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Abstract: Both rainfall characteristics (rainfall intensity and duration) and saturated permeability of soil may influence the type and mechanism of surficial slope failures. In general, the failures can be initiated by two mechanisms, i.e. loss of matric suction through propagation of wetting front, and rise of water table. Up to date, there are still no clear indicators to identify the dominant parameters that control the type of failure for these shallow landslides. This paper investigates the hydraulic responses of soils to the variations of rainfall characteristics and soil permeability through numerical analyses. The results showed that the hydraulic responses to rainfall for a homogeneous infinite slope underlain by an impermeable layer can be divided into two stages: 1) the propagation of wetting front, and 2) the rise of water table. Based on these hydraulic responses, the type and mechanism of failures were deduced from the analytical analyses. Both the rainfall characteristics and saturated permeability were found to be predominant in controlling the hydraulic responses of soil, and hence the occurrence time, depth of failure plane, and type of surficial slope failures. [Copyright &y& Elsevier]

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]

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. [Copyright &y& Elsevier]