This paper is based on research undertaken at five outburst-prone coal mines of Huainan Mining Group. Onsite experiments were carried out at four cross-cut working faces and in floors roadway related to coal seams of different outburst potentials. Comprehensive study combined with theoretical analysis and onsite experimental investigation, the technical parameters of hydraulic flushing were optimized and determined according to analysis and evaluation of observations and measurements of stress release and permeability improvement and outburst potentials after hydraulic flushing. The results show that the critical water pressure and the effective water pressure for coal breakage for Huainan mining area are in the ranges of (10~15) and (12~20)f, respectively, where f is the strength coefficient. For the majority of drill holes, coal flushed out is between 0.5 and 1.0t/m with a maximum of 2.85 t/m. The effective radius of hydraulic flushing is in the range of 5.5 to 9.0m, and coal permeability has been increased by 254 to 524%. Hydraulic flushing has the effect of stress release and permeability improvement, and thus leading to large scale release of both elastic and gas potential energy entrapped in coal seams.
China is the most severely affected country in the world by coal and gas outbursts. The primary causes are: the geologic structure is complex, the mining depth is extensive, gas pressure (content) is high, the seam permeability is low, and, the drilling of coal bed is difficult. Specially high crustal stress participation, not only renders the coal and the gas outburst hazard more serious, but also makes it more difficult to prevent outburst. In 2008, of 12722 coal mines, 754 gas outburst mines were identified, accounting for 5.9%. In 885 state-owned key coal mines, there are 176 coal and gas prominent mines, or 19.9% [1]. Some of the deepest mines in Huainan coal area extend to 600m, some even reach to 800m ~ 1000m. Among the new mines, the majority of mines also belong to deep mines. The deeper seam depth causes increased stress and gas pressure, more gas content, and lower permeability. There are 11-pair coal and gas outburst mines out of 13-pair mines in Huainan coal area, 2 pairs for high gas mine. From mining, the coal and gas outburst accidents had happened 145 times, since 1998, a total of coal and gas accidents of 28, 41 lives were lost, coal and gas had been a major safety hazard all the time. Hydraulic flushing measures by Chongqing divisional mining Office with Nantong cooperation in 1965 at the Fort in Nantong fish tin ore pilot is successful, then in Umeda, Lianshao, Sixzhi, including Beipiao, Jiaozuo [3,4]. But because of the hydraulic flushing technical parameters and the evaluation system is not perfect, the coal rushed out and gas is relatively small, and uniformity is not good, in uncovering coal seam, the coal and gas outburst had happened 3 times. In addition, previous hydraulic flushing measures are only used for outburst elimination of uncovering coal seam.
