ABSTRACT
Engineers are often faced with the problem of constructing on black cotton soils, because of their unsuitable behaviour for construction.The preliminary investigation conducted on the natural black cotton soil collected fromDeba, Deba Local Government Area of Gombe State shows that it falls under A-7-6(16) AASHTO classification and CL according to Unified Soil Classification System (USCS). The soil was treated with bagasse ash (BA) in stepped concentrations of 0, 2, 4, 6 and 8 % by dry weight of soil. The effect of elapsed time after mixing on modified soil was studied with respect to specific gravity, particle size distribution, Atterberg limits, cation exchange capacity, compaction characteristics and shear strength parameters using three (3) compactive efforts namely, British Standard light (BSL), West African Standard (WAS) and British Standard heavy (BSH). Test results of the black cotton soil - bagasse ash mixtures considered showed a decrease in the specific gravity and percentage of fines of the natural soil with increase in bagasse ash treatment it also showed adecrease in the cation exchange capacity (CEC) and plastic limit values with increase in bagasse ash treatment till an optimum mix of4 % BAand consequently increased with further increase in BA treatment.The liquid limit values increased with increase in bagasse ash treatment till an optimum mix of 4 % BAand consequently decreasedwith further increase in BA treatment.The plasticity indices of all the test samples were within the 30 % value prescribed for sub grade by the Nigeria General Specifications. Test results showed that values of maximum dry density (MDD)and cohesion decreased while optimum moisture content (OMC) and angle of internal friction increased with an increase in elapsed time after mixing for BSL, WAS and BSH compactions. Statistical analysis was carried out on results obtained using analysis of variance (ANOVA) with the Microsoft Excel Analysis Tool Pak Software Package. An optimal 4 % BA treatment of black viii cotton soil when compacted with British Standard heavy energy is recommended for use as a sub-grade material of lightly trafficked roads and a maximum elapsed time of 2 hours after mixing should not be exceeded.