Contenido Triaxial Testing of Soils

Triaxial Testing of Soils explains how to carry out triaxial tests to demonstrate the effects of soil behaviour on engineering designs. An authoritative and comprehensive manual, it reflects current best practice and instrumentation.References are made throughout to easily accessible articles in the literature and the book s focus is on how to obtain high quality experimental results.

Table of Contents

Preface

About the Author

1 Principles of Triaxial Testing

1.1 Purpose of triaxial tests
1.2 Concept of testing
1.3 The triaxial test
1.4 Advantages and limitations
1.5 Test stages – consolidation and shearing
  1.5.1 Consolidation
  1.5.2 Shearing
1.6 Types of tests
  1.6.1 Simulation of field conditions
  1.6.2 Selection of test type

2 Computations and Presentation of Test Results

2.1 Data reduction
  2.1.1 Sign rule – 2D
  2.1.2 Strains 13
  2.1.3 Cross?sectional area
  2.1.4 Stresses
  2.1.5 Corrections
  2.1.6 The effective stress principle
  2.1.7 Stress analysis in two dimensions – Mohr’s circle
  2.1.8 Strain analysis in two dimensions – Mohr’s circle
2.2 Stress–strain diagrams
  2.2.1 Basic diagrams
  2.2.2 Modulus evaluation
  2.2.3 Derived diagrams
  2.2.4 Normalized stress–strain behavior
2.2.5 Patterns of soil behavior – error recognition
2.3 Strength diagrams
  2.3.1 Definition of effective and total strengths
  2.3.2 Mohr–Coulomb failure concept
  2.3.3 Mohr–Coulomb for triaxial compression
  2.3.4 Curved failure envelope
  2.3.5 MIT p–q diagram
  2.3.6 Cambridge p–q diagram
  2.3.7 Determination of best?fit soil strength parameters
  2.3.8 Characterization of total strength
2.4 Stress paths
  2.4.1 Drained stress paths
  2.4.2 Total stress paths in undrained tests
  2.4.3 Effective stress paths in undrained tests
  2.4.4 Normalized p–q diagrams
  2.4.5 Vector curves
2.5 Linear regression analysis
  2.5.1 MIT p–q diagram
  2.5.2 Cambridge p–q diagram
  2.5.3 Correct and incorrect linear regression analyses
2.6 Three?dimensional stress states
  2.6.1 General 3D stress states
  2.6.2 Stress invariants
  2.6.3 Stress deviator invariants
  2.6.4 Magnitudes and directions of principal stresses
2.7 Principal stress space
  2.7.1 Octahedral stresses
  2.7.2 Triaxial plane
  2.7.3 Octahedral plane
  2.7.4 Characterization of 3D stress conditions
  2.7.5 Shapes of stress invariants in principal stress space
  2.7.6 Procedures for projecting stress points onto a common octahedral plane
  2.7.7 Procedure for plotting stress points on an octahedral plane
  2.7.8 Representation of test results with principal stress rotation

3 Triaxial Equipment

3.1 Triaxial setup
  3.1.1 Specimen, cap, and base
  3.1.2 Membrane
  3.1.3 O?rings
  3.1.4 Drainage system
  3.1.5 Leakage of triaxial setup
  3.1.6 Volume change devices
  3.1.7 Cell fluid
  3.1.8 Lubricated ends
3.2 Triaxial cell
  3.2.1 Cell types
  3.2.2 Cell wall
  3.2.3 Hoek cell
3.3 Piston
  3.3.1 Piston friction
  3.3.2 Connections between piston, cap, and specimen
3.4 Pressure supply
  3.4.1 Water column
  3.4.2 Mercury pot system
  3.4.3 Compressed gas
  3.4.4 Mechanically compressed fluids
  3.4.5 Pressure intensifiers
  3.4.6 Pressure transfer to triaxial cell
  3.4.7 Vacuum to supply effective confining pressure
3.5 Vertical loading equipment
  3.5.1 Deformation or strain control
  3.5.2 Load control
  3.5.3 Stress control
  3.5.4 Combination of load control and deformation control
  3.5.5 Stiffness requirements
  3.5.6 Strain control versus load control
3.6 Triaxial cell with integrated loading system

