The mechanism of damage in gas-bearing coal-rock combination bodies and gas seepage in coals

With the gradually exhausted shallow coal resources, many coal mines have been carried forward to the deep mining stage. Meanwhile, the outburst-rockburst coupling dynamic disaster is becoming more and more serious, which is becoming a major threat to national energy security. This kind of disaster is essentially induced by the coupling instability of damage in gas-bearing coal-rock combination body and gas seepage in coal. First, based on the study of force analysis and failure criterion of gas-bearing coal-rock combination body, the mechanical failure mechanism of gas-bearing coal-rock combination body is obtained and verified. It is found that the existence of additional stress generated at the interfaces of the coal body and rock body is the fundamental reason for the extremely complex damage and failure of combination body. In the actual mining process, due to the larger strength of roof than coal, the major stress state during the instability and failure of combination body is as follows: with the increase of axial stress, the strength of rock body at contact surface decreases but still greater than that of coal body far from the contact surface. Meanwhile, the strength of coal body at the contact surface increases but still less than that of rock body far from the contact surface. The theoretical failure mode of gas-bearing coal-rock combination body is mainly the failure of coal part, which only in some cases will lead to the failure of local rock part. Then, based on the permeability model of single coal in the full stress-strain process, the permeability model of coal under the influence of coal-rock combination body is established and verified. Finally, the gas-solid coupling instability mechanism of gas-bearing coal-rock combination body is discussed.