Experimental Investigation of Sandstone Damage Evolution and Failure Mechanisms Under Freeze–Thaw Cycles

Elucidating the evolution of sandstone pore structure and the mechanisms of macroscopic damage and failure under freeze–thaw cycling is an important scientific issue for the safe construction and operation of rock engineering projects in cold regions. In this study, sandstone collected from a slope along the Qinghai–Tibet Railway in a seasonally frozen region of western China was investigated. Micro-, meso-, and macroscale experiments were conducted on sandstone specimens subjected to different numbers of freeze–thaw cycles. Scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), computed tomography (CT), and digital image correlation (DIC) were employed to investigate changes in the physical and mechanical properties, pore structure evolution, and macroscopic fracture characteristics of sandstone. Freeze–thaw cycling continuously weakened intergranular cementation and promoted the propagation and interconnection of microcracks. Meanwhile, the pore structure gradually shifted from micropores (r < 0.1 μm) toward mesopores (0.1 μm ≤ r < 10 μm) and macropores (r ≥ 10 μm). With increasing numbers of freeze–thaw cycles, the total number of pores identified by CT decreased from 256,237 to 194,910, whereas the number of connected pores increased from 222 to 1308. The connected porosity increased from 2.25% to 10.40%, the gas-measured porosity increased from 10.63% to 12.45%, and the permeability increased from 0.28 mD to 0.96 mD. In addition, the dry mass of the monitored specimen decreased from 464.53 g to 461.52 g, representing a reduction of 0.65%. The P-wave velocity decreased from 2.632 km/s to 2.309 km/s, corresponding to a reduction of 12.30%. During uniaxial compression, strain localization became more pronounced with increasing freeze–thaw damage. The high-strain zones observed by DIC, defined as regions of localized strain concentration, gradually expanded from localized concentrations to multiple regions, ultimately leading to specimen failure. The results indicate that freeze–thaw damage is essentially a progressive process involving the activation of microdefects, pore expansion and coalescence, crack interconnection, and reconstruction of the interconnected pore network, ultimately resulting in the continuous deterioration of the structural integrity of sandstone.