Critical Role of Borehole Orientation in Governing the Mechanical Degradation and Microstructural Evolution of Red Sandstone in Cold Regions

The stability of tunnel-surrounding rock in high-altitude and cold regions is severely affected by extreme low temperatures and frequent freeze–thaw cycles. These conditions facilitate the expansion of cracks, reduce rock strength, and accelerate the progressive degradation of its microstructure, ultimately intensifying the failure process. Triaxial compression tests and nuclear magnetic resonance (NMR) analysis are utilized in this study to comprehensively assess how hole orientation influences the physical, mechanical, and microscopic characteristics of red sandstone under different cycles. The results demonstrate that temperature fluctuations significantly accelerate the physical deterioration of red sandstone, as evidenced by a progressive reduction in mass and wave velocity, coupled with a gradual increase in volume. The mechanical properties of red sandstone, such as elastic modulus, cohesion, internal friction angle, and compressive strength, are significantly compromised by freeze–thaw cycles. The orientation of boreholes is a critical factor influencing the degree of deterioration, with specimens containing axial boreholes exhibiting substantially greater strength reductions compared to those with longitudinal boreholes or complete samples. The interplay between temperature variations and confining pressure introduces complexity to the failure mechanisms of the rock. NMR analysis shows that freeze–thaw processes enlarge micropores and mesopores, enhance pore connectivity, and accelerate microcrack growth, ultimately compromising the rock’s structural integrity. Energy evolution analysis highlighted that low confining pressures and the presence of boreholes increase energy dissipation and reduce total energy storage, highlighting the synergistic effect of borehole orientation and freeze–thaw cycles in driving energy loss and structural degradation.