Study on deformation and energy of coal and gas outburst coal seam mined without coal pillar

The failure characteristics, crack propagation of gas-containing coal seam and variation of effective stress and energy of coal seam before and after working face excavation were revealed in this paper. The results demonstrated that: (1) The damage along any direction with more cracks, formation of an X-shaped failure surface, and formation of a monoclinic failure surface were observed after loading in gas-containing coal samples with confining pressures of 0 MPa, 1.5 MPa and 2 MPa, respectively. (2) As the confining pressure increased, the peak strength, peak volume strain, difference between volume strain and crack volume strain, crack axial strain rate and time required to reach peak stress of gas-containing coal samples successively increased. After the gas-containing coal sample reached the peak stress, crack radial strain rate was greater than crack volume strain rate, while the average crack volume strain rate and the average crack radial strain rate successively increased. (3) The numerical simulation results reveled that energy and effective stress in coal seam decreased, when roof of transportation roadway was cut. The energy and effective stress significantly reduced when the inclined length of advanced working face was in the range of 50–110 m. (4) The field monitoring showed that there was no gas overrun in roadway after the implementation of the roof-cutting and pressure-relief technology without coal pillar. The average gas extraction concentration in the gas extraction pipeline increased by 23.3% in advance working face of transportation roadway during the period of roadway retention, which improved gas extraction concentration.