Major reasons of sliding failures, especially in Korea, are known as the stability loss by soil cutting and the shear strength loss by weathering and the infiltrating rainfall. In this paper, the goal is to develop a slope stability analysis method and program which can effectively consider the rainfall behavior. By considering the influence of rainfall infiltration into unsaturated soils and appling an optimized technique that searches for the critical section representing a minimum safety factor, a slope stability analysis can be executed. A program adopting the finite element method is used for obtaining the knowledge on the negative pore pressure change that greatly influences the slope stability. As a result, a system which is able to analyze a slope stability problem more accurately, is developed. Besides, design variables, specifying the behavior of unsaturated soils and also influencing the decrease in safety factor, are confirmed by experiments. Using those design variables, a method taking into account the actual slope failure mechanism is proposed. For a given slope, the critical failure surface can be effectively estimated by the application of the optimization technique. Finally, the proposed method was verified.
