Experimental evaluation of fly ash and lime stabilization on the geotechnical characteristics of expansive clay for road subbase applications

Expansive clay soils pose significant challenges in road construction due to their high plasticity, swelling potential, and low bearing capacity. This study investigates the combined use of fly ash, lime, and waste tyre granules as sustainable stabilizing agents, addressing a gap in existing research. Laboratory tests were conducted to assess the geotechnical properties of treated and untreated soils. The results revealed that the natural soil was classified as A-6 according to AASHTO, with 86.8% fines, indicating poor suitability for subgrade applications. Stabilization with 5% combined lime and fly ash (by dry soil weight) reduced the plasticity index from 14.37% to 2.01%, representing an 86% reduction in swelling potential. It also improved the maximum dry density to 1.85 kg/m 3 at an optimum moisture content of 8.62%. The CBR value increased significantly from 2 to 6% (untreated soil) to 16%, indicating an approximate 167–700% improvement in load-bearing capacity. In contrast, a higher additive content of 10% led to reduced performance, suggesting over-stabilization effects due to excessive binder content. The addition of waste tyre granules (2% to 6% by weight) increased the optimum moisture content and reduced the dry density, indicating their influence on compaction behavior, while also enhancing the ductility and energy absorption capacity of the stabilized soil matrix. Overall, this study demonstrates that a mixture of 5% lime and fly ash, combined with controlled tyre content, significantly improves subgrade performance. Furthermore, the integration of industrial by-products and waste tyre materials shows strong potential for sustainable construction through waste valorization, reduced environmental impact, and alignment with circular economy principles. This research supports sustainable construction practices while addressing a critical gap in combined stabilization methods.