To describe the time-dependent

over-consolidated and structural characteristics of natural soft clay

a simple elasto-viscoplastic constitutive model including the influence of structural behavior of clay is proposed. In the new model

the Asaoka's superloading and Hashiguchi's subloading yield surface is used as a reference yield surface at a specified strain rate. A relative overstress relation is used to obtain the dynamic loading surface at any strain rate based on the reference yield surface. A dynamic equilibrium condition

using the current stress

viscoplastic strain and viscoplastic strain rate as state variables

is adopted as a convergence criterion. Two material parameters and about the rate sensitivity are added into the new model

which can be determined by triaxial compression tests with different strain rates. The new model has been implemented into ABAQUS by a stress integration algorithm using Newton-Raphson iteration. The validity of the model and the reliability of the stress integration algorithm are verified by numerical calculation. The numerical results show that the model can describe the time-dependent behavior of clay such as rate sensitivity

structural characteristics and creep. The material parameters are easy to understand and measure with clear physical meaning. Furthermore

the prediction results of the model are in good agreement with experimental data. The model can be used in a finite element calculation under the complex boundary value problem.