Geotechnical and geological study of roadside landslides at Khamvitta and Lete, Kaligandaki Corridor (NH-48), Nepal

Recurring roadside landslides along Himalayan highway corridors pose a critical threat to infrastructure and livelihoods, yet paired investigations of distinct co-located failure modes remain scarce. This study investigates two active landslides, Khamvitta and Lete Khola, along the Kaligandaki Corridor (NH-48), Nepal, with Khamvitta directly tested in the field and laboratory and Lete Khola assessed by geophysics alone. Two-dimensional Electrical Resistivity Tomography (ERT) was deployed at both sites; Standard Penetration Tests, Dynamic Cone Penetration Tests, and laboratory characterisation were additionally conducted at Khamvitta, while Lete Khola relied solely on a single ERT profile, with shear strength parameters drawn from published analogues to inform limit equilibrium modelling. Low-resistivity zones (< 100 Ωm) were interpreted as saturated clay-silt horizons and validated against borehole data at Khamvitta. Monsoon-season SEEP/W-coupled SLOPE/W modelling gave a factor of safety (FOS) of 1.071 at Khamvitta and two critical surfaces at Lete Khola, FOS = 0.724 (upper) and 1.298 (lower), confirming groundwater-induced effective stress reduction as the principal destabilising mechanism. Mitigation comprising subsurface drains, anchored RCC walls, gabion walls, soil nailing with 3D mesh, and slope re-profiling improved the FOS to 1.847 at Khamvitta, 1.378 (upper), and 2.357 (lower) at Lete Khola respectively. The study demonstrates that comparable hydrological forcing produces fundamentally different failure modes, shallow rotational failure in phyllitic colluvium at Khamvitta and compound translational failure in cohesionless alluvium at Lete Khola, governed by contrasting lithology, slope geometry, and anthropogenic modification; the Lete Khola interpretation is a literature-constrained conceptual model. These findings offer transferable guidance for roadside slope management across trans-Himalayan corridors.