Freeze-thaw damage mechanism and deterioration model of unsaturated sandstone in open-pit coal mines in cold regions

In cold regions,rock masses in open-pit coal mines remain unsaturated for long periods,and their freeze-thaw (F-T) damage evolution differs significantly from that of saturated rocks. To reveal the deterioration mechanism of unsaturated sandstone under F-T cycles,sandstone from the Shitoumei No. 1 Open-Pit Coal Mine was taken as the research object. Physical and mechanical tests and acoustic emission (AE) monitoring were conducted under four saturation levels (0,30%,60%,and 100%) and 0,15,and 30 F-T cycles. The saturated water absorption,microcrack density,P-wave velocity,uniaxial compressive strength,and crack evolution characteristics were analyzed to determine the critical saturation for F -T damage. Based on the “micro- pillar-microcrack” mesostructure, a deterioration model coupling saturation and F-T cycles was established. The results show that the F-T deterioration of unsaturated sandstone has an obvious saturation threshold effect. When the saturation is below the critical value, F-T cycles cause almost no damage; once the critical value is exceeded, damage increases nonlinearly with increasing water content. The strength loss rate of 100% saturated specimens reaches 33. 37% after 30 F-T cycles. F-T cycles promote continuous microcrack propagation, and water absorption and microcrack density increase synchronously. The P-wave velocity, elastic modulus, and uniaxial compressive strength decrease rapidly at first and then slowly,and the deterioration amplification effect is significant at high saturation. Dry and low-saturation sandstone is dominated by tensile cracks; at medium and high saturations, cracking changes from tension-dominated to synergistic tensile-shear development,and fracture surface complexity increases significantly. Under low saturation and few F-T cycles,the theoretical values of the established damage model agree well with the experimental data;under high saturation and multiple F-T cycles, the prediction deviation increases somewhat, but overall the model can still reflect the evolution law of F-T damage in unsaturated sandstone well.