Dynamic response mechanism of initial failure of coal mass induced by in-situ stress in an outburst inoculation process of the working face for coal mining

There is still a great challenge to reveal the micro-macro dynamic mechanical behavior of initial coal failure induced by mining stress field in working face as a necessary condition for the outburst occurrence. The multivariate stress loading paths for the damage and instability of mining coal were constructed based on the typical coal and gas outburst accident of mining face. PFC3D discrete software was used to carry out the visual simulation of damage and instability of coal under multiple working conditions and scales. The dynamic response law of damage and instability of mining coal was revealed, the dynamic evolution behavior of cracks in mining coal was clarified, and the prospect of outburst prevention technologies was put forward based on the initial failure process of coal induced by in-situ stress under mining. The results shown that the failure type and strength of the mined coal varied significantly with the loading and unloading rate of the principal stress. In the process of gradual unloading of stress, the macroscopic fracture surface of coal presented the form of single inclined plane or conjugate shear plane, and the failure strength of coal decreased with the increase of unloading rate. With the gradual loading in both directions of stress at the same rate, unidirectional sudden unloading or a certain residual stress maintaining state presented a macro fracture surface parallel to the direction of the intermediate principal stress. And the failure strength of coal decreased with the increase of the unloading degree or the decrease of the residual stress. Under different stress loaded, the tension-shear failure process of coal appeared successively with shear and tension cracks. In the process of instability caused by mining coal damage, the dynamic evolution of cracks presented intermittent, progressive and paroxysmal composite characteristics. The overall development process of cracks can be characterized as the initial appearance of new cracks (intermittent-sudden increase stage), crack expansion (gradual-slow increase stage), penetration and expansion (paroxysmal-slow increase stage), and the overall tearing process of coal (sudden increase stage). The mechanical strength of coal was regarded as one of the main controlling factors affecting the occurrence of outbursts. The difficulty of initial failure of coal induced by in-situ stress depended critically on the stress loading and unloading path. The coal was most prone to initial failure when the unidirectional principal stress was suddenly unloaded or the bidirectional principal stress was gradually unloaded. Based on this mechanical law, the “ideal mining mode of outburst prediction in mining working face” was proposed to represent the most dangerous state of coal in the working face.