In this paper, a large plasticity deformation finite element modeling is presented for threedimensional dynamic analysis of unsaturated soils with special reference to the failure of lower San Fernando dam under the 1971 earthquake. The finite element method is applied to the governing equations for the spatial discretization, followed by a generalized Newmark scheme used for the time domain discretization. Time stepping scheme is used in the fully implicit coupled method and a direct solution procedure is used for the coupled equation system. The framework of generalized plasticity is presented and the numerical results of unsaturated soils are demonstrated based on the Pastor-Zienkiewicz model (Int. J. Numer. Analyt. Meth. Geomech., 14: pp. 151–190, 1990), Bolzon-Schrefler-Zienkiewicz model (Geotechnique, 46: pp. 279–289, 1996), and enhanced-BSZ model (Transport in Porous Media, 65: pp.1–30, 2006). Finally, 3D dynamic analysis of the failure of lower San- Fernando dam is presented based on the modified Pastor-Zienkiewicz plasticity model.
