Abstract: The Zhaotong gas tunnel faces various challenges, such as high ground stress, low permeability of coal seams, high gas content, and soft coal layers. Herein, three tunnel gas drainage schemes—two-phase gas drainage in the parallel guide tunnel and main tunnel, single-phase gas drainage in the parallel guide tunnel and main tunnel, and one-time gas drainage using high- and low-level drilling fields—are theoretially analyzed and compared. The optimal gas drainage scheme using high- and low-level drilling fields is selected, and a gas drilling hole layout for the Zhaotong tunnel′s M7－M9 coal seams is implemented. Based on this, high-pressure water jet fracturing technology is employed to enhance coal seam permeability. The field study shows that: (1) Compared to other two schemes, the drilling workload of the high and low-level drilling field scheme is reduced by approximately 44% and 19%, respectively, and the construction period is shortened by 0.5 and 3 months, respectively. (2) The monthly cumulative pure gas drainage volume is approximately 200 000 m³, the gas extraction efficiencies in the high- and low-level drilling fields are 83% and 85%, respectively, reaching a 84% comprehensive gas drainage efficiency. The scheme has successfully controlled the gas outburst volume and guaranteed the construction safety of the project. 

Key words: high-speed railway tunnel, gas outburst, high ground stress, high- and low-level drilling fields, gas-outburst prevention measures, hydraulic fracturing