diagrams 412.2.4 Normalized stress–strain behavior 482.2.5 Patterns of soil behavior – error recognition 492.3 Strength diagrams 512.3.1 Definition of effective and total strengths 512.3.2 Mohr–Coulomb failure concept 512.3.3 Mohr–Coulomb for triaxial compression 542.3.4 Curved failure envelope 552.3.5 MIT p–q diagram 572.3.6 Cambridge p–q diagram 592.3.7 Determination of best‐fit soil strength parameters 602.3.8 Characterization of total strength 602.4 Stress paths 612.4.1 Drained stress paths 612.4.2 Total stress paths in undrained tests 612.4.3 Effective stress paths in undrained tests 612.4.4 Normalized p–q diagrams 662.4.5 Vector curves 682.5 Linear regression analysis 722.5.1 MIT p–q diagram 722.5.2 Cambridge p–q diagram 742.5.3 Correct and incorrect linear regression analyses 752.6 Three‐dimensional stress states 762.6.1 General 3D stress states 762.6.2 Stress invariants 762.6.3 Stress deviator invariants 802.6.4 Magnitudes and directions of principal stresses 812.7 Principal stress space 832.7.1 Octahedral stresses 832.7.2 Triaxial plane 842.7.3 Octahedral plane 862.7.4 Characterization of 3D stress conditions 872.7.5 Shapes of stress invariants in principal stress space 892.7.6 Procedures for projecting stress points onto a common octahedral plane 902.7.7 Procedure for plotting stress points on an octahedral plane 962.7.8 Representation of test results with principal stress rotation 973 Triaxial Equipment 993.1 Triaxial setup 993.1.1 Specimen, cap, and base 993.1.2 Membrane 1033.1.3 O‐rings 1053.1.4 Drainage system 1063.1.5 Leakage of triaxial setup 1123.1.6 Volume change devices 1133.1.7 Cell fluid 1133.1.8 Lubricated ends 1203.2 Triaxial cell 1253.2.1 Cell types 1253.2.2 Cell wall 1273.2.3 Hoek cell 1283.3 Piston 1283.3.1 Piston friction 1293.3.2 Connections between piston, cap, and specimen 1323.4 Pressure supply 1333.4.1 Water column 1333.4.2 Mercury pot system 1343.4.3 Compressed gas 1353.4.4 Mechanically compressed fluids 1363.4.5 Pressure intensifiers 1373.4.6 Pressure transfer to triaxial cell 1373.4.7 Vacuum to supply effective confining pressure 1383.5 Vertical loading equipment 1393.5.1 Deformation or strain control 1393.5.2 Load control 1403.5.3 Stress control 1413.5.4 Combination of load control and deformation control 1413.5.5 Stiffness requirements 1433.5.6 Strain control versus load control 1433.6 Triaxial cell with integrated loading system 1434 Instrumentation, Measurements, and Control 1454.1 Purpose of instrumentation 1454.2 Principle of measurements 1454.3 Instrument characteristics 1474.4 Electrical instrument operation principles 1494.4.1 Strain gage 1494.4.2 Linear variable differential transformer 1514.4.3 Proximity gage 1534.4.4 Reluctance gage 1534.4.5 Electrolytic liquid level 1544.4.6 Hall effect technique 1544.4.7 Elastomer gage 1544.4.8 Capacitance technique 1554.5 Instrument measurement uncertainty 1554.5.1 Accuracy, precision, and resolution 1564.5.2 Measurement uncertainty in triaxial tests 1564.6 Instrument performance characteristics 1584.6.1 Excitation 1584.6.2 Zero shift 1594.6.3 Sensitivity 1594.6.4 Thermal effects on zero shift and sensitivity 1594.6.5 Natural frequency 1594.6.6 Nonlinearity 1594.6.7 Hysteresis 1594.6.8 Repeatability 1594.6.9 Range 1594.6.10 Overload capacity 1604.6.11 Overload protection 1604.6.12 Volumetric flexibility of pressure transducers 1604.7 Measurement of linear deformations 1604.7.1 Inside and outside measurements 1604.7.2 Recommended gage length 1624.7.3 Operational requirements 1624.7.4 Electric wires 1634.7.5 