The construction of petroleum pipeline corridors across the expansive Vertisol clays of Upper Nile State, South Sudan, presents extreme geotechnical hazards driven by the high shrink-swell behaviour of smectite-dominated soils with liquid limits of 55–85% and plasticity indices of 25–50%. This study investigates the principal geotechnical challenges — excessive heave, trench instability, differential settlement, and pipe stress concentrations — through an integrated programme of in-situ investigation, laboratory testing, numerical analysis, and mitigation design. Dynamic Cone Penetrometer (DCP) surveys and borehole logging at five test sites along a 180-km corridor from Malakal to Renk documented subgrade CBR values of 3–8%, confirming A-7-6 classification throughout. Free swell values ranged from 43% to 112% and swell pressures from 118 to 248 kPa, placing all sites in the high-to-very-high swelling risk category. Seasonal variation in water content of ±18% in the upper 0.5 m generates cyclic volumetric strains up to 8.4%, sufficient to cause bending stresses in the pipeline approaching the ASME B31.4 design limit of 72% SMYS when differential heave exceeds 95 mm across a 30-metre span. Five ground improvement strategies were evaluated — lime mass stabilisation, geotextile reinforcement, sand replacement, lime column stabilisation, and pile-supported sleeper — and compared on heave reduction effectiveness (40–98%), unit cost (USD 18–260/m), and long-term life-cycle cost over a 30-year design life. Lime stabilisation at 6% content achieved optimal performance: reducing plasticity index to 25%, CBR to above 80%, swell pressure below 80 kPa, and unconfined compressive strength above 510 kPa at a competitive unit cost of USD 28–45/m. A corridor risk zoning methodology inte