This paper presents an improved three-dimensional anisotropic elasto-viscoplastic constitutive model which is based on overstress theory of Perzyna and critical state theory. In this model a rotational hardening law of Wheeler is also adopted to account for initial anisotropy and changes in anisotropy due to stress. The viscoplastic strain will not occur when the stress state is located within the static yield surface. A power-type scaling function is adopted for the viscoplastic strain-rate. The constitutive model is programmed in ABAQUS software by writing UMAT subroutine

which is integrated with Return Mapping Algorithm. Anisotropically consolidated undrained (CU) triaxial creep test for Sackville clay is simulated by proposed model and the suitable integration time step is determined. Then other CU triaxial creep tests and CU triaxial constant strain rate tests are simulated. By switching the anisotropic features off

the proposed model is degraded to the isotropic model. The simulation results show (1) The CU triaxial creep tests with rotational hardening law is more accurate than isotropic model in the high level of the shear stress; (2) The CU triaxial constant strain rate tests rationally reflect that the undrained strength of soil increase with loading rate.