Introduction

A complete description of the stress:strain behaviour of an elastic material can be embodied in a rather small number of parameters. For example, an isotropic linear elastic material (Section 2.1) requires only two independent parameters: Young's modulus and Poisson's ratio, or shear modulus and bulk modulus. A cross-anisotropic or transversely isotropic linear elastic material (Section 2.3) requires five parameters: which might be two Young's moduli, two Poisson's ratios, and a shear modulus. Once these elastic parameters have been determined, the response of the material to any changes in stress can be predicted. For the simple case of an isotropic elastic soil, the elastic parameters could be obtained from a uniaxial compression test, but the use of these parameters would in no way be confined to the prediction of the response in such tests. The testing programme required to determine the five elastic parameters for a cross-anisotropic elastic soil is rather more complex: simple compression tests in a triaxial apparatus are no longer sufficient, and tests which include rotation of principal axes are required (Graham and Houlsby, 1983). Nevertheless, once the values of the five parameters have been established, there is no limit to the range of stress paths to which the model can legitimately be applied.

A distinction has to be drawn between the description of the behaviour of inelastic soils and the description of the model which may be used to represent certain aspects of their behaviour.