In this paper, effects of antecedent rainfall on stability of the unsaturated weathered granite slope at Inje, Korea were investigated. Effect of antecedent rainfall was considered in the numerical analysis using initial condition of matric suction, which was induced from field measurement. We also investigated case histories of slope failure, rainfall data, geotechnical properties, soil-water characteristic curve (SWCC) and hydraulic conductivity in those areas. Several sets of numerical stability analysis were performed on unsaturated slope with different initial matric suction. Result of the analyses indicated that the higher initial matric suction (less antecedent rainfall) of the unsaturated weathered granite slope delayed the slope failure. Also, unsaturated slope with higher saturated permeability is more vulnerable to rainfallinduced landslide than that with lower saturated permeability due to rainfall infiltration. Numerical analysis also indicated that a slope with wetting SWCC reached the failure condition earlier than a slope with drying SWCC.
Unsaturated geotechnical properties and other factors(rainfall characteristic, geomorphology, etc) have to be considered in order to evaluate the stability of real slope. Even though antecedent rainfall is also an important factor on the slope stability, it is rarely considered in analysis of slope stability. In this paper slope stability analysis for unsaturated weathered granite soil in Inje was conducted considering antecedent rainfall. Effect of antecedent rainfall was considered in analysis by using initial matric suction, which was induced from field measurement. Unsaturated geotechnical properties(soil-water characteristic curve, unsaturated conductivity, unsaturated shear strength) of weathered granite soil in Inje were also investigated. Effects of antecedent rainfall on variation of factor of safety(FS) of slope, failure time and cumulative rainfall were evaluated. For a slope with high initial matric suction, FS slowly decreased and failure time delayed than a slope with low initial matric suction. Numerical analysis also indicated that a slope with wetting SWCC more rapidly approached to failure than a slope with drying SWCC.
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