Dynamic resilient modulus deterioration of Pisha sandstone filler under freeze-thaw cycles: Experimental and microstructural insights

The long-term mechanical stability of Pisha sandstone (PS) under freeze-thaw (FT) conditions and heavy loads remains insufficiently understood, significantly limiting its application in road engineering. To address this issue, a series of cyclic loading and microscopic tests were conducted on typical Pisha sandstone filler (PSF) from Ordos, Inner Mongolia, considering multiple influencing factors. These tests aimed to elucidate the degradation mechanism of its dynamic resilient modulus (M_R) under FT cycles in seasonally frozen regions. Additionally, a M_R prediction model was developed to account for the combined effects of heavy traffic loading and FT conditions. Furthermore, the microscopic damage mechanism of M_R deterioration was analyzed to provide a deeper understanding of the material's structural evolution under FT cycles. The results indicate that the M_R of PS increases significantly with rising confining pressure (σ₃) and cyclic dynamic stress (σ_d) and exhibits a monotonic increase with load frequency (f), particularly under high σ₃ and large σ_d. However, M_R gradually deteriorates with successive FT cycles, with the most severe damage occurring during the first cycle. The degradation process can be classified into three stages: rapid attenuation, slow attenuation, and stabilization. Additionally, σ_d and f are positively correlated with the damage factor (D_FT) and negatively correlated with the damage rate (V_FT). Microstructural analysis reveals a 47.73 % increase in porosity after the first FT cycle, leading to the fracture of cemented particles and the degradation of soil particle structure, thereby reducing M_R. After seven FT cycles, porosity increases by only 17.31 %, indicating that M_R stabilizes. This microstructural evolution closely aligns with the macroscopic degradation trend of M_R. Furthermore, based on the analysis of the influence of various factors, a predictive model for the M_R of PS under heavy loads and FT cycles was established, demonstrating high prediction accuracy. Finally, an internal mechanism explaining the strength degradation of PS under FT cycles was proposed, based on the evolution of its pore structure characteristics. The findings contribute to the sustainable design and optimization of PS subgrade in seasonally frozen regions, promoting its effective utilization as a reliable subgrade material.