Effect of hysteresis on the stability of residual soil slope

Abstract

Soil-water characteristic curve (SWCC) is an essential parameter in unsaturated soil mechanics. Matric suction in unsaturated residual soils changes with varying climatic conditions associated with cyclic drying-wetting conditions that result in hysteresis in the SWCC. The soil mechanical behaviour under wetting is crucial since numerous rainfall-induced slope failures occur during the wetting process. However, many slope stability analyses were carried out using drying SWCC. Consequently, the factor of safety (FoS) calculation may not represent the actual field condition. This paper presents the effect of hysteresis in SWCC on the stability of unsaturated residual soil slopes from Bukit Timah Granite in Singapore. The study focused on the analyses considering the differences in pore-water pressure and water content variations under drying and wetting conditions by performing numerical seepage and stability analyses. Each analysis was carried out on a slope subjected to dry and rainy periods under three different conditions: i) using only drying SWCC; ii) using only wetting SWCC; iii) using combined drying and wetting SWCCs. The results indicated that the FoS variations obtained from the numerical analyses incorporating combined SWCC matched more closely those obtained by incorporating only wetting SWCC than those obtained by incorporating only drying SWCC, regardless of the wetting or drying processes that the soil experienced. Moreover, the numerical analyses under high rainfall intensity by incorporating only wetting SWCC gave a more conservative FoS as compared to those obtained by incorporating combined SWCC. Numerical analyses incorporating only drying SWCC gave the most conservative FoS regardless of rainfall intensity.