Implementation of a plasticity model for large post-liquefaction deformation of sand in FLAC3D

The purpose of the study is to improve the simulation of liquefaction of sands and variations in effective stress when using FLAC(3D) 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 FLAC(3D) code in Visual C++ environment. After testing the calculation stability, the implemented schematics allow subzones of the mixed discretization process in FLAC(3D) 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.