Modeling of suction distributions in an unsaturated heterogeneous residual soil slope

Abstract

The formation of residual soil of Grade V and Grade VI due to tropical weathering process introduces small hydraulic heterogeneities in the soil mantle which greatly alter the suction distribution during rainfall infiltration, and hence the stability of the residual soil slopes. This paper presents field evidences of suction distributions in a heterogeneous residual soil slope. Several modeling approaches were attempted to simulate the observation by considering the presence of thin layer of Grade VI, the variation in the hydraulic conductivity of Grade V layer as well as the effect of evaporation. The soil hydraulic heterogeneity in Grade V layer was modeled by adopting continuum method, in which the residual soil was subdivided into three zones of average hydraulic conductivities. The analysis results show that the presence of thin layer of Grade VI residual soil and the relict discontinuities in Grade V soil must be considered in the analysis as these features introduced permeability disparity and thus a natural capillary barrier effect that limited the downward movement of infiltrated rainwater even during the exceptionally wet condition. The results also show that the inclusion of evaporation effect provided a better prediction to the suction distributions during wet condition than dry condition.