Reliability-Based Slope Stability Analysis Along the Bonga-Felegeselam Road, Ethiopia

Slope instability is a common problem in mountainous road corridors, particularly in regions characterized by steep terrain, weak soil formations, and intense seasonal rainfall. The Bonga-Felegeselam road in southwestern Ethiopia has experienced several slope failures during construction due to these unfavorable geological and hydrological conditions. This study investigates the causes of instability and evaluates appropriate stabilization measures for two critical cut slopes located between Stations 16+900 to 16+980 (left-hand side, LHS) and Stations 35+200 to 35+300 (both LHS and RHS) of LOT I: Bonga Asphalt Road Project. Detailed site characterization was conducted through field observations, geometric configuration, soil classification, and laboratory testing to determine the relevant geotechnical properties. Slope stability was assessed using both deterministic and probabilistic approaches based on the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM). The analyses considered different groundwater conditions to evaluate the influence of rainfall-induced saturation on slope stability. The results indicate that the slopes become highly unstable under fully saturated conditions, with factors of safety ranging from 0.657 to 0.916 and failure probabilities between 41.8% and 95.3%. Sensitivity analysis further showed that slope stability is more sensitive to variations in the friction angle than cohesion. To mitigate the instability, a combination of masonry retaining walls, surface drainage, and subsurface drainage systems was proposed. Post-remediation analyses demonstrated a significant improvement in slope stability, increasing the factor of safety to values between 1.57 and 1.90. The findings highlight the importance of integrating deterministic and probabilistic approaches to develop reliable stabilization strategies for rainfall-prone mountainous regions.