A numerical model for outburst including the effect of adsorbed gas on coal deformation and mechanical properties

The complexity of the interactions between coal and gas is an obstacle to creating a quantitative description of the outburst mechanism. In addition to free gas acting in the form of pore pressure for the coal mass, the adsorbed gas can also induce coal deformation and changes in the mechanical properties of coal. To analyse the effect of gas desorption on outburst initiation, a model is established for gas migration and mechanical processes during excavation. The permeability is significantly changed during excavation and is zonally redistributed. The coal mass near the coal wall undergoes stress release and cracks develop, which creates a sharp increase in permeability. Behind this area, the permeability of stress-concentrated coal mass drops quickly, causing a steeper pressure gradient and therefore creating greater potential for an outburst initiation. The mechanical behaviour of the coal mass is influenced not only by the free gas but also by the absorbed gas, which affects the coal deformation and mechanical properties in gas emission. To analyse the effect on outburst initiation, the results obtained consider the desorption-induced shrinkage and mechanical changes and are compared with those obtained with consideration of the effect of only free gas. This comparison demonstrates that desorption-induced shrinkage affects the stress state of the coal mass and that this influence becomes increasingly obvious as the desorption proceeds. The plastic area and the maximum plastic strain of the coal mass are also altered. The increase in strength due to the decrease of adsorbed gas decreases the plastic area and the maximum plastic strain of the coal mass; it also minimises the tendency towards coal collapse, thus enabling convergence with higher gas pressure. This result indicates that the effect of adsorbed gas on the mechanical behaviour of the coal mass is non-negligible. The effects of coal-seam depth, gas pressure and strength on outburst are analysed using this model.