Study on mechanical properties of composite rock mass slopes in cold region open-pit coal mines

To address the issue of mechanical property degradation of composite rock mass slopes in open-pit coal mines in cold regions under the action of dry-wet and freeze-thaw cycles, orthogonal test design is adopted to optimize the proportioning scheme, and three types of similar rock mass samples are prepared to explore the mechanical response laws and failure mechanisms of hard rock （sandstone）, soft rock（mudstone） and composite rock mass. Through dry-wet and freeze-thaw cycle tests, uniaxial compression tests, combined with high-speed photography observation, the apparent degradation characteristics, mass loss rate, uniaxial compressive strength and failure mode of the samples under different cycle times are systematically analyzed. The test results show that with the increase of cycle times, the three groups of samples exhibit different apparent characteristics, among which the soft rock group first develops obvious cracks due to its water absorption and expansion properties. In terms of mass loss rate, the soft rock group has the largest mass loss rate after the cycles, reaching 5.15%, followed by the composite rock mass group（3.04%） and the hard rock group（2.60%）. In terms of strength changes, the hard rock group has the largest cumulative decrease in compressive strength, with a strength attenuation of 42.36%, and its degradation process is divided into two stages: microcrack propagation and structural penetration failure. The composite rock mass group, due to the synergistic effect of interface debonding and soft rock damage, has a strength attenuation of 30.87%, and its failure mode evolves from layered peeling to plastic flow. The soft rock group undergoes a linear decrease in strength by 29.77% because the water absorption and expansion of bentonite lead to the loosening of cementation. During the compressive failure process, the samples of the hard rock group are mainly characterized by axial splitting in the early stage of the cycles, and their failure mode changes from single crack to multi-branch crack failure in the middle and late stages. With the increase of cycle times, the samples of the soft rock group change from uniform compressive failure in the early stage to failure with debris falling off in the middle and late stages. The failure of the composite rock mass group combines the failure characteristics of both hard and soft rock samples, showing plastic failure in the soft rock part and local fragmentation in the hard rock skeleton. This study reveals the dry-wet and freeze-thaw degradation mechanism of composite rock mass slopes in cold regions, providing experimental support for slope stability evaluation and protection design.