**2. Engineering Background**

Zhangji coal mine is a large scale underground colliery located at Huainan, Anhui province, China (shown in Figure 1a) with an annual output capacity of 13 million tons of raw coal. The working horizontal extends from −450 m to −1000 m. Zhangji coal mine covers seven mining areas which are the East-I, East-II, East-III, West-I, West-II, West-III, and North-I mining areas. The primary minable coal seams within Zhangji coal mine include No. 13-1, No. 11-2, No. 8, No. 6, as well as No. 1 and the total thickness of coal seams is 21.08 m. The measured resources and minable reserves of Zhangji coal mine are 17.78 billion and 8.77 billion tons of raw coal, respectively [20].

**Figure 1.** Simplified map of Zhangji coal mine and layout of a Tunnel Boring Machine (TBM)-excavated coalbed methane (CBM) drainage roadway. (**a**): location of Zhangji coal mine (**b**): position of TBM-excavated roadway (**c**): geological sketch of TBM-excavated roadway.

The West-II mining area is situated in west wing of Zhangji coal mine with the No.1 coal seam as its primary minable coal seam, and the mining operations within the West-II mining area are conducted from −480 m to −625 m. The No.1 coal seam has an average thickness of 6.5 m, a dip angle of 3◦ to 5◦, and a calorific value of 6000 kcal/kg. The coal is classified as high volatile bituminous coal and coking coal and the thickness of overburden is about 500 m. The coal seam gases (mainly include CH4, CO, H2S, CO2, etc.) content of the No.1 coal seam is 8.05 to 11.22 m<sup>3</sup>/t and the gas pressure of coalbed methane (CBM) ranges from 1.4 to 4.35 MPa. Generally, gas content greater over 8 m3/t or a gas pressure greater than 0.74 MPa is considered sufficient to initiate an outburst if other conditions are favorable. Therefore, the No.1 coal seam is considered to be an outburst-prone coal seam. In order to eliminate coal and gas outburst risk, CBM drainage roadways were excavated prior to coal mining operations.
