Strength behaviour including stress-strain relations of clays-

Abstract

Stability problems for their analysis and design necessitate an assessment of appropriate strength parameters of a given soil system. In soil masses, since deformations result largely from slippages between soil particles, shear strength is synonymous with the strength of soils. In order to understand several mechanical properties and the interrelationships among them, it is desirable to analyze the material response in more detail from a mechanistic viewpoint. Stress-strain relationships as well as the failure or yield criterion characterize the material behavior. In this investigation, an attempt has been made to study the strength behavior of saturated clays mechanistically (with particular reference to strength parameters), including the characterization of stress-strain response.

A simple three-parameter hyperbolic stress-strain relationship to predict peak deviator stress under triaxial loading conditions has been formulated in Chapter 2. It is intended to take into cognizance the strain at failure for the prediction of peak stress. An analysis is presented to account for the differences:

(i) between experimentally determined values and the values determined by hyperbolic representation.

The results of this study show that the pore pressure-strain and volumetric strain-axial strain relationships can also be represented in the same way as the deviatoric stress-strain relationships. The potential use of these findings in developing a new multistage triaxial test to obtain strength parameters from tests on a single sample has been brought out.

In Chapter 3, the various factors affecting the strength parameters and their limitations have been critically reviewed. A comprehensive testing program has been carried out to investigate the influence of various factors on Hvorslev parameters, and the results have been reported in Chapter 4. The results obtained have been analyzed using different existing methods. This investigation reveals that Hvorslev parameters are not unique and can have a wide range of values depending upon the method by which they are determined, the initial conditions (stress history and water content) of the sample, and the stress level. A significant observation from the analysis of the data is that there seems to be a unique relationship between tan and C/ irrespective of the sample state, stress level, and stress history in both undisturbed and remolded conditions for all the procedures adopted.

In Chapter 5, the undrained strength behavior has been studied. The dependency of various factors such as consolidation pressure, mode of consolidation, and lateral constraints on undrained strength behavior has been discussed. The variation in S_u/ ratio has been studied in relation to changes in the pore pressure parameter, A. Having realized that the effect of several above factors is to give a band rather than a simple unique relationship between undrained strength and effective normal stress, the well-known correlation of linearly increasing S_u/ ratio with plasticity index, as proposed by Skempton, has been critically examined in this chapter.

In Chapter 6, the predominant influence of clay fabric due to initial molding water content, and mode of preshear consolidation stress history (isotropic and K_0-consolidation) on shear strength of remolded clays has been studied. The effect of cyclic loading on the kaolinite clay is reflected as cohesion intercept and reduced values of �. Strength test data on samples extruded in mutually perpendicular directions (vertical and radial) are also examined for possible influence, or otherwise, of material anisotropy on shear strength.

In Chapter 7, a comparison has been made of undrained and drained strength parameters. A change in the volume of the test specimen during drained tests signifies adsorption or expenditure of surface energy. It has been brought out that the volume change is made up of two components:

(i) isotropic stress, and (ii) shear stresses.

A procedure to separate out volume changes due to isotropic and shear stresses has been evolved. The existence of micro-dilatancy has been brought out. It has been shown that to account for the volume change component in the drained test, shear-induced volume changes need to be considered. The final difference between undrained and drained strength, after accounting for shear-induced volume changes, is attributed to differences in interference effect.

Chapter 8 deals with the influence of system chemistry on shear strength characteristics of kaolinite and montmorillonite monomineralic clays. Trace additives, viz., aggregents and dispersants, have been used to alter the system chemistry. The effective stress, modified to encompass the interparticle bonding and disruptive forces, has been used to explain the strength behavior. The influence of the system chemistry is significant on montmorillonite, while the effect is relatively less on kaolinite.

In Chapter 9, the influence of compositional factors (clay type and system chemistry) and environmental factors on residual strength of remolded clays has been examined. The uniqueness of residual strength parameters with respect to environmental factors and the influence of system chemistry have been explained.

In the last Chapter (10), the major conclusions of this investigation are critically examined and summarized.