Investigation of factors influencing the slope failure on road: Case study “Huye – Nyamagabe road

Abstract

This study examines the geotechnical factors contributing to slope instability along the Huye– Nyamagabe road in southern Rwanda. Comprehensive field investigations and laboratory tests were carried out to characterize the soil properties, while numerical simulations using GTS NX were employed to analyze slope behavior under seismic loading and stabilization measures using geogrid reinforcement and bench terrace in progressive steps. Slope stability was assessed using both the Bishop Method and the Strength Reduction Method (SRM), which revealed that most multi-layered and artificially cut slopes exhibited a Factor of Safety (FoS) below 1.35, indicating inherent instability. This instability is further exacerbated by the steep slope inclinations observed along the corridor The seismic slope stability assessment involved both time history analysis and permanent deformation evaluation across varying Peak Ground Acceleration (PGA) levels. The findings indicated a progressive increase in displacement with higher seismic intensity: 0.030 m at 0.10g, 0.0595 m at 0.16g, and 0.066 m at 0.20g. Under static conditions, the slope demonstrated marginal stability with a low Factor of Safety (FoS) of 1.06. the road embankment fill material, which initially had a safety factor of 1.16, improved to 2.2 after reinforcement with geogrid. Rainfall and seismic loading significantly affect slope stability. Rainfall infiltration raises porewater pressure, reducing the factor of safety from 1.504 to 1.068 at 45 mm/hr (Partha Pratim Boruah V. R., 2025). Vegetation, such as vetiver grass, improves stability by reducing displacement by about 36%. Seismic vibrations also generate excess shear stresses and pore pressures, triggering landslides (Partha Pratim Boruah J. T., 2024). Hence, integrating rainfall and seismic effects is vital for accurate slope stability assessment The study highlights the substantial impact of seismic activity on slope failure, emphasizing the need for mitigation measures that address both soil strength and earthquake-induced stresses. These insights will inform future slope stabilization and road safety enhancements in Rwanda’s hilly regions.