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

广泛分布于日本九州岛南部的白石坝，由于抗水性较弱，致使天然边坡多次发生冲刷破坏和地表滑动破坏。评价风化侵蚀作用引起的破坏基本要素的变化，对于制定防灾减灾对策具有重要意义。通过室内试验和现场试验，对石拉苏地区边坡风化侵蚀阻力随时间的变化规律进行了评价，并在此基础上提出了预测边坡失稳风险等级的方法。主要结论如下：1)反复干湿对白拉苏土体剪切性能的影响主要表现在变形性能上。即岩石材料的破坏公式转变为土质材料的破坏公式。2)湿润作用下，石拉苏处于平衡状态，施加的剪应力大于饱和崩塌时的残余剪切强度。3)石拉苏只由密度较小的土粒组成，但水流很弱，石拉苏混合不同密度的土粒很难被侵蚀，直到高密度的粗粒开始流出。4)切断四福石拉苏边坡后，Yamanaka型土壤硬度计的阻力随时间的推移而减小。5)从人工固化砂土的风化侵蚀特性可以判断原状砂土的性质。破坏的基本因素、动力因素和扩张抑制因素。在考虑这些因素为风险因素的基础上，采用多元Logistic分析方法对防灾潜力进行了评价。

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.