Dynamic Resilient Modulus and Microstructure Degradation of Compacted Loess under Wetting–Drying–Freezing–Thawing Cycles

This study investigated the impact of temperature and humidity variation on loess subgrade performance by conducting dynamic triaxial tests on compacted loess subjected to wetting–drying (WD) and wetting–drying–freezing–thawing (WDFT) cycles. The results show that the dynamic resilience modulus decreased exponentially with cycle number and was linearly related to confining pressure. Additionally, increasing confining pressure and reducing the dynamic deviator stress helped slow down the modulus reduction. Scanning electron microscope (SEM) analysis showed that under WD cycles, the relationship between cycle number and pore area percentage was parabolic, while under WDFT cycles it was linear. As the number of cycles increased, mesopores in loess tended to shift to an irregular elliptical shape. The fractal dimension of the pore space was largely determined by mesopores. The study also found that WDFT cycles caused greater degradation of loess performance than WD cycles, as indicated by larger dominant particle inclination intervals. Overall, the coupling effects of historical temperature and humidity variations played a significant role in the degradation of loess subgrades, with WDFT cycles having a more intense impact than WD cycles.