Soil Settlement and the Concept of Effective Stress and Shear Strength Interaction

Mohd Jamaludin Md Noor

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000262131
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

This book is about the principal concept of soil mechanics that become the basis in explaining the soil mechanical behaviours. It is the extended concept of effective stress of Terzaghi and it is known as "the concept of effective stress and shear strength interaction."

This new concept incorporates the role of mobilised shear strength developed within the soil body in resisting the compressive effect. Based on this new concept a comprehensive soil volume change framework has been developed known as Rotational Multiple Yield Surface Framework (RMYSF). This RMYSF is able to explain and quantify the puzzled and complex soil volume change behaviours. The main advantage of this RMYSF is that it is able to make a good prediction of soil and rock stress-strain responses at any effective stress. This will lead to accurate prediction of soil and rock settlements.

Due to its simplicity and the comprehensive nature of this new fundamental concept in soil and rock mechanics, it will eventually be included in soil and rock mechanics syllabus for undergraduate and postgraduate courses. This book would be very useful for geotechnical engineers dealing with soil settlement, underground excavation, computer modelling, rock mechanics, road engineering, earth and rock dam engineering and tunnel engineering.

Chapter 1

The true soil stress–strain response and shear strength behaviour

1.1 Introduction to soil stress–strain response and shear strength behaviour

A stress–strain curve is one of the very important characteristics of soil. It reflects how a soil would respond to the change in the subjected stress. It is the basic property of a soil. It governs how a soil would respond when it is subjected to a stress increase or decrease. In contrast to the stress–strain curve of a metal where under a low-stress range, the response is linear-elastic, in the soil, even though the curve is linear at low-stress levels, the response is not purely elastic.

The soil shear strength failure envelope is defined from the stress–strain curves where the maximum deviator stress is taken as the failure condition. The failure envelope can be either linear or non-linear. The maximum deviator stresses will be the diameter of the Mohr circles at various effective confining pressures. The output shear strength parameters will be the unique intrinsic properties of the soil whether it is for a linear or a non-linear failure envelope. Different soils would have a different set of stress–strain curves and obviously, they produced shear strength parameters, which are specific for the soil only.

The effective confining pressure or the subjected effective stress is the prime factor that influences the way the response of soil to stress increases or decreases. In other words, the depth below the ground, which governs the magnitude of the confining effective stress, would have a strong influence on how the soil responds to stress. The deeper the depth, the higher the confining effective stress. Thence, it is very important for any soil volume change framework to be developed or formulated based on the soil stress–strain curves at various effective stresses.

Figure 1.1, Figure 1.2, Figure 1.3 show stress–strain curves for various soils, which are (1) saturated Ham River sand at normal and elevated net confining pressures (Bishop, 1966), (2) greywacke rockfill in large-scale triaxial testing by Indraratna et al. (1993) and (3) gneiss rock residual soil at horizon C by Futai and Almeida (2005) at a suction of 100 kPa, respectively. The works by Bishop (1966) and by Indraratna et al. (1993) dealt with granular soil and obviously, in the interpretation of the shear strength failure envelope, there will be no cohesion intercept. However, the same goes to the work of Futai and Almeida (2005) where under the saturated condition, there will be no cohesion base on effective stress analysis. Nonetheless, the failure envelope would intercept the shear strength axis at a certain apparent shear strength, because the soil is partially saturated. This apparent shear strength may be mistakenly thought of as the cohesion c′ when viewed in two dimension, i.e., shear strength versus net stress.

Figure 1.1 Stress–strain and volume change relationships for drained triaxial tests on saturated Ham River sand at normal and elevated net confining pressures (Bishop, 1966). (Note: 1 lb/in.². = 6.896 kN/m².)

Figure 1.2 Stress–strain and volume change behaviour of greywacke rock fill in large-scale triaxial testing (Indraratna et al., 1993).

Figure 1.3 Constant suction triaxial tests performed on gneiss rock residual soil at horizon C (Futai and Almeida, 2005) at a suction of 100 kPa.

Mostly, it can be noted that these stress–strain curves exhibit in common the normal soil characteristics, which are:

1. Stress–strain curves become steeper as the confining pressure increases.
2. Axial strain at failure increases with the increase of the effective confining pressure. In other words, the axial strain at failure is not the same for each stress–strain curve. The maximum deviator stress occurs at different axial strains for each stress–strain curve.
3. Maximum deviator stresses increase with the increase in the effective confining pressures.
4. The responding strain is higher under the same increase in stress at a lower effective confining pressure. This will be further illustrated in the following paragraph.

The slope of the stress–strain curve is termed as stiffness but for soil, it cannot be regarded as the modulus of elasticity, because the behaviour is not purely elastic even though the curve is linear at the low-stress range. The fact that it is not purely elastic can be seen when unloading is carried out. Essentially, the unloading curve does not retrace the initial loading curve. Thence, soil behaviour is a unique behaviour and it is not comparable to the stress–strain curve of a metal.

Figur...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• About the author
• 1 The true soil stress–strain response and shear strength behaviour
• 2 Concept of effective stress and shear strength interaction in governing soil settlement
• 3 Rotational Multiple Yield Surface Framework
• 4 Normalised Strain Rotational Multiple Yield Surface Framework
• 5 Modelling inundation settlement and loading collapse settlement using RMYSF
• 6 Anisotropic and elastic–plastic rock deformation model for accurate prediction of intact rock stress–strain response
• Index