Experimental study on impact tendency characteristics of coal under unloading and gas action

The study on impact tendency of coal body containing gas under unloading damage plays a key role in exploring the formation mechanism of coal and gas combined rock burst in deep underground mine. In order to explore the influence of unloading action on impact tendency of coal body containing gas, this paper carried out cyclic loading and unloading of coal body containing gas with varying upper limit under different unloading amplitude conditions. The test results show that: gas has a deterioration effect on mechanical properties of coal. Under 9 MPa confining pressure, the average compressive strength of coal specimens under 1 MPa and 2 MPa methane action decreases by 11.2% and 18.7%, respectively. Under the unloading amplitude of 3 MPa and 6 MPa, the compressive strength σ_c decreases by 12.6% and 36.5%, and the residual elastic energy index C_EF decreases by 10.4% and 16.7%, respectively. For coal containing 1 MPa gas, σ_c decreases by 13.5% and 39.8%, and C_EF decreases by 15.9% and 43.6%, respectively. For coal containing 2 MPa gas, σ_c decreases by 16.7% and 47.2%, and C_EF decreases by 20.3% and 48.5%, respectively. With the increase of unloading range, the mechanical property of coal specimen decreases continuously, and the gas action will further aggravate the unloading deterioration degree, and the deterioration degree will gradually deepen with the increase of gas pressure. In addition, pressure relief can effectively reduce the impact tendency of coal body. Meanwhile, the deterioration degree of impact tendency index of coal body containing gas after unloading is higher than that of original coal specimen, and with the increase of gas pressure, the reduction degree of residual elastic energy index is higher.