Performance of Lateritic Soil Slopes at the Onset of Rainfall Infiltration

Rainfall-induced slope instability is one of the major concerns in tropical regions. Many rainfall-induced slope failures were observed in lateritic soil slopes in Kerala, during monsoon seasons. In the present study, an attempt was made to investigate the factors affecting the stability of soil slopes at the onset of rainwater infiltration; and to assess the relative importance of various factors on the slope stability by using the properties of soil collected from areas where rainfall-induced slope failures were observed. A total of about 200 numerical models were investigated by considering 10 different rainfall intensities, 9 different rainfall durations for each rainfall intensity, 5 different configurations of initial water table, 5 different slope angles and 3 different types of soils. It was observed that the slope failures in unsaturated lateritic soil during rainfall are due to the loss of suction due to the advancement of wetting front rather than the rise in ground water table. For any particular slope angle in the unsaturated lateritic slopes, there exists a critical duration of the rainfall to cause slope failure; irrespective of the rainfall intensities varying from 0.1 to 132 mm/h. And, it decreases as the slope inclination increases. Also, there exists a limiting ratio, (q/k(sat))(lim) which initiates a reduction in factor of safety and a threshold value (q/k(sat))(thr) which causes a maximum reduction in the initial factor of safety of the soil slopes at the onset of rainfall infiltration. This was found to be applicable to all the three different soil types (classified as CL, MH and SP) used in the present study. Findings from the present study will be useful in understanding the failure mechanism in unsaturated residual soils and predict the initiation of slope failures for any region with respect to a rainfall scenario, slope geometry and soil permeability.