ABSTRACT
An expansive black cotton soil taken from a borrow pit located along Numan - Ngurore road in the North Eastern part of Nigeria was treated with an agricultural waste ash produced from the burning of the husks of ripe fruits of the African Locust Bean tree that are abundantly available in Nigeria. The soil was classified as A-7-6 and CH in accordance with the AASHTO Soil Classification and Unified Soil Classification System (USCS), respectively. The study focused on the effect of up to 15% locust bean waste ash (LBWA) by weight of dry soil on the geotechnical properties of expansive black cotton soil. Test specimens were subjected to particle size analysis, compaction, unconfined compressive strength (UCS), California bearing ratio (CBR) and durability tests. The compactions were carried out at the energy levels of the British Standard Light (BSL),West African Standard (WAS) and British Standard heavy (BSH). The maximum dry density values of 1.59, 1.68 and 1.8 Mg/m3 obtained for the natural soil when compacted at the three energy levels of the BSL,WAS and BSH, respectively, increased to peak values 1.7, 1.72 and 1.8 Mg/m3, respectively, on treatment with 7.5% locust bean waste ash content. The optimum moisture content values of 18.53, 16.83 and 14% recorded for the natural soil when compacted at the three energy levels of the BSL. WAS and BSH, respectively, decreased to minimum values of 17.7, 16.6 and 13.7%, respectively, on treatment with 5 to 7.5% locust bean waste ash content. This trend is good in the tropics as less moisture is needed to achieve high compaction. The CBR values of 3, 6 and 8% for the natural soil compacted at the energy levels of the BSL, WAS and BSH slightly increased to peak values of 6, 8 and 12%, respectively when treated with 7.5% locust bean waste ash. The peak CBR value of 12% recorded for the treated soil using the BSH compactive effort does not meet the minimum requirement of 15% prescribed by the Nigerian General Specification (1997) for subgrade materials compacted at the BSL energy level. The unconfined compressive strength (UCS) values for BSL, WAS and BSH compactive efforts obtained for the natural soil at 7 days curing period were 114.65 299.96 and 401.50 kN/m2, respectively. For the same curing period the UCS values of the treated soil increased to 321, 325.15 and 384.5 kN/m2 for the three compaction energies, respectively, at 7.5% locust bean waste ash content. Considering the strength values recorded at an optimal 7.5% locust bean waste ash treatment of deficient black cotton soil, the LBWA cannot be used as a 'stand alone' stabilizer.