Effect of Freeze–Thaw Cycles (FTCs) on the Mechanical Behavior of Highway Clay Subgrade Soils Stabilized with Lime and Polypropylene Fibers

High-plasticity soils pose significant problems in road infrastructure, adversely affecting structural safety due to their unfavorable engineering properties. Lime stabilization is one of the most widely used methods for improving such soils. However, lime addition may cause brittleness of these soils, resulting in a sudden loss of strength. To overcome this weakness, this study investigated using polypropylene fibers in combination with lime stabilization. Accordingly, the plasticity, compressibility, and strength properties of soil mixtures containing 3%, 6%, 9%, and 12% lime, along with mixtures prepared with a constant 0.5% polypropylene fiber content, were systematically evaluated in a laboratory environment. Additionally, the influence of freeze–thaw cycles (FTCs), which induce long-term strength degradation in highway subgrades, on these mixtures was investigated. The results indicated that lime addition reduced the plasticity index by up to 38% without causing a significant change in dry unit weight. It was also determined that FTCs could lead to a strength loss of up to 84% in natural soil, and this loss was substantially reduced by adding lime and fibers. These findings highlight that the lime-fiber combination represents an effective and sustainable method for increasing the performance of high-plasticity soils.