Geotechnical Investigation and Stability Assessment of Landslide-Prone Slopes along the Mugling-Narayanghat Highway of Nepal

The Mugling-Narayanghat highway corridor in central Nepal is a major trade route frequently disrupted by rainfall induced landslides. This study presents a site-specific geotechnical investigation aimed at characterizing soil properties and assessing the stability of unstable sections along the corridor. Representative soil samples were collected from five active landslide locations (Chainages 16 + 600, 20 + 400, 32 + 300, 34 + 800, and 35 + 600) to determine particle size distribution, Atterberg limits, compaction characteristics, and shear strength parameters. Laboratory results classifies the slope materials as Non-Plastic (NP) soils, ranging from Well-Graded Gravels (GW) to Poorly Graded Sands (SP) and Well-Graded Sands (SW). These characteristics render the soil matrix highly permeable and susceptible to rapid shear strength loss upon saturation due to the absence of cohesive clay minerals. Shear strength parameters determined using direct shear tests yielded friction angles ranging from 26.1° to 29.1° and apparent cohesion values between 38.2 kPa and 102.5 kPa. Analytical factor of safety (FOS) calculations were performed to evaluate slope vulnerability under dry and saturated conditions. The assessment indicates that saturation reduces the FOS by approximately 30% to 36%, with values dropping below unity (FOS ranges from 0.89 to 0.96) in critical sections. These results quantitatively align with field observations of recurrent failures driven by monsoon rainfall and hydraulic forcing. Consequently, site-specific mitigation measures are proposed. This study demonstrates that index-property testing combined with analytical stability assessment provides a cost-effective framework for prioritizing maintenance in highway corridors with similar geotechnical conditions.