ABSTRACT
Black cotton soil (BCS) obtained from Baure; Yamaltu Deba Local Government Area was treated with up to 12 % palm kernel shell ash (PKSA) by weight of dry soil to determine the suitability of the treated soil for use in road construction. Index tests were carried out to classify the natural soil, while the moisture-density relationships were determined by compaction tests on the natural and treated soils using three energy levels viz, British Standard light (BSL), West African Standard (WAS) and British Standard heavy (BSH). Black Cotton Soil used in the study was classified as A-7-5 (20) using the American Association of State Highway and Transportation Officials (AASHTO) and CH group in the Unified Soil Classification System (USCS). Tests results show that specific gravity of the soil increased from 2.29 for the natural to 2.34 at 12 % treatment. Liquid limit decreased from 76.2 % for natural to 73.4 % at 10 % PKSA content. Plastic limit increased from 40 % for the natural soil to 47.1 % at 12 % PKSA content treatment. Maximum dry density (MDD) values decreased from1.44 Mg/m3, 1.5Mg/m3 and 1.65 Mg/m3 for the natural soil to 1.38 Mg/m3 at 10 %, 1.45 Mg/m3 at 10 % and 1.56 Mg/m3 at 6 % PKSA content for BSL, WAS and BSH compaction energy levels, respectively. On the other hand, optimum moisture content (OMC) value decreased from 28.5 % and 22.4 % for the natural soil to 22.4 % and 21.0 % at 12 % PKSA content for BSL and WAS energies, respectively, while the value for BSH energy increased from 18.2 % to 19.0 % at 8 % PKSA content. Peak 7 days unconfined compressive strength (UCS) values of 351.59, 362.64 and 539.17 kN/m2 at 8 % PKSA content were recorded for BSL, WAS and BSH compactive efforts, respectively. The peak unsoaked California bearing ratio (CBR) values of 10, 15 and 27 % as well as soaked CBR values of 3, 3 and 7 % at 10 % PKSA were obtained for BSL, WAS and BSH compactions, respectively. Peak durability (resistance to loss in strength) values of 17.6 % ( i.e., 82.4 % loss in strength), 14.1 % (i.e., 85.9 % loss in strength) and 11.9 % (i.e., 88.1 % loss in strength) were recorded for specimens compacted with BSL, WAS and BSH energies, respectively. Results obtained show that 8 % PKSA treatment of BCS improved the strength characteristics of BCS, but it can only be used as a subgrade material in road construction. The benefit of the technique includes the elimination of the negative impact of PKSA on the environment