# Investigation into Influences of Hydraulic Fracturing for Hard Rock Weakening in Underground Mines
## 4 In Situ Testing Results
### Roadway Displacement and Pressure Analysis  
To verify the effect of hydraulic fracturing on the hard roof of the working face, mining pressure monitoring points were arranged in the hydraulic fracturing section and the unfractured section of the return air roadway. The monitoring content included the displacement of the surrounding rock surface of the roadway and the vertical stress of the protective coal pillar. The cross-point method was used in conjunction with the top and bottom plate displacement and infrared rangefinder to monitor the top and bottom plate displacement and the two-sided displacement. The displacement sensors were installed at a distance of 15 m from the long-wall mining surface in the return air roadway. The deformation of the surrounding rock in the hydraulic fracturing section and the unfractured section were recorded for 100 days, with the results shown in Figure 9. According to the deformation monitoring results, it could be seen that the surface deformation of the roadway without hydraulic fracturing on the hard roof was basically stable after 80 days, the average displacement of the roadway top and bottom plate was 680.5 mm, and the average displacement of the two sides was 493.8 mm. For the hydraulic fractured section, the stability time of the surrounding rock was significantly earlier than that without hydraulic fracturing, and it tended to stabilize after 65 days. Compared to that without hydraulic fracturing, the surface deformation of the roadway's surrounding rock decreased, and the convergence speed of the roadway was significantly reduced. The displacement of the top and bottom plates decreased by 48.7%, and the displacement of the two sides decreased by 53.0%. This observation also indicated that the stress concentration around the roadway was eliminated, and pressure relief and stress transfer were realized, which also reduced the deformation of the roadway in the hydraulic fractured section. Four sets of borehole stress gauges were installed at a distance of 2.5 m from the surface of the coal pillar in the return air roadway, with a spacing of 2 m between each set of stress gauges. The continuous monitoring system was used to collect stress data from coal pillars. When the borehole stress gauge was located in the front of the working face, its distance from the working face was recorded as positive; when the borehole stress gauge was located behind the working face, its distance from the working face was recorded as negative. Coal pillar stress monitoring stopped when the working face advanced about 30 m past the monitoring point. The stress monitoring results of the coal pillar in the working face return air roadway are shown in Figure 10. Figure 10a shows the stress monitoring results of the coal pillar in the non-fractured section of the working face. It can be seen from the figure that when the borehole stress gauge was more than 50 m away from the working face, the vertical stress of the coal pillar increased slowly. As the distance from the working face decreased, the stress of the coal pillar increased significantly. When the working face advanced to the stress monitoring point, the stress growth rate of the coal pillar increased. Figure 10b shows the stress monitoring results of the coal pillar in the hard roof fracturing section of the working face. It can be seen that when the borehole stress gauge was more than 50 m away from the working face, the vertical stress of the coal pillar  
Figure 9: Surrounding rock deformation of return air roadway. changed slightly. However, as the distance from the working face decreased, the stress of the coal pillar gradually increased. When the working face advanced to 15 m behind the stress monitoring point, the stress of the coal pillar reached its peak due to the influence of the advanced support stress, and then the stress rapidly decreased. When it was 20 m behind the working face, it was basically stable.