Hydro-mechanical behaviour of a residual soil slope in Malaysia

Abstract

Climate change poses real threats to the sustainability of slopes, particularly in the tropical region of the world. Its effects have caused a greater occurrence of extreme climate events that are reflected in a greater occurrence of slope failure incidents for this region. The hydro-mechanical characteristics of soils linked with climate variation are factors that can explain deterioration in slope stability. Therefore, the ability to analyse the hydro-mechanical behaviour properly is worthy of investigation and this can be done by the use of experimental investigation and numerical modelling using both saturated and unsaturated soil properties. In this thesis, the description of the important effects of climate impacts on slope stability has been made for a failed tropical residual soil slope located in Precinct 9, Putrajaya, Malaysia. Part of the work involved soil sampling for the acquisition of undisturbed soil samples from the slope. Series of advanced saturated and unsaturated laboratory testing for both hydrological and mechanical properties have also been implemented and were used in transient, unsaturated numerical modelling of slope stability analysis (using Plaxis 2D). The results demonstrate that within a slope the mobilised shear strength drops quickly during a rainfall event (due to rainfall infiltration) but recovers much more slowly during drying. This shows how a series of regular rain storms with short periods of drying in between can cause a ratcheting effect, with rapid loss of strength during each period of rain that is not recovered during the intermediate drying periods. In addition, the results also show that the adoption of critical state soil parameters is more suitable to match the observed failure. The failure was due to a very extreme amount of rainwater infiltration in the two days before the incident, including the largest daily rainfall in 2007 of 140mm.