Triaxial Testing of Soils

Poul V. Lade

• Publisher: Wiley-Blackwell
• Year: 2016
• Print ISBN: 9781119106623
• eBook ISBN: 9781119106593
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

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.

1
Principles of Triaxial Testing

1.1 Purpose of triaxial tests

The purpose of performing triaxial tests is to determine the mechanical properties of the soil. It is assumed that the soil specimens to be tested are homogeneous and representative of the material in the field, and that the desired soil properties can in fact be obtained from the triaxial tests, either directly or by interpretation through some theory.

The mechanical properties most often sought from triaxial tests are stress–strain relations, volume change or pore pressure behavior, and shear strength of the soil. Included in the stress–strain behavior are also the compressibility and the value of the coefficient of earth pressure at rest, K₀. Other properties that may be obtained from the triaxial tests, which include time as a component, are the permeability, the coefficient of consolidation, and properties relating to time dependent behavior such as rate effects, creep, and stress relaxation.

It is important that the natural soil deposit or the fill from which soil samples have been taken in the field are sufficiently uniform that the soil samples possess the properties which are appropriate and representative of the soil mass in the field. It is therefore paramount that the geology at the site is well-known and understood. Even then, samples from uniform deposits may not “contain” properties that are representative of the field deposit. This may happen either (a) due to the change in effective stress state which is always associated with the sampling process or (b) due to mechanical disturbance from sampling, transportation, or handling in the laboratory. The stress–strain and strength properties of very sensitive clays which have been disturbed cannot be regenerated in the laboratory or otherwise obtained by interpretation of tests performed on inadequate specimens. The effects of sampling will briefly be discussed below in connection with choice of consolidation pressure in the triaxial test. The topic of sampling is otherwise outside the scope of the present treatment.

1.2 Concept of testing

The concept to be pursued in testing of soils is to simulate as closely as possible the process that goes on in the field. Because there is a large number of variables (e.g., density, water content, degree of saturation, overconsolidation ratio, loading conditions, stress paths) that influence the resulting soil behavior, the simplest and most direct way of obtaining information pertinent to the field conditions is to duplicate these as closely as possible.

However, because of limitations in equipment and because of practical limitations on the amount of testing that can be performed for each project, it is essential that:

1. The true field loading conditions (including the drainage conditions) are known.
2. The laboratory equipment can reproduce these conditions to a required degree of accuracy.
3. A reasonable estimate can be made of the significance of the differences between the field loading conditions and those that can be produced in the laboratory equipment.

It is clear that the triaxial test in many respects is incapable of simulating several important aspects of field loading conditions. For example, the effects of the intermediate principal stress, the effects of rotation of principal stresses, and the effects of partial drainage during loading in the field cannot be investigated on the basis of the triaxial test. The effects of such conditions require studies involving other types of equipment or analyses of boundary value problems, either by closed form solutions or solutions obtained by numerical techniques.

To provide some background for evaluation of the results of triaxial tests, other types of laboratory shear tests and typical results from such tests are presented in Chapter 11. The relations between the different types of tests are reviewed, and their advantages and limitations are discussed.

1.3 The triaxial test

The triaxial test is most often performed on a cylindrical specimen, as shown in Fig. 1.1(a). Principal stresses are applied to the specimen, as indicated in Fig. 1.1(b). First a confining pressure, σ₃, is applied to the specimen. This pressure acts all around and therefore on all planes in the specimen. Then an additional stress difference, σ_d, is applied in the axial direction. The stress applied externally to the specimen in the axial direction is

(1.1 )

and therefore

(1.2 )

In the general case, three principal stresses, σ₁, σ₂ and σ₃ may act on a soil element in the field. However, only two different principal stresses can be applied to the specimen in the conventional triaxial test. The intermediate principal stress, σ₂, can only have values as follows:

(1.3 )

(1.4 )

The condition of triaxial extension can be achieved by applying negative stress differences to the specimen. This merely produces a reduction in compression in the extension direction, but no tension occurs in the specimen. The ...

Table of contents

• Cover
• Title Page
• Table of Contents
• Preface
• About the Author
• 1 Principles of Triaxial Testing
• 2 Computations and Presentation of Test Results
• 3 Triaxial Equipment
• 4 Instrumentation, Measurements, and Control
• 5 Preparation of Triaxial Specimens
• 6 Specimen Saturation
• 7 Testing Stage I
• 8 Testing Stage II
• 9 Corrections to Measurements
• 10 Special Tests and Test Considerations
• 11 Tests with Three Unequal Principal Stresses
• Appendix A: Manufacturing of Latex Rubber Membranes
• Appendix B: Design of Diaphragm Load Cells
• References
• Index
• End User License Agreement