Comprehensive investigation of rainfall infiltration and progressive failure mechanism of the Kavalappara Landslide in Kerala, comprising geophysical and geotechnical factors

Landslides triggered by heavy rainfall pose a serious threat globally, endangering infrastructure and lives. Many previous landslide studies lack comprehensiveness and site specificity. Thus, a comprehensive investigation is essential to understand the failure mechanisms and contributing factors for assessing potential future hazards. This study aims to investigate the debris flow landslide that occurred in Kavalappara, Kerala, India, on August 8, 2019, through an integrated approach combining geophysical test, weathering characterization, geotechnical, and numerical analyses. Shear wave velocity (V_s) was determined using the Multi-Channel Analysis of Surface Waves (MASW) test to obtain the substrata of the slope. Residual and unsaturated soil properties were obtained through ring shear and dew point potentiometer tests. The mineralogical composition of the soil was identified using Field-Emission Scanning Electron Microscopy (FE-SEM), Energy Dispersive X-Ray Analysis (EDAX), and X-Ray Diffraction (XRD) patterns. These investigation results focused on slope stability during rainfall infiltration using Limit Equilibrium (LEM) and Finite Element Analysis (FEM) for both low and high-intensity rainfall. Finally, the progressive failure mechanism of the landslide was analysed using the Finite Difference program (FDM). The soil profile showed a variation from loose to dense, with a V_s range of 172.85 m/s to 440.53 m/s. No rock layers were identified down to a depth of 15 m. The landslide area consists of migmatite as a parent rock, and the soil was identified as silty clay, comprising quartz and clay minerals. The FEM and LEM analyses reveal that the factor of safety was reduced to 0.83 due to increased pore water pressure and the degree of saturation. The pore water pressure ratio (r_u), estimated at 0.32, was used in the FDM. The landslide, initiated at r_u of 0.35, reached maximum velocities of 15.4 m/s horizontally and 12.4 m/s vertically. This study helps disaster management to analyse debris flow and find effective mitigation strategies for hilly areas.