Experimental investigation on the dynamic characteristics of expansive soil stabilized with polyvinyl alcohol–amended red mud

Abstract

Black cotton soil (BCS) exhibits severe swell–shrink behaviour and poor resistance to cyclic loading, which limits their suitability for pavement subgrade and foundation applications. This study investigates the static and dynamic characteristics of expansive soil stabilized using red mud (RM) amended with polyvinyl alcohol (PVA). The novelty of this work lies in evaluating the combined RM-PVA stabilization system under both static and dynamic triaxial loading, whereas most previous studies have examined RM or polymer additives separately. RM was added at 0–40% and PVA at 0–4% by dry mass of soil. Consolidated undrained triaxial tests were conducted under confining pressures of 100, 200, and 300 kPa to determine the optimum mix, followed by dynamic triaxial tests at loading frequencies of 1, 2, and 3 Hz. The optimum mixture was identified as 30% RM + 3% PVA based on peak static shear strength. At this dosage, cohesion increased from 62.5 kPa to 82.4 kPa, while the internal friction angle increased from 17.10° to 19.35°, indicating improved interparticle bonding and shear resistance. Dynamic test results showed pronounced nonlinear stress–strain behaviour, with stiffness increasing under higher confinement and loading frequency. The initial dynamic elastic modulus increased from 44.52 MPa at 100 kPa and 1 Hz to 135.42 MPa at 300 kPa and 3 Hz. The fitted dynamic modulus–strain relationship showed strong agreement with the experimental data, with R 2 = 0.97. The damping ratio ranged approximately from 0.05 to 0.22, increasing with dynamic strain but decreasing with higher confining pressure and loading frequency. XRD and FE-SEM observations qualitatively confirmed mineralogical modification, pore filling, and formation of a denser bonded soil matrix. Overall, the results demonstrate that 30% RM + 3% PVA is a promising sustainable stabilization combination for improving the strength, stiffness, and dynamic performance of expansive soil for pavement subgrade applications.