The purpose of the study is to improve the simulation of liquefaction of sands and variations in effective stress when using FLAC3D for the dynamic analysis of liquefiable ground and structures. Based on the physics of large post-liquefaction deformation of sand

a bounding surface plasticity model with appropriate mapping rules for plasticity and dilatancy in three-dimensional stress space is developed. A state parameter for compatibility with critical state soil mechanics enables the simulation of sand at various densities and confining pressures with a same set of parameters. According to requirements and procedures of user-defined-model (UDM)

the plasticity model is successfully implemented into FLAC3D code in Visual C++ environment. After testing the calculation stability

the implemented schematics allow subzones of the mixed discretization process in FLAC3D share mapping center and simultaneously enter and leave liquefaction status. Using the program

undrained and drained triaxial tests

cyclic triaxial tests

undrained cyclic torsional tests are simulated and a three-dimensional site dynamic response analysis is also performed. The results show that the model and the implemented program have superior and reasonable capability to simulate small and large deformation in pre- to post-liquefaction phase of sand.