Abstract: The extraction of gas at the bottom of coal seams with low-angle high-pressure gas outbursts is difficult, which reduces the extraction efficiency. To address this issue, a case study is conducted on the Taoziya tunnel on the Zheng′an-Xishui expressway in Guizhou, China. Based on the variation in the spatial position relationship between the excavation section and coal seam during tunnel excavation, a step-by-step and zoned coal seam gas extraction plan is proposed. The similarity between heat conduction theory and gas flow theory is leveraged by the thermodynamic finite difference calculation software to simulate and analyze gas outburst prevention and extraction as well as the effect of constructing a tunnel in the small-angle coal seam. Results are as follows: (1) The step-by-step and zoned coal seam gas extraction scheme prevents the drilling of super-long discharge boreholes in the bottom coal seam, thereby reducing the total drilling length by 31% compared to the overall one-time extraction scheme; it is also proving more efficient and reliable. (2) Gas extraction simulation results show that when the extraction boreholes are evenly distributed on the coal seam section, the middle drilling and extraction areas reach the gas outburst elimination target earlier and the edge area lags behind. Therefore, adopting a dense edge and sparse middle extraction and extraction borehole layout method is reasonable. (3) A comparison of simulation results of drainage boreholes drilled along the coal seam dip and those drilled obliquely through the coal seam shows that the former improves drainage efficiency by ~1/3, with more uniform gas pressure dissipation. (4) The gas extraction period can be estimated via simulation based on the extraction plan, which is roughly between the time when the average gas pressure in the extraction area reduces to the critical pressure of the outburst and the time when the maximum gas pressure decreases to the critical pressure of the outburst.

Key words: gas outburst tunnel, small-angle coal seam, coal mining and prevention, gas extraction, numerical simulation