Rain triggered slope failures in unsaturated residual soils

This paper is a continuation ofthe research paper titled "Rainfall Infiltration Analysis in Unsaturated Residual Soil slopes", which discussed the changes in the pore pressure regime ofa slope made of unsaturated residual soils, due to prolonged rainfall. This paper discusses the consequent changes in the safetymargins of the slope. Residual soils, which remain at the location of the parent rock after weathering are characterizcd by different degrees of weathering. difference in the weathered product based on the mineralogical composition of the parent rock and significant variations within a short distance due to above reasons. Zones of different levels of weathering will have significantly different hydraulic and shear strength properties. Rain induced slope failures are a common geotechnical problem in the tropics. In this study the complex geological situations in a residual soil slope is idealized by two states; a uniform soil slope and a slope of weathered rock underlying the soil. Two typical cut slopes from the Southern Transport Development Project were used in the analysis. The variation in the safety margins of the slope with the progression of the rainfall was analyzed by the Bishop's simplified method and the Spencer's method. The analysis was done with the SLOPEW computer software and the data on the changes of the pore pressure regime computed with SEEPW analysis were incorporated. Initially, the safety margins were assessed through the computation of factor of safety and subsequently a probabilistic analysis was done accounting for the uncertainties in the properties of soil such as, friction angle. cohesion. and pore water pressure. Monte-Carlo approach was used in the probabilistic analysis