Geotechnical Behavior and Microstructural Analysis of Expansive Subgrade Soils Stabilized With Coffee Husk Ash

Abstract

This study examines the enhancement of engineering properties in expansive subgrade soils through stabilization with coffee husk ash (CHA), an agricultural byproduct obtained from coffee husk combustion. CHA was incorporated at 5%, 10%, 15%, 20%, and 25% by dry soil weight, and the soil–CHA mixtures were cured for 7, 14, and 28 days. Mechanical and index properties of the virgin soil, including compaction characteristics, plasticity limits, soil compressive strength (unconfined compressive strength [UCS]), and soil penetration resistance index (California bearing ratio [CBR]), were first evaluated. The effect of CHA dosage on the heavy compaction test, UCS, and CBR was then assessed, along with the influence of curing time on strength and bearing capacity. Replacing 20% soil with CHA reduced plasticity while enhancing mechanical performance. Performance declined beyond this optimum. UCS increased markedly with curing, reaching 327.5 kPa (2.72‐fold improvement) after 28 days at the optimal dosage. Two‐way ANOVA revealed that both curing time and CHA content, along with their interaction, had highly significant effects on UCS (p < 0.001), explaining nearly all of its variability (R² = 0.998). The soaked CBR at 20% CHA satisfied IRC:37 (2018) requirements for high‐volume roads. Microstructural analyses (X‐ray diffraction [XRD], scanning electron microscopy [SEM], energy dispersive X‐ray spectroscopy [EDS], and Fourier‐transform infrared [FTIR]) verified cementitious compound formation, validating pozzolanic stabilization. Pavement design using IITPAVE software, and cost analysis indicated a 21.1% lower construction cost per kilometer compared to untreated soil. The novelty of this research lies in ANOVA validation of UCS, integration of microstructural and mechanical results, and extension to pavement design with cost–benefit analysis. CHA offers an eco‐friendly and cost‐effective solution to mitigate swell–shrink behavior and improve load‐bearing capacity in sustainable road construction.