# Numerical Study on the Pressure Relief Characteristics of a Large-Diameter Borehole
## 6 Engineering Application  
### Test Roadway Deformation and the Failure State and Stress Environment  
(1) Test roadway deformation and failure state:  
During tunneling, the deformation is large, and the deformation rate is fast. The deformation of the coal and rock is mainly concentrated on the side wall of the coal pillar and the shoulder socket of the roof near coal pillar. The roof of the shoulder socket on the side of the coal pillar is crushed near the shoulder socket of the coal pillar, and the roof sinks and bulges, and the anchor nets twist. The roadway side of the coal pillar bulges, and the maximum extent of the bulging is 420 mm.  
In the mining stage, the roadway is strongly affected by mining. The amount of movement of the two sides of the roadway 10 m ahead of the working face is 900 mm, and the amount of movement of the roof and floor is 450 mm. The failure forms of the coal and rock  
Figure 16: Borehole pressure relief mechanism.  
roadway are mainly a split type of collapse and a lateral bulging type of collapse. In the part of the roadway wall with good coal integrity, the split type of collapse mainly occurs, and in the part with poor integrity, the lateral bulging type of collapse mainly occurs.  
Test roadway stress environment:  
The test roadway is the transportation roadway of the W1123 fully mechanized top-coal caving face section in the west wing of Kuangou coal mine, +1255 level 1 mining area, with a total length of 1469 m. Five drilling stress measuring points are arranged on the inner side of the coal pillar at 1300 m in the transportation lane of the W1123 section. The monitoring points are arranged at 1300 m, 1290 m, 1280 m, 1270 m, and 1260 m, and the depths of the measuring points in the coal seam are 2 m, 4 m, 6 m, 8 m, and 10 m, respectively. The monitoring points of the solid coal side monitoring station are symmetrically arranged with the coal pillar side. The specific monitoring scheme is shown in Figure 17.  
As can be seen from Figure 18, with the working face approaching the monitoring position, the stress inside the coal body at the monitoring position at different depths on the side of the coal pillar showed a trend of slowly increasing to the peak stress at first, and then rapidly decreasing. With the increase in the depth of the monitoring position, the peak stress inside the coal pillar increased from 9.7 MPa to 12.8 MPa; when the depth of the coal pillar reached about 4-5 m, the stress inside the coal pillar changed violently. At 5 m, the peak value of the stress inside the coal pillar reached 12.8 MPa and the stress concentration coefficient reached 2.33. In addition, compared with the trend in the stress change of the roadway side, the stress change trend of the coal pillar was more severe due to the superposition of the leading abutment pressure and the lateral abutment pressure of the working face, and the coal pillar side was in danger of impact due to the mining during the mining of the working face. In order to ensure the safety and stability of the roadway during mining, it is necessary to drill the coal pillar to relieve pressure and prevent an impact.