Investigation of rainfall-induced landslide on unsaturated lateritic residual soil slope in Nilgiris, Western Ghats, India using deterministic and reliability analysis

The study investigates an earth slide type rain-induced landslide that occurred at Madithorai, Nilgiris District, India in 2009. It consists of detailed field investigations such as topographical survey and borehole investigation, laboratory and numerical investigations. Based on the topographical survey the slope was modelled in SEEP/W programme to perform the uncoupled transient seepage analysis. The cumulative rainfall of 586 mm was given as an input for a duration of 5 days that caused landslide at Madithorai. The results of transient seepage analysis were used to determine the stability of the slope based on the infinite slope method for unsaturated soil. Based on the analysis the failure mechanism of Madithorai landslide was observed. The influence of antecedent rainfall using three different idealized 5-day antecedent rainfall was studied. A threshold intensity was established from the different 5 days antecedent rainfall and the corresponding factor of safety which can be used as for landslide early warning. The factor of safety from deterministic analysis was greater than one which does not imply that the slope is stable. Therefore, the performance of the slope was studied based on the reliability index and probability of failure using the Monte Carlo simulation. The results highlight that the slope is in hazardous condition with a reliability index of 1.15 and the probability of failure of 11.7%. Hence, it is recommended to adopt a landslide early warning system and suitable remedial measures at Madithorai location.