Research on creep mechanical properties and damage mechanisms of water-saturated coal-bearing sandstone under freeze-thaw cycles

To reveal the long-term instability mechanism of rock slopes in open-pit coal mines in cold regions, this study takes water-saturated coal-bearing sandstone from the Antaibao Open-pit Coal Mine as the research object. Systematic uniaxial compression creep tests were conducted under different freezing temperatures (−5°C, −10°C, −15°C, −20°C) and freeze–thaw cycles (5, 10, 15, 20 times). Combined with longitudinal wave velocity measurements, porosity tests, and scanning electron microscopy (SEM) meso -analysis, the coupled damage evolution law under freeze–thaw-creep conditions was elucidated. The results show that: With decreasing freezing temperature and increasing freeze–thaw cycles, the reduction rate of longitudinal wave velocity and the increment of porosity significantly increase (e.g., wave velocity reduction rate reached 29.58% at −20°C/20 cycles). The number of freeze–thaw cycles exhibits higher sensitivity to damage than freezing temperature. The creep failure stress significantly attenuates (decreased by 21.8% at −20°C compared to −5°C, and by 53.0% after 20 cycles compared to 5 cycles). The ratio of long-term strength to peak strength remained stable at approximately 80%. Microscopically, a sequential damage mechanism is identified, initiating with ice-expansion cracking, progressing to thermal fatigue accumulation, followed by pore networking, and culminating in particle spalling. This process leads to a transition in the macroscopic failure mode from pure shear to a tensile-shear composite. This study provides a theoretical basis for stability assessment and prevention of coal mine slopes in cold regions.