Laboratory Study on the Geotechnical Behaviour of Expansive Soil Treated with Coconut Husk Ash and Silica Fume

Extensive subgrades present severe structural challenges globally due to their inherent tendency to swell upon moisture intake and undergo severe volumetric contraction during dry seasons, causing structural distortion and excessive rehabilitation expenses. This experimental investigation appraises the stabilization performance achieved by combining an agrowaste residue, Coconut Husk Ash (CHA), alongside an industrial byproduct, Silica Fume (SF), on the index and strength parameters of problematic clayey soils. The testing scheme incorporated a fixed dosage of 6% CHA mixed with varying proportions of SF (0%, 2%, 4%, 6%, and 8% relative to dry soil weight), evaluating the matrix via Atterberg limits, Standard Proctor compaction, and Unconfined Compressive Strength (UCS) testing. Results demonstrate that co-supplementing CHA and SF significantly suppresses liquid limits and plasticity indices while enhancing maximum dry densities and compressive strengths. Peak geotechnical improvement—manifested by maximum dry unit weight and structural strength—was secured at a composite proportion consisting of 88% expansive soil, 6% CHA, and 6% SF, confirming its high viability for subgrade enhancement.