The aim of the present paper is to study the instability and strain localization of water saturated clay. In particular, effects of dilatancy and strain rate are discussed using an elasto-viscoplastic constitutive model. The model is based on the non-linear kinematic hardening theory and a Chaboche type of viscoplasticity model. The elasto-viscoplastic model for both normally consolidated and overconsolidated clays can address both negative and positive dilatancies. Firstly, the instability of the model under undrained creep conditions is presented in terms of the accelerating creep failure. The analysis shows that clay with positive dilatancy is more unstable than clay with negative dilatancy. Secondly, a finite element analysis of the deformation of water-saturated clay is presented with focus on the numerical results under plane strain conditions. From the present numerical analysis, it is found that both dilatancy and strain rate prominently affect shear strain localization behavior.
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