# Pressure-Relief Gas Cooperative Drainage Technology in a Short-Distance Coal Seam Group
## 5 Engineering Application
### Results and Analysis  
Before the treatment measures were taken, the gas concentration in many working faces and upper corners of No. 1-2 coal seam often exceeded the limit, rendering normal operation and production impossible. In the working faces of No. 1201 and No. 1209, a control measure of buried-pipe in the goat and directional drilling in the roof was adopted. The working face of No. 1211 adopted cooperative drainage technology. The gas concentration in the upper corner and the return air roadway with the above three control measures are reported in Table 4. The test site gas concentration was tested using a KG9701B low-density methane sensor. The test instrument is shown in Figure 9.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **1201 Working Face** & **1209 Working Face** & **1211 Working Face** \\ \hline The gas concentration in the upper corner/\% & 0.9 & 0.8 & 0.7 \\ The gas concentration in the return air roadway/\% & 0.8 & 0.6 & 0.5 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Gas concentration in upper corner and return airway of working face when three kinds of control measures were adopted, respectively.  
Figure 8: Layout of pressure–relief gas coordinated drainage boreholes.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Hole Number**} & **Design Hole** & **Hole** & **Height of Final Hole to Roof of** & **Horizontal Distance from the Final** \\  & **Depth/m** & **Azimuth/\({}^{\circ}\)** & **No. 1-2 Coal seam/m** & **Hole to the Air Return Side/m** \\ \hline
1\# & 380 & 275 & 15 & 20.50 \\
1-2\# & 380 & 275 & 17 & 22.25 \\
2\# & 380 & 278 & 19 & 24.00 \\
2-2\# & 380 & 278 & 21 & 25.75 \\
3\# & 380 & 281 & 23 & 27.50 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Parameters of roof directional long drillings.  
From Table 4, compared with working faces 1201 and 1209, which only adopted one treatment measure, the gas concentration in the upper corner of the 1211 working face with cooperative drainage technology was reduced to less than 0.7%, and the gas concentration in the return air roadway was reduced to less than 0.5%. The governance effect was best. In order to better understand the situation of pressure-relief gas control, the gas volume fraction of the return air roadway and upper corner of the 1211 working face and the gas extraction volume in the drainage pipes were monitored on site during the mining period from May 5 to 11-15, 2020, as shown in Figure 10.  
As can be seen in Figure 10, the average extraction volumes of the directional long drilling in the roof and the staggered buried-pipe in the goaf were 7.4 m\({}^{3}\)/min and 4.8 m\({}^{3}\)/min, and the average extraction concentrations were 73.2% and 5.7%, respectively. The average gas drainage rate of the collaborative drainage technology was 83.2%, which meets the requirement that the gas drainage rate of the working face should be greater than 40%. From further analysis, several tendencies can be observed in Figure 10.  
First, in the initial stage of mining, because the fracture zone of the overlying coal and rock strata has not developed to the position of the directional long drilling in the roof, the  
Figure 10: Investigation of cooperative extraction effect.  
Figure 9: KG9701B low-density methane sensor and technical parameters.  
-drainage effect is relatively poor, but, with the technical measures of staggered buried pipe drainage in the goal, the volume fraction of the upper corner can still be stabilized below 0.7%.  
Second, with the increase in mining distance, the directional drillings in the roof are located in the fracture zone, the drainage measures are implemented efficiently, and the drainage effect is remarkable.  
Third, in the later stage of drainage, the integrity of directional long drilling is damaged by mining, and the drainage effect is slightly reduced.