Regional Modelling of a Rainfall-Induced Landslide in Idukki, Kerala by Incorporating Unsaturated Soil Properties: Case Study

Idukki is constantly hit by rainfall-triggered landslides, which lead to loss of precious lives of people apart from other various types of allied losses. Residual soils in tropical regions are mostly found in an unsaturated condition. In unsaturated conditions, soil will have different hydrological and mechanical properties, which are quite often ignored in traditional soil mechanics approach due to either insufficient data or for the reasons of avoiding complex analyses. This paper attempts to analyse the rainfall-induced shallow translational landslide at Idukki district of Kerala state in India by conducting regional modelling with incorporation of unsaturated soil properties prior to the occurrence of the landslide. This landslide was triggered by a heavy rainfall which lasted for several days and it claimed seven people lives besides damaging seven houses. Transient Rainfall Infiltration and Grid-based Regional Slope Stability (TRIGRS) was used to model this event by incorporation of saturated and unsaturated soil properties. Site visits were carried out and several field and laboratory tests were performed for the determination of various saturated and unsaturated geotechnical properties of soil required for the analysis. Seepage and stability analyses were performed to determine the conditions that lead to the failure of the slope. It was observed that the rainfall leads to the gradual reduction in suction in the soil along with the rise in ground water table which caused the shear strength to reduce along the slip surface and hence led to a shallow translational landslide.