Experimental study of evolution law of mechanical properties of coal containing gas under unloading confining pressure

Taking axial stress and radial strain under fixed axial strain as study parameters, the evolution law of mechanical properties of coal containing gas under unloading confining pressure is experimentally studied; and the relationship between test conditions and study parameters is fit. The results show: the axial stress of coal containing gas decreased with two stages if confining pressure is gradually unloaded under fixed axial strain; the relationship between decrease rate of axial stress and the difference between confining pressure and gas pressure is positive. The total decrease of axial stress is larger when the confining pressure is unloaded by unit value as gas pressure increasing; and sensitivity of the influence is stronger, the relationship between axial stress and confining pressure unloaded amounts can be show by quadratic function. The radial strain of coal containing gas shows gradually increases tendency when confining pressure is gradually unloaded under fixed axial strain; the total increasing of radial strain is larger when the confining pressure is unloaded by unit value as gas pressure increasing; the variable quantity of radial strain is influenced by sum of the confining pressure unloaded and gas pressure; the relation between radial strain and confining pressure unloaded can be show by modified logarithm function.