Influence of transport Layer on Transient Suction Distribution in a Two-Layered Slope

Residual soil slope failure due to rainfall infiltration is one of geotechnical hazards receiving much attention in many tropical climate countries. The infiltrating water eliminates matric suction in the residual soil slope and results in slope failure. A capillary barrier is used to prevent excessive rainfall infiltration and preserve matric suction in the residual soil slope and hence prevent rainfall-induced slope failure. A numerical study to examine the performance of a transport layer in a two-layered slope using capillary barrier principle was presented in this paper. Material properties of tropical residual soils consisting of Grade V (silty gravel) and Grade VI (sandy silt) were used and modelled a two-layered slope. These material properties were obtained from representative soil sample of Balai Cerapan slope in Universiti Teknologi Malaysia, Johor Bahru campus. A granite chips (Gravel) was also incorporated to act as a transport layer in the numerical model. The simulated slope model was then subjected to three different rainfall intensities of 9 mm/h (rainfall 1), 22 mm/h (rainfall 2) and 36 mm/h (Rainfall 3) representing short, medium and high intensity rainfalls, respectively. A total of six numerical schemes were performed by restricting the thickness of the transport layer to 0.1 m. However, to assess the effect of the transport layer thickness on suction distribution; the thickness was increased to 0.2 m. The results of the study show that inclusion of gravelly transport layer enables the top layer of fine sandy silt residual soil to retain the infiltrating water as a result of capillary break developed at the interface and also divert it above the interface towards the direction of the toe of the slope. Similarly the transport layer is found to be effective in preventing water breakthrough occurrence into the underlying coarser soil layer of the two-layered slope, especially when the thickness of the transport layer is optimum.