Stability Assessment of Landslide in Himalayan Terrain: A Spectral Element Method and Back Analysis‐Based Approach

ABSTRACT Securing slope stability in the Nepalese Himalayas today prevents against future disasters. This research integrates numerical and experimental approaches for the analysis of the slope stability of the Ramche landslide using a combination of Back Analysis (BA) and Spectral Element Method (SEM) implemented in the open source Specfem3D_Geotech software. This study focuses on the determination of the factor of safety (FoS) under fully saturated and fully dry conditions, excluding vegetation and earthquake loading. Laboratory tests on collected soil samples provided cohesion and friction angles. The Ramche landslide is a post‐failure slope geometry and soil profile data was used from such an area as a case study to develop a SEM model using AutoCAD and SW_DTM, meshing in CUBIT software and Specfem3D_Geotech tools is used for displacement analysis at different Strength Reduction Factors (SRF). A BA method was used to determine representative soil parameters under dry conditions, assuming zero cohesion and adjusting the friction angle until the SRF reached unity, yielding insitu friction angles between 33° and 36° for different sections. Displacement of the slope and SRF was visualized in Paraview, and corresponding graphs were plotted using Tecplot. Additionally, Phase2 was utilized for the validation of numerical simulation results, yielding a correlation coefficient of 99.02% for SRF and 87.04% for maximum displacement with low statistical errors (RMSE = 0.22266, MSE = 0.04957, MAE = 0.2 for SRF and RMSE = 2.73959, MSE = 7.50539, MAE = 2.16685, for displacement). This study concludes that due to the steep slope and water table fluctuation, the landslide remains highly vulnerable, particularly during the monsoon season. The results indicate that the FoS remains below one, with stability achieved only under fully dry conditions. Mitigating measures such as groundwater draw down and slope modification were found effective in improving slope stability.