Evolution law of coal and gas outburst hole shapes with varying underground stress conditions: Numerical analysis and on-scene evidence

A rock-coal-rock excavation model was established, and a coal and gas outburst (Abbreviated as CAGO) numerical simulation was carried out, which takes into account the dual-pore model and the desorption of gas. The plastic zone expansion was simulated under the different gas pressures and lateral pressure coefficients, and the variation in hole shape was characterized by the change in the plastic zone and compared with the actual engineering. The results indicate that: (1) There is a critical time in the CAGO process that divides the gas action into two stages: the first stage is the energy that causes the destruction of the coal body and throws out the crushed coal, the pressure difference between atmospheric and gas provides energy for this. The second stage is that the pressure difference will only provide energy for the throwing of broken coal. (2) Under the same loading conditions, gas pressure is set from 0.5 MPa to 0.9 MPa in 0.1 MPa increments, the area of the plastic zone around the exposed surface increases from 1.323 m2 to 1.3238 m2, and the peak velocity is reached faster. Meanwhile, the plastic zone increased from 1.319 m2 to 1.3238 m2 under the gas pressure of 0.9 MPa. This suggests that gas also has an effect on coal seam damage, but it is relatively weak compared to the geo-stress. (3) Pre-destruction of coal by geo-stress determines the size and shape of the hole, and the lateral pressure coefficient determines the pre-destruction situation. When the coefficient is small, the hole develops into an asymmetrical butterfly shape with a larger upper part and a smaller lower part, and when the coefficient is close to 1, the hole tends to develop into a semi-elliptical shape. Finally, the simulation result was compared with the experiment and the on-scene evidence. This study has guiding significance for the prevention and control of CAGOs at different depths.