Effects of soil porosity and hydraulic conductivity on slope stability: case study of slope failure on mountain ranges

Abstract

In Malaysia, landslides have posed serious threats to settlements and structures that support transportation, natural resource management and tourism. More than 100 hillslopes had been identified by Malaysian Public Works Department (PWD) as risky for possible landslides. The hillslopes are found predominantly in Fraser's Hill, Cameron Highlands, Genting Highlands (all in Pahang), Gunung Raya (Langkawi), Paya Terubung Valley (Penang), the mountain ranges in Ulu Kelang, Selangor, and several limestone hills in Ipoh. As a tropical country, most landslides in Malaysia are triggered by heavy rainfall. Modeling rainwater infiltration in slopes is vital to the analysis of slope failure induced by heavy rainfall. In this study, a numerical model was developed to estimate the extent of rainwater infiltration into an unsaturated slope, the formation of a saturated zone, and the change in slope stability. This model is then used to analyze the effects of the soil porosity parameters and soil thickness on the occurrence of slope failure. Results showed that when the surface soil of a slope has a relatively large ESP value, it has a greater capacity for holding rainwater, and therefore delays rainwater infiltration into the subsurface layer. In this manner, a relatively large surface layer ESP value contributes to delaying slope failure. In addition, the thickness of soil is also a significant parameter in slope stability analysis. A shallow soil depth resulted in greater discharge volume and a lower peak pore water pressure during the major rainfall event, and consequently the slope failure tends not to occur. However a deeper soil depth increased the weight of solids and the soil moisture conditions in the slope and the pore water pressure which the increased likelihood of slope failure.