# Pressure-Relief Gas Cooperative Drainage Technology in a Short-Distance Coal Seam Group
## 5 Engineering Application
### Optimization Design of Pressure-Relief Gas Collaborative Drainage  
To prevent the hidden danger of gas accumulation caused by a large amount of pressure-relief gas from the upper and lower adjacent seams pouring into the 1211 working face during the mining of the No. 1-2 coal seam, it is proposed that directional long drilling in the roof combined with the staggered buried-pipe in the goat be used to drain the pressure-relief gas from the adjacent seams.  
First, based on the above optimization design scheme of pressure-relief gas collaborative drainage, the height range of the directional long drilling in the roof should be arranged in the overlying strata above 10.3 m and below 23.2 m of the No. 1-2 coal seam roof. At the same time, considering the construction feasibility and the integrity of the final borehole, a reasonable layout horizon of the directional long borehole is in the sandy mudstone 14.7 m away from the roof working face to ensure a longer drainage time and higher drainage efficiency.  
Second, based on the Formula (11) calculation for the arrangement of the pressure-relief gas drainage holes in the adjacent seam, the dip angle \(\alpha\) of the coal seam was taken as 4\({}^{\circ}\), the included angle \(\beta\) between the lines connecting the fracture boundary, mining boundary, and the coal seam was taken as 65\({}^{\circ}\), and the distance from the borehole to the outer boundary of the \"O\"-ring was taken as 15.1 m. The horizontal distance between the directional long borehole in the roof of the 1211 working face and the return air roadway was calculated to be 20.5-27.5 m.  
Based on the theoretical calculation and combined with the characteristics of the overlying coal and rock mass of the No. 1-2 coal seam roof, directional long drillings were performed in the No. 2 drilling field, 350 m away from the cutting hole of the 1211 return air roadway. Three main boreholes and two branch boreholes were arranged. The borehole spacing was 1.75 m, and the layer spacing was 2.0 m. The inclination of the opening was 9\({}^{\circ}\), and the borehole aperture was 96 mm. The borehole construction parameters are shown in Table 3. After drilling was completed, the borehole was sealed by cement grouting; the negative pressure of drainage was 13 kPa.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Lithology** & **Compressive Strength/MPa** & **Tensile Strength/MPa** & **Modulus of Elasticity/GPa** \\ \hline Coarse sandstone & 76.3 & 4.8 & 7.9 \\ Sandy mudstone & 30.1 & 1.9 & 3.1 \\ Medium and coarse sandstone & 61.3 & 3.6 & 7.4 \\ Sandy mudstone & 34.8 & 2.4 & 3.2 \\ Coal & 14.7 & 1.3 & 1.1 \\ Sandy mudstone & 29.1 & 1.7 & 3.0 \\ Coarse sandstone & 76.4 & 4.1 & 8.1 \\ \hline \end{tabular}
\end{table}
Table 2: Overburden physical and mechanical parameters of the No. 1-2 coal seam.  
Finally, to intercept the pressure-relief gas from the lower adjacent seam into the goat for the second time, two pipelines with a diameter of 300 mm were arranged at the upper corner of the return air roadway at a staggered distance; one pipeline was located at a height of 1.1 m from the floor and a depth of 11.3 m into the goat, and the other was arranged in parallel, at a height of 1.7 m from the floor and a depth of 20.8 m into the goat. The negative pressure of the drainage was 20 kPa. The two drainage pipelines were arranged in a staggered manner in space and always located in the gas emission zone of the goat; this ensured the continuity of drainage and effectively improved the effect of gas drainage. The borehole layout of pressure-relief gas cooperative extraction is shown in Figure 8.