Prediction of Failure Risk Level based on Weakness caused by Weathering and Evaluation of Resistance for Erosion on Shirasu Slope

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Shirasu widely distributed in southern Kyushu in Japan is weak in the resistance for water so that erosion failure and surface slide failure on natural slopes have occured repeatedly. It is very important for making up a counterplan of preventing disasters to evaluate the transition of basic factors of failure caused by the weathering and erosion actions. In this study, the time transition of resistance force for weathering and erosion on Shirasu slopes is evaluated by the laboratory tests and field tests, and the method predicting risk level of slope failure is proposed based on their results. The main conclusions are summarized as follows :1) The influence of repetition of wetting and drying on the shear properties of Shirasu appears in the deformation property. namely, the failure formula as a rock materials changes into that as a soil material.2) Shirasu in the equilibrium under applied shear stress larger than residual shear strength at saturation collapse when taking the action of wetting.3) Shirasu constituted by only soil particles with small value of density, however, is weak in water flow, Shirasu mixing the soil particles with different densities is hard to erode until the coarse particles with high value of density start flowing out.4) After cutting off in-sifu Shirasu slope, the resistance force for soil hardness tester of Yamanaka type decreases with time passing.5) The properties of undisturbd Shirasu can be estimated from weathering erosion properties of artificially solidified Shirasu.6) The occurrence of slope disaster can be dividd into three groups ; basic factor of failure, motive factor and expansion-restrain factor. Disaster prevention potential can be evalated from the multiple logistic analysis based on the consideration that these factors are regarded as risk factor.