Performances of two instrumented laboratory models for the study of rainfall infiltration into unsaturated soils

Rainfall infiltration and suction variation in unsaturated soils must be taken into consideration in the analysis of most slope stability problems, particularly in the tropical regions where the annual precipitation is high. The process of rainfall infiltration into unsaturated soils is an extremely complex problem attributed to the non-linearity of the hydraulic property functions of the unsaturated soils. This paper describes in detail two instrumented laboratory models, i.e. one-dimensional soil column, and two-dimensional slope model used to provide experimental evidences for the transient suction variations in the unsaturated soils under certain rainfall conditions. The performances of the laboratory models were tested on four typical types of residual soils, i.e. sand-gravel, silty gravel, sandy silt, and silt (kaolin), and a two-layered soil system, i.e. sandy silt underlain by silty gravel. The results showed that the suction distributions for the single-layered homogeneous soils obtained from the simpler one-dimensional soil column were almost identical to that of two-dimensional slope model. However, the two-dimensional slope model should be employed for the two-layered soil system because of the dominant effect of the lateral flow mechanism. The capillary barrier effect was observed when a less permeable soil layer was underlain by a more permeable soil layer. The minimum suction value in soil is governed by the rainfall intensity, rainfall duration, and the saturated permeability of soil. The infiltration rate of the fine-grained soils that subjected to shrink and crack was independent of the soil permeability, but was significantly governed by the preferential flows developed in the soils.

Research Highlights►The simpler one-dimensional soil column could give an equally good result with the two-dimensional slope model for single-layered homogenous soil. ►The two-dimensional slope model, however, should be employed for the two-layered soil system because of the dominant effect of horizontal flow mechanism. ►The capillary barrier effect was observed when a less permeable soil layer (sandy silt) is underlain by a more permeable soil layer (silty gravel). ►The minimum suction value in a soil is governed by the rainfall intensity, rainfall duration, and the saturated permeability of the soil. ►The flow of water in the fine-grained soil that subjected to shrink and crack is independent of the mass permeability of the soil matrix.