# Pressure-Relief Gas Cooperative Drainage Technology in a Short-Distance Coal Seam Group
## 4 Optimization Design of Pressure-Relief Gas Drainage by Mining
### Position of the Pressure-Relief Gas Drainage Hole in Short-Distance Coal Seam Group  
In the gas-conducting fracture zone, the separation fractures and broken fractures are extremely developed. The pressure-relief gas of the coal and rock mass around the stop continues to flow to it and gathers into a gas enrichment zone under the action of airflow. This area is the best space for extracting pressure-relief gas from adjacent layers. Mastering the width of the \"O\"-ring fracture zone is the key to determining the position of the pressure-relief gas extraction hole. According to previous studies [33], the width of \"O\"-ring fracture zone in the gas-conducting fracture zone can be calculated according to Formulas (9) and (10):  
\[W(M)=k_{2}W^{\prime}(M) \tag{9}\]  
\[W^{\prime}(M)=0.011M^{3}-0.640M^{2}+10.220M+13.514. \tag{10}\]  
\(W\)(\(M\))--width of the \"O\" ring crack area, m;  
\(k_{2}\)--constant is taken as 0.8\(\sim\)1.1. When the overburden is less and the mining depth is larger, a small value is taken; otherwise, a large value is taken;  
\(W^{\prime}\)(\(M\))--average width of \"O\" ring, m;  
\(M\)--height of the mining layer, m.  
Optimized Design Scheme of Pressure-Relief Gas Collaborative Drainage in the Short-Distance Coal Seam Group  
The cooperative extraction mode of pressure-relief gas in the short-distance coal seam group was established based on the \"three zones\" division of gas pressure-relief extraction in the short-distance coal seam group and the gas migration characteristics in the \"O\"-ring area of the gas-conducting fracture zone, as shown in Figure 7.  
(1) Directional long drilling pressure-relief in the far field gas-conducting fracture zone is observed. The roof directional long drillings are arranged in the gas-conducting fracture zone. Under the action of negative pressure and airflow, the pressure-relief gas in the gas-conducting fracture zone was effectively intercepted for the first time and prevented the accumulation of gas in the upper corner. The spatial arrangement position of the pressure-relief gas drainage holes in the adjacent layers of the short-distance coal seam group should be close to the return air roadway to avoid the stress recovery zone, and the horizontal distance should be calculated according to Formula (11):  
\[HD=[h-(h_{2}+h\cot\beta)\tan\alpha]\sin\alpha+\frac{h_{2}+h\cot\beta}{\cos\alpha}. \tag{11}\]  
\(HD\)**--horizontal distance from the directional long borehole in the roof to the return air roadway, m;  
\(h\)**--vertical height of the pressure-relief gas drainage hole from the roof of the coal seam of the mining seam, m;  
\(\alpha\)**--dip angle of the coal seam;  
\(\beta\)--included angle between the lines connecting the fracture boundary, mining boundary, and the coal seam;  
\(h_{2}\)--distance from the borehole to the outer boundary of the \"O\"-ring fracture zone, which can be taken as 0.5 \(W(M)\), m.  
(2) Extraction of staggered buried-pipe in the near field goat was performed. The buried pipes were arranged in the staggered distance of the return air roadway to extract the pressure-relief gas in the gas emission zone of the goat. The gas in the low gas accumulation area flowing to the working face due to the destruction of the integrity of the directional long borehole of the roof was intercepted twice, which effectively reduced gas emission in the goat.