Experimental investigation of mechanical properties, impact tendency, and brittleness characteristics of coal mass under different gas adsorption pressures

Understanding the mechanical properties of coal after gas and coal interaction can better guide the mining technology of gas-bearing coal seams and drainage technology of coalbed methane. In this study, triaxial compression tests of coal mass under different gas adsorption pressures are carried out to evaluate the mechanical properties, impact tendency, and brittleness of coal with different gas adsorption pressures. Results show that due to the adsorption of gas, the mechanical parameters, such as elastic modulus and strength of coal are significantly decreased. In addition to the effect of fluid pressure, the decrease of elastic energy absorbed by coal, the increase of ductile deformation and the change of molecular structure of coal particles caused by gas adsorption also significantly affect the mechanical properties of coal. The influence of gas adsorption pressure on energy input becomes gradually evident when the coal enters the stage of plastic deformation and yield failure. Then the deformation and stress bearing capacity of coal are affected, and the energy accumulation and dissipation process of coal are further affected. The gas adsorption pressure generally develops the energy absorption density and elastic energy density of three characteristic points (initial damage point, volume expansion point and peak stress point), but reduces the dissipation energy density of these points. Gas adsorption reduces the storage of deformation energy of coal, strengthens the energy dissipation, and develops the plastic failure characteristics of coal. It is not prone to appear sudden brittle failure in coal mass. Although high pressure reduces the risk of rock burst, it increases the risk of gas outburst. The chemical effects of pore/fracture fluid caused by the combined action of adsorbed and free gas lead to the further decline in coal brittleness, thus changing the deformation and failure mode of coal.