Low-Volume Road Design Using Laterite and Stabilised Soils in South Sudan's Equatoria Region

Low-volume roads constitute over 85% of South Sudan's rural road network and are the primary means by which farming communities in the Equatoria Region access markets, healthcare, and educational institutions. Despite their importance, these roads are largely constructed without reference to engineering design standards, using locally available laterite and gravelly soils whose suitability and geotechnical behaviour under tropical loading conditions are poorly characterised in the published literature for this specific geographic context. This study presents a comprehensive geotechnical and pavement engineering investigation of laterite and stabilised soils from three borrow areas in Central, Eastern, and Western Equatoria States, aimed at developing practical design guidelines for low-volume road construction using local materials. Forty-two soil samples were collected and subjected to physical, mechanical, and chemical characterisation, including particle size distribution, Atterberg limits, compaction, California Bearing Ratio (CBR) at 0, 7, 28, and 90-day curing intervals, X-Ray Fluorescence (XRF) geochemical analysis, and free swell. Stabilisation trials were conducted with hydrated lime (2–8%), ordinary Portland cement (2–8%), and rice husk ash (2–8%) to evaluate improvement in CBR and plasticity reduction. Accelerated loading trials at a field wheel-track facility assessed rutting performance over a simulated 3,000 × 10³ ESAL loading history. Results indicate that natural laterites from Equatoria meet the minimum CBR threshold for unsealed base course (CBR ≥ 80%) in two of three borrow areas after moisture conditioning, but all three require stabilisation to serve as a structural subbase above weak subgrade soils. Cement stabilisation at 4–6% produces the highest