Slope Stability Analysis of Road Embankments in the Mountainous Terrain of Eastern Equatoria, South Sudan

Road embankment failures in the mountainous terrain of Eastern Equatoria, South Sudan, represent a persistent threat to regional connectivity, humanitarian access, and economic activity in one of sub-Saharan Africa's most geotechnically challenging environments. The region's steep escarpment terrain, deeply weathered tropical soils, intense monsoon rainfall (1,200–1,800 mm/year), and near-total absence of site investigation data have historically resulted in embankment designs based on empirical rules-of-thumb with no formal factor of safety analysis. This study presents the first systematic slope stability assessment of road embankments along five critical routes in Eastern Equatoria (Torit–Kapoeta, Nimule–Juba, Kapoeta–Narus, Chukudum–Budi, and Lafon–Moli corridors), encompassing 28 embankment cross-sections identified as unstable or potentially unstable through field reconnaissance. Geotechnical characterisation included Standard Penetration Tests (SPT, n = 86 boreholes), laboratory triaxial and direct shear tests, and soil classification per USCS. Slope stability was analysed using the Bishop Simplified Method and Spencer's Method within the Slide2 software package, for both dry-season and post-rainfall saturation conditions. Results indicate that 61% of assessed embankments have Factors of Safety (FS) below the critical threshold of 1.25 under post-rainfall conditions, with five sections recording FS < 1.0 (indicating active or imminent failure). A rainfall-triggered pore water pressure model calibrated to local piezometer data reveals that embankment FS drops by 28–45% within 48 hours of a 100 mm/day rainfall event. Stabilisation recommendations including geosynthetic reinforcement, drainage improvements, slope regrading, and soil nailing are evaluated through par