Effect of Atili Shell Ash on the Geotechnical Properties of Lateritic Soil

Abstract

The use of agricultural waste materials as soil stabilizers continues to attract attention for its economic and environmental benefits. This study investigated the effect of Atili shell ash (ASA) -- an agro-waste pozzolan derived from the shells of Canarium schweinfurthii (African elemi) -- on the index, gradation and compaction properties of a lateritic soil, with a view to its use as a sustainable, low-cost stabilizer. Lateritic soil obtained from Shika, Zaria, Kaduna State, Nigeria, was treated with 0, 4, 8, 12 and 16% ASA by dry weight of soil. The natural soil classified as A-7-6 (GI = 12) under AASHTO and CL under USCS, with 64.00% fines, a liquid limit of 47.00%, a plastic limit of 27.30%, a plasticity index of 19.70%, and a specific gravity of 2.61. Oxide analysis showed ASA to be a silica-rich pozzolan (57.2% SiO₂, with SiO₂ + Al₂O₃ + Fe₂O₃ exceeding 70%) and only marginally calcareous (1.78% CaO), indicating a slower, silica-driven pozzolanic mechanism rather than the rapid calcium-based cation exchange typical of lime- or cement-type stabilizers. Incorporation of ASA progressively coarsened the soil gradation, with the fines fraction falling from 3.15% to 1.14% under British Standard Light (BSL) compaction and from about 2.40% to 1.21% under West African Standard (WAS) compaction over the 0-16% ASA range. The specific gravity decreased slightly from 2.61 to 2.58 at 16% ASA. The liquid limit rose from 44.50% to a peak of 48.02% at 12% ASA before falling to 46.54% at 16% ASA, while the plastic limit fell from 25.00% to a minimum of 22.14% at 4% ASA. The plasticity index increased to a maximum of 25.54% at 8% ASA before reducing to 23.40% at 16% ASA. Maximum dry density decreased consistently with ASA content, from 1.60 to 1.55-1.57 Mg/m³ (BSL), 1.68 to 1.63-1.64 Mg/m³ (WAS), and 1.73 to 1.68-1.69 Mg/m³ (BSH), while optimum moisture content increased correspondingly, from 16.60% to 17.77% (BSL), 15.70% to 16.45% (WAS), and 14.64% to 15.41% (BSH). These results show that ASA modifies lateritic soil primarily through particle flocculation, aggregation and progressive pozzolanic bonding rather than densification, and indicate that 4-8% ASA offers the most favourable balance of improved gradation and consistency without excessive loss of compacted density, supporting its potential as an effective, sustainable and economical soil-stabilizing material for low-volume road and embankment applications.