Effect of soil layering on suction distribution in unsaturated residual soil slope

Rainwater infiltration into unsaturated residual soil slope is the most significant triggering factor to slope instability. The role of rainwater infiltration in destroying or reducing additional shear strength provided by matric suction especially in homogeneous soil slope profile have been studied extensively. However, the effect of soil layering on suction distribution in unsaturated residual soil is often neglected. Therefore, this paper examines the effect of soil layering on transient suction distribution and hence on slope stability using a laboratory slope model and numerical modeling approach. Two types of soil (sandy silt and silty gravel) obtained from Balai Cerapan Slope in Universiti Teknologi Malaysia, Johor Bahru campus are used in this study. The homogeneous slope consists of sandy silt and the layered slope system consists of the sandy silt as the upper layer and the silty gravel as the lower layer. Three rainfall intensities of 1.7196 × 10-5m/s of 1 hour duration, 1.7694 × 10-6m/s of 24 hour duration and 4.2600 × 10-7m/s of 7 day duration were applied to the slope model. The effects of these rainfalls on factor of safety (FOS) of the analyzed slopes were determined with the SLOPE/W using pore water pressure obtained from the laboratory slope model. The results obtained from this study shows that the layered soil system maintained the soil suction more than the homogeneous soil system due to the effect of capillary forces in the upper layer before breakthrough occurs. Similarly; layered soil system increases the factor of safety of the slope. However after breakthrough occurrence, the factor of safety of the layered soil system decreases more compared to the homogenous soil system. © 2014 ejge.