Study on the impact force evolution law of coal and gas outburst under high ground stress

With the increase of mining depth and intensity, the dynamic disaster of coal and gas outburst was frequent. The disaster-causing mechanism of impact force has become the main direction of current research. In order to further reveal the impact force evolution law and failure mechanism of coal and gas outburst under high ground stress. The self-developed simulation roadway system in the whole process of coal and gas outburst was adopted, and the monitoring technology of impact force and acoustic emission were introduced. The gas pressure in coal seam was simulated by mixture pressure of 45% CO₂ and 55% N₂. The stress of overlying strata and surrounding rock was simulated by axial and confining stress, respectively. Taking the outburst coal seam of Pingding shan No.11 mine as the research object to conduct the simulation test of coal and gas outburst. The ground stress with buried depths of 600 m, 800 m, 1 000 m, 1 200 m, 1 400 m and 1 600 m were considered. The migration process of coal-gas two-phase flow, distribution of pulverized coal and evolution characteristics of impact force were analyzed. The influence between impact force and gas pressure, critical gas pressure, effective stress of test, acoustic emission signal were obtained, respectively. Transformation characteristics of gas internal energy to impact kinetic energy, i.e., gas pressure to impact force, was analyzed from the viewpoint of energy conversion of coal and gas outburst. The results shown that, (1) The force condition and damage degree in embryonic stage of outburst affected the propagation characteristics of impact force in the roadway after outburst. As the simulated buried depth increased, the impact force evolution became more complex, accompanied by obvious pulse characteristics, and the impact force value increased with the pulse characteristics. (2) The pulse characteristics was divided into the stage of high and low frequency. Coal-gas two-phase flow in high frequency stage had the characteristics of rapid speed, high strength and strong outburst hazard. The outburst hazard in low frequency stage gradually weakened with the development of outburst. (3) The two-phase flow energy of outburst was mainly from gas internal energy. Part of gas pressure was converted into impact force. The strength of impact force was mainly determined by gas pressure. Deep coal, with high ground stress, was more prone to coal and gas outburst than shallow one. (4) At the beginning of outburst, the peak point of acoustic emission ringing count preceded that of impact force, i.e., the acoustic emission signal detected outburst hazard earlier. But the impact force was more specific to coal fracture. A steep increase in acoustic emission ringing count was accompanied by pulse characteristics. However, the appearance of the pulse characteristic did not necessarily correspond to a steep increase in acoustic emission ringing count.