Clip gages 1634.7.6 Linear variable differential transformer setup 1674.7.7 Proximity gage setup 1684.7.8 Inclinometer gages 1704.7.9 Hall effect gage 1714.7.10 X‐ray technique 1714.7.11 Video tracking and high‐speed photography 1714.7.12 Optical deformation measurements 1724.7.13 Characteristics of linear deformation measurement devices 1744.8 Measurement of volume changes 1784.8.1 Requirements for volume change devices 1784.8.2 Measurements from saturated specimens 1804.8.3 Measurements from a triaxial cell 1894.8.4 Measurements from dry and partly saturated specimens 1924.9 Measurement of axial load 1954.9.1 Mechanical force transducers 1954.9.2 Operating principle of strain gage load cells 1974.9.3 Primary sensors 1974.9.4 Fabrication of diaphragm load cells 1984.9.5 Load capacity and overload protection 1984.10 Measurement of pressure 1994.10.1 Measurement of cell pressure 1994.10.2 Measurement of pore pressure 1994.10.3 Operating principles of pressure transducers 2014.10.4 Fabrication of pressure transducers 2014.10.5 Pressure capacity and overpressure protection 2014.11 Specifications for instruments 2014.12 Factors in the selection of instruments 2024.13 Measurement redundancy 2024.14 Calibration of instruments 2034.14.1 Calibration of linear deformation devices 2034.14.2 Calibration of volume change devices 2044.14.3 Calibration of axial load devices 2044.14.4 Calibration of pressure gages and transducers 2044.15 Data acquisition 2064.15.1 Manual datalogging 2064.15.2 Computer datalogging 2064.16 Test control 2064.16.1 Control of load, pressure, and deformations 2064.16.2 Principles of control systems 2075 Preparation of Triaxial Specimens 2115.1 Intact specimens 2115.1.1 Storage of samples 2115.1.2 Sample inspection and documentation 2125.1.3 Ejection of specimens 2145.1.4 Trimming of specimens 2155.1.5 Freezing technique to produce intact samples of granular materials 2175.2 Laboratory preparation of specimens 2175.2.1 Slurry consolidation of clay 2175.2.2 Air pluviation of sand 2195.2.3 Depositional techniques for silty sand 2225.2.4 Undercompaction 2275.2.5 Compaction of clayey soils 2325.2.6 Compaction of soils with oversize particles 2345.2.7 Extrusion and storage 2355.2.8 Effects of specimen aging 2355.3 Measurement of specimen dimensions 2355.3.1 Compacted specimens 2355.4 Specimen installation 2355.4.1 Fully saturated clay specimen 2365.4.2 Unsaturated clayey soil specimen 2376 Specimen Saturation 2396.1 Reasons for saturation 2396.2 Reasons for lack of full saturation 2396.3 Effects of lack of full saturation 2406.4 B‐value test 2416.4.1 Effects of primary factors on B‐value 2416.4.2 Effects of secondary factors on B‐value 2436.4.3 Performance of B‐value test 2466.5 Determination of degree of saturation 2496.6 Methods of saturating triaxial specimens 2506.6.1 Percolation with water 2506.6.2 CO2‐method 2516.6.3 Application of back pressure 2526.6.4 Vacuum procedure 2586.7 Range of application of saturation methods 2627 Testing Stage I: Consolidation 2637.1 Objective of consolidation 2637.2 Selection of consolidation stresses 2637.2.1 Anisotropic consolidation 2647.2.2 Isotropic consolidation 2677.2.3 Effects of sampling 2687.2.4 SHANSEP for soft clay 2687.2.5 Very sensitive clay 2727.3 Coefficient of consolidation 2727.3.1 Effects of boundary drainage conditions 2727.3.2 Determination of time for 100% consolidation 2728 Testing Stage II: Shearing 2778.1 Introduction 2778.2 Selection of vertical strain rate 2778.2.1 UU‐tests on clay soils 2778.2.2 CD‐ and CU‐tests on