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Abstract: 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. [Copyright &y& Elsevier]

Abstract: Slope failures in the tropical regions, particularly Malaysia are commonly triggered by frequent rainfall. The tropical rainfall can be characterized as short and intense throughout the year, and prolonged and less intense during monsoon seasons. Under such circumstances, various rainfall patterns should be included in the analysis of rainfall-induced slope failure in the tropical regions. This paper is aimed to demonstrate a simple model for preliminary evaluation of rainfall-induced slope failure. The critical rainfall patterns for four typical types of soil were first determined. Seepage finite element analyses were conducted using the extreme rainfall of ten-year return period for Johor Bahru, Malaysia. The results showed that the ratio of rainfall intensity to soil saturated permeability (i.e., I/k sat) plays an important role in determining the critical rainfall pattern. Two critical combinations of antecedent rainfall and major rainfall, 1-day, 2-day, 3-day, 5-day, 7-day, 14-day, and 30-day antecedent rainfalls and the redistribution of the critical combination of antecedent rainfall and 1-day major rainfall were responsible for the formation of suction envelope in soil. The suction envelope, representing the worst suction distribution in soil, was used for the computation of factor of safety of soil slope through the modified infinite-slope–limit-equilibrium method. A model, PERISI, was developed based on the findings from numerical simulation. The suction envelope and factor of safety computed from the PERISI model showed good agreements with the results obtained from Seep/W and Slope/W computer programs and the results derived from the model of Rahardjo et al. developed in 1995. [Copyright &y& Elsevier]