ABSTRACT
Two-dimensional and three-dimensional non-linear finite element analyses have become popular in practice. However, unless a constitutive model which can describe fundamental features of soil behaviors in three-dimensional stresses properly, it is not possible to obtain reliable results in the analyses. The Cam clay model, developed at Cambridge in 1960s (e.g., Schofield andWroth 1968; Roscoe and Burland, 1968) was the first model to unify relevant aspects of soil behavior under shear or under isotropic and anisotropic compression. Since the Cam clay model, many constitutive models have been proposed. Most of these models including the Cam clay model are formulated using the stress invariants (p and q). It is known that such models cannot describe uniquely the soil behaviors under three different principal stresses. To describe the soil behavior under three different principal stresses, some material parameters in the models are given by a function of the relative magnitude of the intermediate principal stress. However, such method is not essential in rational modelling. Stress invariants (p and q) come from the metal plasticity in which microscopic fabric change does not occur except plastic region near failure. On the other hand, it has been experimentally shown that the fabric of soils – e.g., distribution of contact normal directions between soil particles – is changed from the beginning of shear
deformation. Hence, it is important to define stress invariants which reflect real fabric change of soils.
