Development of rainfall hazard envelope for unsaturated infinite slope

Abstract

Rainfall during summer season is one of the major causes of slope failure in Korea, especially in unsaturated weathered granite soils. The current slope design procedure for the rainy season is to just raise the ground-water level, which is not adequate for the unsaturated soil slope. Previous rainfall hazard envelopes only tend to use rainfall intensity and duration as the main variables, not considering mechanical and hydraulic characteristics of unsaturated soils. The general equation for Factor of Safety (FS) in infinite unsaturated slopes is derived to consider the rainfall infiltration. The three important factors governing the equation are as follows: rainfall characteristics such as intensity and duration to determine a wetting front depth using an infiltration model, shear strength theory for unsaturated soil, and variation of suction profile during rainfall. The minimal FS is calculated from the FS profile with depth for a given rainfall intensity and duration, and a rainfall hazard envelope considering soil infiltration characteristics is built for a given slope angle. The procedure to develop a hazard envelope for various slope angles is applied to three unsaturated weathered granite soils in Korea.