A two-surface plasticity model for clay

This paper presents a two-surface plasticity constitutive model for clays based on critical-state soil mechanics. The model reproduces the mechanical response of clays under multi-axial loading conditions and predicts both drained and undrained behavior at small and large strains. The constitutive model also captures both the strain-rate-dependent behavior of clays and the drop in strength towards a residual value at very large shear strains using novel approaches. The paper also describes a hierarchical process for the determination of the model parameters relying more on simple curve fitting of model equations to experimental data points corresponding to specific soil states instead of trial-and-error simulations of entire experiments. Model parameter values are determined for London Clay, San Francisco Bay Mud, Boston Blue Clay and Lower Cromer Till, and the performance of the model in simulating mechanical response of clays is demonstrated for a variety of initial states and loading conditions.