## 6 Engineering Verification
### Engineering Verification of Roof Self-Stability  
(1) Backfilling hydraulic support strength  
The strength of backfilling hydraulic support needed to be greater than 0.93 MPa to meet the basic requirements for roof self-stability. Xinjulong Coal Mine adopted the C7000/20/38 four-column backfilling hydraulic support with a supporting intensity of 0.94 MPa.  
(2) Backfill ratio  
1. Measuring-point layout  
The roof is supported by the backfilling body after the goat is filled in coal mining. Roof displacement sensors are arranged inside the backfilling body to monitor the self-stable state of the roof after mining.  
There are 25 roof displacement sensors installed in the backfilling body. The first row of sensors is installed at the points that are 350 m from the open-off cut of the working face. When the working face is advanced to 450, 550, 700, and 850 m, the second, third, fourth, and fifth rows are installed, respectively. Figure 12 shows the layout of monitoring points.  
The data transmission lines of roof displacement sensors are easily damaged in the goat. Therefore, the data lines are placed in a special pipeline.  
2. Result analysis  
A roof subsidence monitor is used to monitor the roof subsidence of the goat after backfilling. The monitor is installed in the goat during the mining period to measure and record the roof subsidence of the goat. The monitoring data are transmitted through a wired connection. When the working face is advanced to 450 m, measuring point 1\({}^{\#}\) in the middle of the second row is selected for the monitoring analysis of dynamic roof subsidence. Figure 13 shows the monitoring results of dynamic roof subsidence.  
Figure 13: Roof dynamic subsidence value of measuring point 1\({}^{\#}\).  
Figure 12: Measuring-point layout of roof displacement.  
1. Dynamic roof subsidence above the goaf is divided into three stages with the advancement of the working face: the accelerated subsidence stage (the distance between the working face and measuring point is 0-88 m); the slowly-increased subsidence stage (the distance is 88-188 m); the creep deformation stage (the distance > 188 m). 2. The backfilling body is not compacted in time with the increased advancing distance at the accelerated subsidence stage. The overlying strata cannot be supported by the backfilling body, which accelerates roof subsidence. The dynamic backfill ratio of the roof is 89.78% when the working face is 88 m away from the measuring point (the working face is advanced by 538 m). A stable roof structure leads to its bending subsidence. 3. The backfilling body is gradually compacted with the increased advancing distance at the slowly-increased subsidence stage. The compaction rate continuously decreases. The roof subsidence rate is lower than the compaction rate at the accelerated deformation stage of surrounding rocks. The roof bending amplitude increases, but no caving failure occurs. 4. When the distance between the working face and measuring point is larger than 188 m (the advanced distance is larger than 638 m), the roof subsidence enters the creep deformation stage of surrounding rocks. The backfilling body is gradually compacted to bear a load of overlying strata with the advanced working face, which stabilizes the roof subsidence. When roof subsidence is stable, the foundation coefficient of the backfill material is 4.16 \(\times\) 10\({}^{8}\) Nm\({}^{-3}\); the maximum subsidence at measuring point 1\({}^{\#}\) is 438 mm. The backfill ratio at measuring point 1\({}^{\#}\) is calculated to be 86.3%, which is greater than the roof self-stability condition. Therefore, the roof forms a self-stable structure.