Mechanical behavior and multiscale damage evolution of filled fractured sandstone under freeze-thaw cycles

To address the deterioration of rock engineering stability in seasonally frozen regions, this study investigated the mechanical response and damage evolution of sandstone subjected to the coupled effects of freeze-thaw (F-T) cycles and filled fractures. Uniaxial compression tests were performed on intact specimens and specimens containing prefabricated, through-going filled fractures with dip angles of 30°, 45°, and 60° after 0, 20, 40, and 60 F-T cycles. A multi-technique monitoring framework integrating acoustic emission (AE), digital image correlation (DIC), and nuclear magnetic resonance (NMR) was used to characterize energy, deformation, and pore-structure evolution. The results showed that F-T cycles progressively degraded the physical and mechanical properties of sandstone, with strength decreasing approximately linearly over the tested cycle range. Fracture geometry exerted a pronounced influence on damage evolution. Larger fracture dip angles were associated with a stronger shift of the pore-size distribution toward larger pores, consistent with enhanced transmission and concentration of frost-heave stress. NMR T₂-spectrum analysis identified a five-stage pore-evolution sequence: "micropore reduction → small-pore increase → medium-pore growth → large-pore expansion → macropore initiation." These findings provide a quantitative basis for evaluating the stability of rock masses containing filled fractures and for designing reinforcement measures in cold regions.