Based on the Hvorslev’s envelopebased threedimensional elastoplastic constitutive model for overconsolidated clay, an acoustic tensor of bifurcation for strain localization under undrained and drained conditions was obtained in the explicit form. The influences of permeability, drainage path length and strain rate on the bifurcation were analyzed. Theoretical results show that the strain localization bifurcation is related to the water seepage for watersoil twophase material. The numerical simulation of plane strain tests on a cubic specimen for the bifurcation under drained condition was conducted by using a nonlinear finite element analysis software ABAQUS with the model being implemented. Numerical results show that the greater the permeability, the sooner the bifurcation occurs when the drainage path length and the strain rate are fixed; when the permeability is fixed, the greater the drainage path length and the strain rate, the later the bifurcation occurs; the strain corresponding to the bifurcation is constant when the time factor is given.
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