Bearing Capacity Assessment of Bridge Foundations on Expansive Black Cotton Soils in Warrap State, South Sudan

Expansive black cotton soils (BCS) present a severe geotechnical challenge for bridge foundation design in the Warrap State of South Sudan, where significant seasonal moisture fluctuations drive extreme volume changes and dramatic reductions in bearing capacity. This study presents a comprehensive bearing capacity assessment of shallow and semi-deep bridge foundations resting on BCS profiles encountered along four road corridors in Warrap State. A total of 120 undisturbed and remoulded soil specimens were subjected to standard Proctor compaction, Atterberg limit tests, free-swell index determination, and consolidated undrained triaxial shear tests across three moisture-content regimes corresponding to dry, transitional, and fully saturated field states. Plate load tests at six borehole locations provided in-situ ultimate bearing capacity values that were cross-validated against analytical solutions of Terzaghi, Meyerhof, and Hansen–Vesic. Monte Carlo simulation incorporating probabilistic characterisation of cohesion, friction angle, and soil unit weight was employed to derive reliability indices for four candidate foundation geometries. Results indicate that the ultimate bearing capacity of natural BCS declines from 285 kPa in the dry season to as low as 76 kPa post-flooding — a reduction exceeding 73 percent. Lime stabilisation at 4 percent by weight restored the bearing capacity to 224 kPa under saturated conditions, achieving a reliability index β = 3.12 against a target of 3.0. Recommendations for foundation depth, treatment strategy, and design bearing pressures are provided, offering a region-specific framework for bridge engineers in South Sudan.