Geotechnical Investigation of Stabilized Black Cotton Soil for Flexible Pavement Design: A Case-study of Dadin-kowa, Gombe

This study investigates Black Cotton Soil (BCS), classified as A-7-6 under the American Association of State Highway and Transportation Officials (AASHTO) system and the Unified Soil Classification System (USCS). The result shows that the natural BCS has a liquid limit (LL) of 80% and a plastic limit (PL) of 35.48%. When blended with 10% Ordinary Portland Cement (OPC) (90% BCS), LL and PL decreased to 55.5% and 27.14%, respectively. A comparable blend of 8% OPC, 12% Bagasse Ash (BA), and 80% BCS yielded LL and PL of 57.4% and 28.3%, respectively. The natural BCS also shows a shrinkage limit (SL) of 21.7%, which was reduced to 14.02% in a 10% BA–90% BCS blend, while the 8% OPC–12% BA–80% BCS blend lowered SL to 14.88%. For compaction properties, the natural BCS showed a maximum dry density (MDD) of 1.52 Mg/m³ and optimum moisture content (OMC) of 18.2%. On treatment with 2% OPC, 3% BA, and 95% BCS increased MDD to a peak of 1.78 Mg/m³, while a 10% BA 90% BCS blend raised OMC to 19.1%. For the California Bearing Ratio (CBR) values critical for pavement design, the unsoaked CBR of BCS increased from 36.49% to 42.60%, making it suitable only as a sub-base material for flexible pavements. Soaked CBR improved from 6.80% to 20.03% but remained insufficient for sub-base use.