Coupled disaster-causing mechanisms of strata pressure behavior and abnormal gas emissions in underground coal extraction

The generalization and application of fully mechanized caving with super great mining height (SGMH) face a series of new issues, of which strata pressure behavior and abnormal gas emissions are the most prominent disasters and display a close relationship. The Tashan coalmine was chosen as the experiment base because of its SGMH and the overlying hard and thick main roof (HTMR). This coalmine had experienced many damaging hydraulic support and abnormal gas emission accidents induced by strata pressure behavior. The “Key Strata Theory” and the fracture failure process analysis model established by 3DEC were used to analyze the structural characteristics and fracture failure process of the HTMR. The geostress reduction and permeability enhancement redistribution of the adjacent coal measure strata were studied by the geostress and permeability distribution model established by FLAC 3D. The HTMR controlled the failure extent and the movement evolution of the entire overburden stratum. The HTMR acted as a large double-sided embedded rock beam during primary fracturing and a cantilever-articulated rock beam during periodic fracturing, and it generated a huge hanging space above the gob. The fully mechanized caving with SGMH caused large-scale disturbances and permeability enhancements in the adjacent rock and coal strata. Then, the substantial amounts of gas stored in the remaining coal, surrounding rock strata and adjacent coal seam rushed out and aggregated in the caved and fissured zones of the gob, forming a huge gas warehouse. The strata pressure behavior and abnormal gas emission disasters primarily occurred after the primary and periodic fracturing of the HTMR. These disasters are predominantly caused by the escaping of the abundant gas that existed in the gob during mine pressure behavior processes. When the ultimate broken span was reached, as the HTMR rotated and collapsed, the substantial amounts of gas that accumulated in the gob escaped into the working face, which easily caused abnormal gas emissions. Hydraulic presplitting and gas drainage technologies are proposed to solve these disasters.