<p>Fly ash and coir fiber are solid waste materials commonly used to mitigate the swelling and shrinkage of expansive soil subgrades. While prior research has primarily explored their individual effects, the combined influence of these materials remains underexplored. Through compaction and unconfined compressive strength tests, this study investigated the coupled effects of fly ash and coir fiber content, along with curing time, on the mechanical properties of expansive soil, addressing this critical research gap. The results demonstrate that the addition of fly ash and coir fibers significantly enhances the unconfined compressive strength of expansive soil, with peak strength achieved at fly ash and coir fiber contents of 20 and 0.5%, respectively. Furthermore, the unconfined compressive strength increases notably with longer curing times. Building on the findings from element tests, physical model tests (static and dynamic loading tests) were further conducted to study the mechanical response of improved expansive soil subgrades under geosynthetic reinforcement conditions, such as geogrids and geocells. The optimal embedment depth of the geosynthetics is approximately 0.33 times the width of the foundation. Compared to unreinforced subgrades, the settlement of reinforced subgrades is significantly reduced. Including geosynthetics demonstrates notable diffusion and attenuation effects on vertical stress and peak acceleration, respectively. Furthermore, the stress distribution angle of geocell-reinforced subgrades is higher than that of geogrid-reinforced subgrades, indicating a more pronounced effect on subgrade reinforcement.</p>