Abstract
The Ramche and Dhaibung landslides, located along Nepal's Pasang Lhamu Highway and Jibjibe route, represent persistent slope instability hazards that threaten local infrastructure and settlements. This study integrates geomorphological, geological, geophysical, and geotechnical approaches to investigate the failure mechanisms and stability conditions of these landslides. Detailed field mapping and satellite image interpretation revealed ongoing mass movement characterized by active erosional gullies, surface deformation, and loose colluvial deposits. Subsurface investigations using Electrical Resistivity Tomography (ERT) identified key geoelectrical contrasts, with low-resistivity zones corresponding to saturated, clay-rich or fractured zones and high-resistivity zones linked to dry, coarse colluvium. These discontinuities indicate weak structural layers and potential slip surfaces at depths of 15 – 20 metres. Geotechnical tests confirmed that the slope materials comprise poorly graded, low to medium plasticity soils with low cohesion and strength. Mineralogical analysis (XRD) revealed quartz as the dominant mineral and an absence of expansive clay minerals, suggesting that slope failure is primarily driven by hydrological factors rather than swelling clays. Slope stability analysis using the Limit Equilibrium Method indicated that both landslides are stable under dry conditions but become unstable when saturated, with safety factors dropping below 1.0. Additionally, seismic loading scenarios further reduce slope stability, highlighting the compounded risk during the monsoon season. While both sites share similar geological characteristics, the Ramche landslide is more vulnerable due to deeper tension cracks and greater hydrological influence, whereas Dhaibung presents a higher risk of debris flow during rainfall due to its gully-dominated terrain. The study underscores the critical role of rainfall infiltration, weak lithology, and structural discontinuities in driving slope instability, providing a comprehensive understanding essential for landslide hazard mitigation in mountainous terrains.