Engineering Geological Assessment of Slope Stability Issues for Bridge Sites in the Central Nepal Himalaya

Abstract

The Himalayan region presents major challenges for infrastructure development due to active tectonics, steep terrain, and fractured rock masses. This study investigates slope stability at two adjacent bridge sites in central Nepal within the Kalitar Formation of the Bhimphed Group, comprising mica schists, micaceous quartzites, and chloritic–sericitic schists. Engineering geological mapping, discontinuity surveys, geomechanical analysis, and Electrical Resistivity Tomography (ERT) were conducted. The rock mass is thinly foliated, highly fractured, and variably weathered. Three major discontinuity sets and one random set were identified, with closely spaced joints and occasional argillaceous infill. Kinematic analysis indicated no planar failure potential, moderate wedge failure potential, and high toppling failure potential. Rock mass characterization yielded a Geological Strength Index of 40–45, uniaxial compressive strength of 0.396–4.133 MPa, and deformation modulus of 2311–2362 MPa. Limit equilibrium analysis of an approximately 50 m slope at Pile P4 produced a Factor of Safety of 1.411 under static, dry conditions. ERT surveys identified subsurface moisture zones, weathered horizons, and potential slip surfaces. Recommended measures include extended slope analysis, including areas upslope and downslope of access roads, detailed topographic and subsurface investigations, drainage management, bioengineering, environmentally sensitive construction, and structural assessment of bridge foundations to support safer infrastructure development.