Study on Mechanical Properties and Mesostructure of Subgrade Silty Sand under Freeze-Thaw Cycles

Abstract

Freeze–thaw cycles induce frost heave and thaw settlement in subgrades, degrade their service performance, and jeopardize traffic safety, making them primary concerns for transportation infrastructure in seasonally frozen regions. Silty sand is widely distributed in these areas. Investigating its mechanical behavior and mesostructure under repeated freeze–thaw cycling is therefore essential to improve subgrade durability. Therefore, silty sand was chosen as the study material, and triaxial compression tests together with scanning electron microscopy were performed on silty sand subjected to freeze–thaw cycles to elucidate the correlation between mechanical property evolution and mesostructural change under those cycles. The results indicated that the stress–strain behavior of silty sand shifted from strain-softening to strain-hardening as confining pressure, the number of freeze–thaw cycles, and moisture content increased. At freezing temperatures of -10 °C, -20 °C, and -30 °C, the maximum difference in peak strength was less than 6%, indicating only a minor influence of freezing temperature. Peak strength increased with confining pressure and decreased with both freeze–thaw cycles and moisture content. The elastic modulus followed a similar trend to peak strength. Freeze–thaw cycling degraded the silty sand structure, promoting particle breakage and pore development, and higher moisture content exacerbated this damage. The silty sand was dominated by micropores and small pores, and the total pore area grew with increasing freeze– thaw cycles and moisture content. Freeze–thaw cycles and greater moisture content also reduced the average shape parameter. The average fractal dimension of pores increased with additional freeze– thaw cycles, and under the combined influence of high moisture content and freeze–thaw cycling it exhibited an initial decrease followed by an increase. The grey correlation coefficients between macro-and meso-scale parameters lay between 0.6 and 0.8, indicating a strong linkage between the macroscopic mechanical behavior and the mesostructure of silty sand. These findings offer a theoretical basis for the design and maintenance of silty sand subgrades in seasonally frozen regions.