Forensic geotechnical–geophysical investigation of groundwater-driven recurrent slope failure along the Kali Gandaki Road corridor, Lesser Himalayas, Nepal

Abstract

Recurrent slope failures along Himalayan Road corridors cause substantial infrastructure damage and loss of life, yet systematic forensic investigation of individual failure sites remains rare in Nepal. This study presents an integrated forensic geotechnical–geophysical investigation of the Sirubari landslide, a repeatedly active failure site located on the Kali Gandaki Road corridor (Myagdi District, Lesser Himalayas, Nepal), to determine its failure mechanism, identify primary causal factors, and recommend evidence-based mitigation measures. Three two-dimensional Electrical Resistivity Tomography (ERT) profiles using the Wenner electrode array configuration were conducted across the landslide zone, supplemented by two rotary core boreholes (BH-1 and BH-2) each drilled to 15 m depth with Standard Penetration Testing and comprehensive laboratory characterization. ERT inversion revealed a complex subsurface stratigraphy comprising sandy colluvial deposits exceeding 21 m in thickness overlying saturated fractured phyllite bedrock, with multiple slip surfaces identified at depth. The colluvial mass is predominantly sand (73–97%), with natural moisture content ranging from 17.5 to 23.6%, consistent with extensive groundwater saturation. Slope stability analysis using the limit equilibrium method yielded factors of safety of 0.858 (hillside) and 0.857 (valley side) under monsoon-season groundwater conditions, confirming active slope failure. Subsurface seepage driven by monsoon infiltration into thick permeable colluvium overlying low permeability phyllite bedrock is identified as the primary failure mechanism. Integrated subsurface drainage, slope reinforcement, and bioengineering measures are recommended to achieve long-term slope stability along this critical trade corridor.