4 Instrumentation, Measurements, and Control

4.1 Purpose of instrumentation
4.2 Principle of measurements
4.3 Instrument characteristics
4.4 Electrical instrument operation principles
  4.4.1 Strain gage
  4.4.2 Linear variable differential transformer
  4.4.3 Proximity gage
  4.4.4 Reluctance gage
  4.4.5 Electrolytic liquid level
  4.4.6 Hall effect technique
  4.4.7 Elastomer gage
  4.4.8 Capacitance technique
4.5 Instrument measurement uncertainty
  4.5.1 Accuracy, precision, and resolution
  4.5.2 Measurement uncertainty in triaxial tests
4.6 Instrument performance characteristics
  4.6.1 Excitation
  4.6.2 Zero shift
  4.6.3 Sensitivity
  4.6.4 Thermal effects on zero shift and sensitivity
  4.6.5 Natural frequency
  4.6.6 Nonlinearity
  4.6.7 Hysteresis
  4.6.8 Repeatability
  4.6.9 Range
  4.6.10 Overload capacity
  4.6.11 Overload protection
  4.6.12 Volumetric flexibility of pressure transducers
4.7 Measurement of linear deformations
  4.7.1 Inside and outside measurements
  4.7.2 Recommended gage length
  4.7.3 Operational requirements
  4.7.4 Electric wires
  4.7.5 Clip gages
  4.7.6 Linear variable differential transformer setup
  4.7.7 Proximity gage setup
  4.7.8 Inclinometer gages
  4.7.9 Hall effect gage
  4.7.10 X?ray technique
  4.7.11 Video tracking and high?speed photography
  4.7.12 Optical deformation measurements
  4.7.13 Characteristics of linear deformation measurement devices
4.8 Measurement of volume changes
  4.8.1 Requirements for volume change devices
  4.8.2 Measurements from saturated specimens
  4.8.3 Measurements from a triaxial cell
  4.8.4 Measurements from dry and partly saturated specimens
4.9 Measurement of axial load
  4.9.1 Mechanical force transducers
  4.9.2 Operating principle of strain gage load cells
  4.9.3 Primary sensors
  4.9.4 Fabrication of diaphragm load cells
  4.9.5 Load capacity and overload protection
4.10 Measurement of pressure
  4.10.1 Measurement of cell pressure
  4.10.2 Measurement of pore pressure
  4.10.3 Operating principles of pressure transducers
  4.10.4 Fabrication of pressure transducers
  4.10.5 Pressure capacity and overpressure protection
4.11 Specifications for instruments
4.12 Factors in the selection of instruments
4.13 Measurement redundancy
4.14 Calibration of instruments
4.14.1 Calibration of linear deformation devices
4.14.2 Calibration of volume change devices
4.14.3 Calibration of axial load devices
4.14.4 Calibration of pressure gages and transducers
4.15 Data acquisition
4.15.1 Manual datalogging
4.15.2 Computer datalogging
4.16 Test control
4.16.1 Control of load, pressure, and deformations
4.16.2 Principles of control systems

5 Preparation of Triaxial Specimens

5.1 Intact specimens
  5.1.1 Storage of samples
  5.1.2 Sample inspection and documentation
  5.1.3 Ejection of specimens
  5.1.4 Trimming of specimens
  5.1.5 Freezing technique to produce intact samples of granular materials
5.2 Laboratory preparation of specimens
  5.2.1 Slurry consolidation of clay
  5.2.2 Air pluviation of sand
  5.2.3 Depositional techniques for silty sand
  5.2.4 Undercompaction
  5.2.5 Compaction of clayey soils
  5.2.6 Compaction of soils with oversize particles
  5.2.8 Effects of specimen aging
5.3 Measurement of specimen dimensions
  5.3.1 Compacted specimens
5.4 Specimen installation
  5.4.1 Fully saturated clay specimen
  5.4.2 Unsaturated clayey soil specimen

6 Specimen Saturation

6.1 Reasons for saturation
6.2 Reasons for lack of full saturation
6.3 Effects of lack of full saturation
6.4 B?value test
  6.4.1 Effects of primary factors on B?value
  6.4.2 Effects of secondary factors on B?value
  6.4.3 Performance of B?value test
6.5 Determination of degree of saturation
6.6 Methods of saturating triaxial specimens
  6.6.1 Percolation with water
  6.6.2 CO2?method
  6.6.3 Application of back pressure
  6.6.4 Vacuum procedure 258
6.7 Range of application of saturation methods

