ABSTRACT
The preliminary investigation of the black cotton soil collected from New Marte, Borno State, shows that it belongs to A-7-6 (13) in AASHTO and CH in the Unified Soil Classification System (USCS), respectively. Soil under these groups are usually very poor for engineering use which was why it was treated with an ordinary Portland cement - locust beans waste ash (LBWA) blend in stepped concentration of 0, 2, 4, 6 and 8% by dry weight of soil. Compaction was carried out using three energy levels namely: the British Standard Light, West African Standard and the British Standard Heavy. The liquid limit increased from 63% for the natural soil to 77% at 6% OPC/6% LBWA while the plastic limit decreased from 26.6% for the natural soil to 21.4% at 6% OPC/4% LBWA treatment. Unconfined compressive strength (UCS) values of 151, 324 and 637kN/m2 recorded for the natural soil were increased to 837, 1220 and 1420kN/m2 at 6% OPC/6% LBWA for specimens compaction using British Standard Light (BSL), West African Standard (WAS) and British Standard Heavy (BSH) energies respectively. The unconfined compressive strength (UCS) values for natural soil compacted with BSL, WAS and BSH energies at 7 days curing period are 179, 381 and 750kN/m2 respectively and increased to 986, 1436 and 1650kN/m2 at 6% OPC/6% LBWA treatment. The treated specimens did not attain the 7 days UCS value of 1710kN/m2 recommended for an adequate cement stabilized base, but the value is, however, acceptable for a sub-base material. The CBR values of the natural soil are 5, 6 and 9% for BSL, WAS and BSH energies. For the recommended value of 80%, the unsoaked CBR values of 73% at 6% OPC/6% LBWA for WAS compaction and 83% obtained for BSH compactive efforts also at 6% OPC/6% LBWA could be acceptable. The soaked CBR values obtained was 66% for both WAS and BSH compactive energies both at 6% OPC/6% LBWA; a value which meets the recommendation of 30% sub-base when compacted at optimum moisture and at 100% West African Standard compactive effort. The 6% OPC/6% LBWA blend at BSH compaction attained 71% resistance to loss in strength (when subjected to 7 days curing then 7 days soaking) as against 80% recommended for 7 days curing and 4 days soaking.