Investigation of the partition failure process and energy evolution in coal-rock composite structure under free surface unloading

Unloading failure of the coal-rock (CR) system is the key factor leading to rock burst disaster. Therefore, it is very important to explore the failure mechanism of the CR system by laboratory test. Initially, CR composite samples underwent laboratory tests with unloading pressure at various rates (0.03–0.12 ​MPa/s). However, due to the limitations of the available monitoring equipment, the recorded deformation data were restricted to the coal mass, which may lead to inaccurate conclusions as potential rock deformation was not captured. Subsequently, coal and rock mass deformations were separately monitored by simulating corresponding unloading pressure tests using PFC2D numerical software. Simulation results suggested that the peak of the AE event during the critical stage before sample failure could serve as an indicator of imminent sample destabilization. Post-failure observation revealed a higher degree of damage in the coal mass (35.02%) compared to the rock mass (12.17%), indicating that coal mass destabilization triggers destabilization in CR composite samples. Moreover, faster unloading rates corresponded to deeper damage in the coal mass. Additionally, macroscopic tensile and tensile-shear cracks were observed in the rock mass, while macroscopic shear cracks were present in the coal mass, providing insights into the unloading confining failure mode of CR samples. Finally, the study established a relationship between unloading rate and bursting liability by introducing the elastic energy density difference index. The research results can provide a theoretical basis for the prevention and control of rock burst disasters.