Modelling the effect of heterogeneities on suction distribution behaviour in tropical residual soil

Abstract

The formation of tropical residual soils introduces heterogeneities in soil mantle. While the hydraulic conductivity of Grade VI soil is controlled only by the variation at the material scale i.e. grain size, void ratio, and mineralogy, the hydraulic conductivity of Grade V soil is also influenced by the presence of heterogeneity features at the field scale. One of the important features of heterogeneity is relict discontinuities which affects the saturated hydraulic conductivity, ksat. This study focuses on the effect of the heterogeneities on the mechanism of rainfall infiltration and the resulting suction distribution within residual soil mass when subjected to different rainfall patterns. Three approaches were employed in this research i.e. field observation, laboratory experiment, and numerical modelling. Field observation was carried out at an instrumented site for a period of one year to monitor soil response, in term of suction distribution, when subjected to actual rainfall. Forty two (42) series of laboratory infiltration tests were performed on homogeneous and two-layered soils with relict discontinuities subjected to various rainfall intensities to study the effect of different rainfall patterns on suction distribution. The field response was then simulated numerically based on the mechanism found in the laboratory tests to obtain the most appropriate approach in modelling the heterogeneities within soil mass. The field observation shows rainfall patterns play an important role in the propagation of wetting front and suction variation in the soil slope. There was an upper limit of the soil suction in the residual soil slope, even during prolonged dry period which approximately identical to minimum suction, ?min corresponding to the residual water content, ?r in the soil water characteristic curve, SWCC of the soils. The laboratory study also shows that heterogeneities cause the ksat of the soil to vary from one to five orders of magnitude. The flow mechanisms in the residual soils are controlled by the ratio of rainfall intensity to saturated hydraulic conductivity of the soil, q/ksat, the suction potential at the interface between two layers, and the physical flowing conduit within the heterogeneous soil mass resulting in disparate suction distribution profile. Continuum model is capable of modelling the effect of heterogeneities in Grade V material on the mechanism of rainfall infiltration and suction distribution in tropical residual soil. In this method, the relict discontinuities in Grade V soil were simulated by subdividing the layer into multiple isolated zones with an identical SWCC but with different average ksat. The study indicated that the presence of thin Grade VI layer and the relict discontinuities in Grade V layer should be considered in the analysis of suction distributions in residual soil slope subjected to rainfall infiltration.