Analysis of Deformation, Permeability and Energy Evolution Characteristics of Coal Mass Around Borehole After Excavation

The mining of underground coal seam often induces the dynamic disasters of coal seam, such as rock burst and gas outburst. The determination of evolution characteristics of mechanical parameters and seepage parameters of coal mass after excavation is the basis for analyzing the formation mechanism of dynamic disasters. In this study, a coupled hydromechanical numerical approach is developed and their complex interactions are analyzed using the energy method to study the influence of outburst factors during underground excavation. A finite element model of porous media, which represents the gas flow field, the coal deformation field, the coal damage field and the cross-couplings between them, is established and the reliability of the model is ensured by numerical and experimental validation. Subsequently, during underground excavation, the coal deformation and gas flow properties around the borehole associated with different factors, including gas sorption, the matrix-fracture characteristics of coal and the seepage characteristics, are analyzed quantitatively using numerical simulations. Furthermore, an energy index is developed based on the energy method to analyze the comprehensive influence of in situ stress and gas pressure on coal mass. The results revealed that in the damage zone, coal permeability increases greatly by one to two orders of magnitude and the gas pressure remains low. Gas adsorption not only affects the stress and deformation of coal, but also affects the extent of the damage zone. High in situ stress, high gas pressure and low coal strength cause the failure range to increase.