Abstract：The constitutive equations and FEM formulations of rock masses of large elastoplastic deformation are established by using updated Lagrangian method. The strength reduction method is employed in slope stability analysis. Strength parameters are divided by an increasing factor until the plastic zone forms a sliding surface and the computation program can not converge. Then the safety factors are determined. The results show the reduce of strength parameters of soils induced by rainfall is the major factor leading to instability of the slope，and the increase of the weight of slope body is secondary factor.
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2004, Vol. 23 
