On October 4–5, 2025, an extreme low-pressure system delivered record-breaking rainfall across Ilam District, eastern Nepal, with peak intensities exceeding 332 mm in 24 hours, the highest recorded in over four decades. The event triggered widespread landslides and floods, resulting in 39 fatalities, displacement of over 400 households, and infrastructure losses exceeding NPR 11.81 billion. Concurrent extreme impacts were documented in the adjacent Darjeeling Himalaya under the same meteorological system (Talukdar et al., 2026), reflecting an intensifying pattern of late-monsoon extremes across the Hindu Kush Himalayan region (Lamichhane et al., 2025; KC et al., 2025). This study presents the geotechnical investigation and mitigation design of two critical rainfall-induced landslides: Mil Golai and Banchare Golai, along the Mechi Highway, Ilam District (Fig. 1). Site characterization integrated drone-based topographic surveying, Standard Penetration Testing (SPT), and Electrical Resistivity Tomography (ERT) to delineate subsurface stratigraphy, colluvial deposit geometry, and groundwater conditions. Hydrological analysis established rainfall-induced seepage parameters, which informed limit equilibrium stability modelling under monsoon-critical loading scenarios. Results confirmed that slope instability is primarily governed by saturation of colluvial deposits, unfavorable slope geometry, and inadequate surface drainage, with pre-intervention factors of safety near or below critical thresholds. An integrated mitigation framework comprising soil nail reinforcement, steel wire mesh surface protection, and multi-tier gabion wall systems was designed and numerically validated, yielding post-intervention factors of safety of 1.24–1.26. The findings demonstrate the value of multi-method site investigation in evidence-based landslide mitigation and underscore the growing vulnerability of Himalayan road infrastructure to extreme precipitation events.

Landslide,Road,Nepal,Rainfall,Geotechnical Engineering