granular materials 2778.2.3 CD‐ and CU‐tests on clayey soils 2778.2.4 Effects of lubricated ends in undrained tests 2828.3 Effects of lubricated ends and specimen shape 2828.3.1 Strain uniformity and stability of test configuration 2828.3.2 Modes of instability in soils 2848.3.3 Triaxial tests on sand 2848.3.4 Triaxial tests on clay 2908.4 Selection of specimen size 2928.5 Effects of membrane penetration 2938.5.1 Drained tests 2938.5.2 Undrained tests 2938.6 Post test inspection of specimen 2939 Corrections to Measurements 2959.1 Principles of measurements 2959.2 Types of corrections 2959.3 Importance of corrections – strong and weak specimens 2959.4 Tests on very short specimens 2969.5 Vertical load 2969.5.1 Piston uplift 2969.5.2 Piston friction 2969.5.3 Side drains 2989.5.4 Membrane 3019.5.5 Buoyancy effects 3089.5.6 Techniques to avoid corrections to vertical load 3099.6 Vertical deformation 3099.6.1 Compression of interfaces 3099.6.2 Bedding errors 3099.6.3 Techniques to avoid corrections to vertical deformations 3119.7 Volume change 3129.7.1 Membrane penetration 3129.7.2 Volume change due to bedding errors 3179.7.3 Leaking membrane 3179.7.4 Techniques to avoid corrections to volume change 3199.8 Cell and pore pressures 3199.8.1 Membrane tension 3199.8.2 Fluid self‐weight pressures 3199.8.3 Sand penetration into lubricated ends 3199.8.4 Membrane penetration 3199.8.5 Techniques to avoid corrections to cell and pore pressures 32010 Special Tests and Test Considerations 32110.1 Introduction 32110.1.1 Low confining pressure tests on clays 32110.1.2 Conventional low pressure tests on any soil 32110.1.3 High pressure tests 32210.1.4 Peats and organic soils 32210.2 K0‐tests 32210.3 Extension tests 32210.3.1 Problems with the conventional triaxial extension test 32310.3.2 Enforcing uniform strains in extension tests 32410.4 Tests on unsaturated soils 32610.4.1 Soil water retention curve 32610.4.2 Hydraulic conductivity function 32710.4.3 Low matric suction 32710.4.4 High matric suction 32910.4.5 Modeling 33010.4.6 Triaxial testing 33110.5 Frozen soils 33110.6 Time effects tests 33310.6.1 Creep tests 33310.6.2 Stress relaxation tests 33310.7 Determination of hydraulic conductivity 33510.8 Bender element tests 33510.8.1 Fabrication of bender elements 33610.8.2 Shear modulus 33710.8.3 Signal interpretation 33810.8.4 First arrival time 33810.8.5 Specimen size and geometry 34010.8.6 Ray path analysis 34010.8.7 Surface mounted elements 34010.8.8 Effects of specimen material 34110.8.9 Effects of cross‐anisotropy 34111 Tests with Three Unequal Principal Stresses 34311.1 Introduction 34311.2 Tests with constant principal stress directions 34411.2.1 Plane strain equipment 34411.2.2 True triaxial equipment 34511.2.3 Results from true triaxial tests 34811.2.4 Strength characteristics 35311.2.5 Failure criteria for soils 35511.3 Tests with rotating principal stress directions 36011.3.1 Simple shear equipment 36011.3.2 Directional shear cell 36211.3.3 Torsion shear apparatus 36411.3.4 Summary and conclusion 370Appendix A: Manufacturing of Latex Rubber Membranes 373A.1 The process 373A.2 Products for membrane fabrication 373A.3 Create an aluminum mold 374A.4 Two tanks 374A.5 Mold preparation 374A.6 Dipping processes 374A.7 Post production 375A.8 Storage 375A.9 Membrane repair 375Appendix B: Design of Diaphragm Load Cells 377B.1 Load cells with uniform diaphragm 377B.2 Load cells with tapered diaphragm 378B.3 Example: Design of 5 kN beryllium copper load cell 378B.3.1 Punching failure 379References 381Index 397