Microstructure and energy evolution of cyan sandstone with prefabricated fractures at different angles during failure under freeze–thaw cycles

During tunnel construction in seasonally frozen soil areas, freeze–thaw cycles significantly affect the mechanical properties and structural stability of surrounding rock, particularly under fracture evolution and freeze–thaw damage. This study systematically analyzes intact cyan sandstone (IS) and cyan sandstone samples with prefabricated fractures at different angles (PS) to examine changes in physical and mechanical properties and the microstructural evolution of cyan sandstone under freeze–thaw cycles. The results indicated a substantial reduction in the compressive strength and elastic modulus of cyan sandstone, especially in samples with prefabricated fractures, where the reduction in strength is more pronounced. As the angle between the prefabricated fractures and the horizontal direction decreases, the mechanical strength of cyan sandstone declines more sharply, highlighting the vital role of fracture geometry in determining the rock’s strength. Nuclear magnetic resonance (NMR) analysis revealed that increased porosity and fracture expansion induced by freeze–thaw cycles are primary factors contributing to the degradation of cyan sandstone’s mechanical properties. Freeze-thaw cycles accelerate the expansion and propagation of microfractures within the rock, raise its porosity, and weaken its compressive strength. In addition, energy evolution analysis showed that the mechanical response of cyan sandstone under varying confining pressures exhibits distinct stage characteristics. As confining pressure increases, the energy required to dissipate during rock fracture rises, demonstrating that confining pressure significantly regulates the rock failure mechanism. This study enhances the understanding of mechanical property changes in cyan sandstone under freeze–thaw cycles. It highlights the critical role of factors such as prefabricated fractures and confining pressure in freeze–thaw damage, providing a theoretical basis and experimental support for fracture control and surrounding rock stability assessment in tunnel construction in seasonally frozen areas.