## 5 Hydraulic Fracturing
### Effect of Hydraulic Fracturing  
After hydraulic fracturing was finished, electronic dynamometers were employed to monitor the cable's force on the roof and rib at 120 m and 150 m in the PRR. Meanwhile, instruments were placed to measure the separation amount of the roof rocks in the range of 2 m and 12 m of the PRR. Four sets of instruments were set at 80 m, 120 m, 150 m and 210 m in the PRR, respectively. For analyzing the strength and magnitude of the roof weighting after hydraulic fracturing, the resistance of the supports at the longwall face was collected every 5 min. The monitoring results were as follows:  
1. When the unmanned panel was about 3 m, the stress on the roof cables increased sharply, as shown in Figure 14. At this time, the inby fender started to break and gradually lost its bearing ability. As the roof and floor strata coalesced in the gob, the stress on the roof cables gradually became stable within one day after the entrance of the longwall face to the PRR. The values of the force on the two measuring points were increased by about 50 kN and 133 kN, respectively. The force on the rib cables fluctuated slightly, and the increased value was no more than 10 kN. After hydraulic fracturing, although the stress on the roof cables increased with the transference of the abutment force, the roof remained stable without breaking down and the pillar ribs maintained good stability and bearing capacity. 2. The observation results of the four sets of separation instruments found that the separation amount of the roof within 2 to 12 m of the PRR was 0.67 mm, 0.05 mm, 0 mm and 0 mm, respectively. Hydraulic fracturing eliminated the risk of separation and dislocation of the roof strata in the process of mining-induced stress transference. 3. Figure 15 presents the cloud diagrams of the resistances of the panel supports. At 1:00 a.m. on 1 December 2021, when the PRR and 31108 longwall face were separated  
Figure 14: Axial force increasing curve of anchor cable.  
Figure 13: Borehole layout (**a**) Stereogram (**b**) Vertical view (**c**) Side view.  
by about 30 m, a periodic weighting occurred in the main roof. The stresses on the 30# to 70# supports increased first, and the stresses on the 80# to 120# supports increased later. The weighting step was about 4 m. The average stress on the supports in the weighting area was about 44 MPa. After the longwall face entered the hydraulic fracturing area, extensive stress concentration on the supports did not appear because the gob roof collapsed in time. At 13:00 p.m. on 3 December, when the PRR and 31108 longwall face were separated by about 7-8 m, the stresses on the 23# to 80# supports increased slightly. Compared with the former weighting, the magnitude was smaller and the duration was shorter. In the face stop period (from 0 a.m. on the 4 th to 0 a.m. on the 5 th), the average stress on the supports was about 31.2 MPa, about 6 MPa higher than the initial stress. During extraction of the last 5-m-wide fender, high stresses on the supports did not appear.  
Figure 15: Distribution of the stresses on the hydraulic supports at the panel.  
* In the process of withdrawing the equipment, the maximum shrinkage length of the hydraulic supports at the longwall face was 10 cm, and the maximum descending length of the chock supports in the PRR was 6 cm. Good roof stability reduced the difficulty of removing the equipment. Serious damage to the floor was not observed on site. The removal time was reduced by 3 days. Figure 16 is a field photo after hydraulic fracturing.  
To sum up, hydraulic fracturing lowered the integrity degree of the overlying strata and reduced the strength of the periodic weighting, thereby completely eliminating the risk of roof weighting failures induced by the movement of the fractured roof rocks.