Strain Energy Evolution and Burst Liability of Coal–Rock Combination Materials Under Cyclic Loading Condition
The coal–roof/floor combination system is a special geological structure consisting of both coal and roof/floor materials, which often leads to the occurrence of impact failure. The instability and failure of the coal seam are not caused by a single disturbance, but often occur under repeated loading and unloading processes under the action of mining stress. Thus, research on the impact failure, especially the energy evaluation of coal–rock combination materials under cyclic loading and unloading, is of great significance for understanding the rock burst mechanisms of deep coal mines. This paper focuses on the combination material formed by coal seam and roof and floor rock. Taking the #9 coal seam and roof and floor sandstone from the Zhangshuanglou Coal Mine as the test subjects, conventional triaxial compression and cyclic loading and unloading tests were conducted on sandstone, coal, and coal–rock combination material, respectively. Based on the strain energy evolution characteristics, the failure behavior of the coal–rock combination materials throughout the entire process of energy accumulation, dissipation, and release during cyclic loading and unloading are discussed. The research finds that the macroscopic failure behavior of the tested coal–rock combination specimens is dominated by the weaker coal component, and there are marked differences between the tested sandstone and coal components in their respective energy storage capacities, release characteristics, and dissipation behavior. On the basis of these measured differences and of CT-confirmed failure localization within the coal layer, it is inferred—as a mechanistic working hypothesis rather than a directly demonstrated result—that the main driving energy for the impact failure of the coal component may originate from the elastic strain energy stored in the roof/floor sandstone components, released preferentially toward the coal through their interfaces. For mining and excavation at high-in-situ-stress mining areas, the essence of the impact failure of surrounding rock is the non-coordination of energy storage and release between the roof and floor rock materials and the coal seam. The key to preventing impact failure is to eliminate the differences in energy storage and release between different rock materials in the coal seam.