Geotechnical Challenges of Pipeline Corridor Construction Across Expansive Clay Soils of Upper Nile State

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