7 Testing Stage I: Consolidation

7.1 Objective of consolidation
7.2 Selection of consolidation stresses
  7.2.1 Anisotropic consolidation
  7.2.2 Isotropic consolidation
  7.2.3 Effects of sampling
  7.2.4 SHANSEP for soft clay
  7.2.5 Very sensitive clay
7.3 Coefficient of consolidation
  7.3.1 Effects of boundary drainage conditions
  7.3.2 Determination of time for 100% consolidation

8 Testing Stage II: Shearing

8.1 Introduction
8.2 Selection of vertical strain rate
  8.2.1 UU?tests on clay soils
  8.2.2 CD? and CU?tests on granular materials
  8.2.3 CD? and CU?tests on clayey soils
8.3 Effects of lubricated ends and specimen shape
  8.3.1 Strain uniformity and stability of test configuration
  8.3.2 Modes of instability in soils
  8.3.3 Triaxial tests on sand
  8.3.4 Triaxial tests on clay
8.4 Selection of specimen size
8.5 Effects of membrane penetration
  8.5.1 Drained tests
  8.5.2 Undrained tests
8.6 Post test inspection of specimen

9 Corrections to Measurements

9.1 Principles of measurements
9.2 Types of corrections
9.3 Importance of corrections – strong and weak specimens
9.4 Tests on very short specimens
9.5 Vertical load
  9.5.1 Piston uplift
  9.5.2 Piston friction
  9.5.3 Side drains
  9.5.4 Membrane
  9.5.5 Buoyancy effects
  9.5.6 Techniques to avoid corrections to vertical load
9.6 Vertical deformation
  9.6.1 Compression of interfaces
  9.6.2 Bedding errors
  9.6.3 Techniques to avoid corrections to vertical deformations
9.7 Volume change
  9.7.1 Membrane penetration
  9.7.2 Volume change due to bedding errors
  9.7.3 Leaking membrane
  9.7.4 Techniques to avoid corrections to volume change
9.8 Cell and pore pressures
  9.8.1 Membrane tension
  9.8.2 Fluid self?weight pressures
  9.8.3 Sand penetration into lubricated ends
  9.8.4 Membrane penetration
  9.8.5 Techniques to avoid corrections to cell and pore pressures

10 Special Tests and Test Considerations

10.1 Introduction
  10.1.1 Low confining pressure tests on clays
  10.1.2 Conventional low pressure tests on any soil
  10.1.3 High pressure tests
  10.1.4 Peats and organic soils
10.2 K0?tests 322
10.3 Extension tests
  10.3.1 Problems with the conventional triaxial extension test
  10.3.2 Enforcing uniform strains in extension tests
10.4 Tests on unsaturated soils
  10.4.1 Soil water retention curve
  10.4.2 Hydraulic conductivity function
  10.4.3 Low matric suction
  10.4.4 High matric suction
  10.4.5 Modeling
  10.4.6 Triaxial testing
10.5 Frozen soils
10.6 Time effects tests
  10.6.1 Creep tests
  10.6.2 Stress relaxation tests
10.7 Determination of hydraulic conductivity
10.8 Bender element tests
  10.8.1 Fabrication of bender elements
  10.8.2 Shear modulus
  10.8.3 Signal interpretation
  10.8.4 First arrival time
  10.8.5 Specimen size and geometry
  10.8.6 Ray path analysis
  10.8.7 Surface mounted elements
  10.8.8 Effects of specimen material
  10.8.9 Effects of cross?anisotropy

11 Tests with Three Unequal Principal Stresses

11.1 Introduction
11.2 Tests with constant principal stress directions
  11.2.1 Plane strain equipment
  11.2.2 True triaxial equipment
  11.2.3 Results from true triaxial tests
  11.2.4 Strength characteristics
  11.2.5 Failure criteria for soils
11.3 Tests with rotating principal stress directions
  11.3.1 Simple shear equipment
  11.3.2 Directional shear cell
  11.3.3 Torsion shear apparatus
  11.3.4 Summary and conclusion

Appendix A: Manufacturing of Latex Rubber Membranes

A.1 The process
A.2 Products for membrane fabrication
A.3 Create an aluminum mold
A.4 Two tanks
A.5 Mold preparation
A.6 Dipping processes
A.7 Post production
A.8 Storage
A.9 Membrane repair

Appendix B: Design of Diaphragm Load Cells

B.1 Load cells with uniform diaphragm
B.2 Load cells with tapered diaphragm
B.3 Example: Design of 5 kN beryllium copper load cell
B.3.1 Punching failure
References
Index