The contribution of matric suction to the stability of an unsaturated soil slope must be properly accounted for in order to evaluate the slope stability with reasonable accuracy. It is also essential to develop a better method to analyze the stability of unsaturated soil slope instead of the conventional slice method commonly used in practice so far. In this paper, previous research work on strength of unsaturated soil is reviewed and a strength formulation for unsaturated soil is proposed in the three-dimensional effective stress space. The implementation of the strength reduction method for stability analysis using Finite Element Method (FEM) is described in this paper. A case history of unsaturated soil slope failure is analyzed by the developed strength reduction FEM and the results clearly indicate that the global factor of safety decreases as the matric suction decreases during rainfall leading to final slope failure. The numerical analysis clearly demonstrates the important effect of suction on the stability of unsaturated soil slope.
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