MULTIAXIAL CYCLIC PLASTICITY MODEL FOR CLAYS

A total stress-based bounding surface plasticity model for clays is developed to accommodate multiaxial stress reversals. The model is constructed based on the idea of a vanishing elastic region undergoing pure translation inside a bounding surface, and an interpolation function for hardening modulus which varies with stress distance of the elastic region from the unloading point. Central to the development of the model are the general criteria for loading and unloading, which are phrased based upon the simple argument that with continued loading the hardening modulus should decrease monotonically with deformation. Combined with numerical integration of the elastoplastic constitutive equations in a form suitable for a robust computer implementation, the model is applied to cohesive soils undergoing undrained stress reversals and cyclic loading. With a suitable choice of the interpolation function for the hardening modulus, it is shown that existing one-dimensional nonlinear laws for soils can be replicated, such as the hyperbolic, exponential, the Davidenkov, and even the Ramberg-Osgood models. Specifically, the appropriateness of the exponential hardening function for cohesive soils is investigated and its parameters determined for some clays and silts for use in dynamic soil-structure interaction modeling.