Characterization of Tensile Crack Propagation and Energy Evolution during the Failure of Coal–Rock Samples Containing Holes

The instability and fracturing of gas drainage boreholes are one of the main causes of low drainage efficiency. Based on the rock mass energy principle and the Barenblatt model, the energy evolution of the coal–rock mass around the hole, the conversion characteristics of the dissipated energy U^d, and the propagation pattern of the initial tensile cracks were investigated. The results show that based on the conversion process of the dissipated energy, the failure process of samples containing holes can be divided into an initial dissipation stage, a decelerated dissipation stage, a stable dissipation stage, and an accelerated dissipation stage. The dissipated energy is always greater than the elastic energy during the first half of loading, and it is mainly used for the continuous development and propagation of initial tensile cracks. Then, remote cracks and cracks to the left and right of the hole boundary are generated to inhibit the propagation of the tensile cracks. Later, when the energy storage limit is reached, the elastic strain energy around the hole is released, and the macroscopic failure cracks propagate and coalesce, which causes the stress environment to change and the tensile cracks to reopen and finally propagate. The tensile cracks in the upper and lower ends of the holes undergo an opening–closing–reopening process, and the presence of cohesion c(x) hinders the propagation of the tensile cracks that are formed by the generation and migration of fracture initiation zone, friction zone, and intact zone. The dissipated energy released was related to the different stages of the tensile crack propagation, which could be used for the structure monitoring and flaw predicting of the gas drainage borehole.