## 9 Engineering Case  
### Measures of Stress Relief  
The stress evolution demonstrated that the triangular coal pillar stress began to rise significantly when its distance from the fault was less than 200 m, peaking at a distance of 73 m from the fault. Therefore, it is necessary to implement stress relief measures before the width of the triangular coal pillar affected by this stress increase reaches 200 m. These measures involve drilling large-diameter boreholes (150 mm diameter, 25 m depth) within the coal body, with a step spacing of 3 m. However, in the highly stressed region located within 0-73 m from the fault, the drilling step spacing should be reduced to 2 m to ensure adequate stress relief.  
The energy evolution demonstrated that the triangular coal pillar energy began to rise at a distance of 150 m from the fault, peaking at a distance of 50 m from the fault. Therefore, different stress relief measures should be applied within the width range of 50-150 m and 0-50 m, respectively, in the triangular coal pillar to enhance the stress relief. The stress and energy within the triangular coal pillar is an abnormal concentration due to abutment stress from roof and tectonic stress from faults, both of which are under the influence of barrier effect. Therefore, the TB Coal Mine chose the air-return roadway, which is severely affected by the triangular coal pillar, for roof blasting in order to reduce abutment stress and for coal seam blasting to alleviate tectonic stress, as shown in Figure 13.  
The maximum abutment stress generated by the sub key stratum under the influence of the barrier effect is 17.2 MPa, therefore blasting measures need to be taken. The blasting measures were implemented at a distance of 0-150 m from the fault in the air-return roadway, to enhance the stress relief of the triangular coal pillar. The spacing between blasting hole steps was 10 m. Each group included 2 blasting holes with a diameter of 75 mm, with elevation angles of 75\({}^{\circ}\) and 89\({}^{\circ}\), and lengths of 49 m and 55 m, respectively. The blasting holes finally reached the middle of the sub key stratum. The length of explosives were 28.5 m and 32 m, respectively, and the amounts of the explosives were 50 kg and  
Figure 13: Measures of stress relief. (**a**) stress relief blasting for roof. (**b**) stress relief blasting for coal seam.  
56 kg, respectively. The application of roof blasting in high-stress areas of the working face may induce rock burst, while the restoration of surrounding rock stress can diminish the stress relief efficacy of blasting holes located far from the working face. Based on historical experience, it has been established that roof blasting measures should precede the mining face by approximately 200 m. Given that the average mining speed of the 501 working face is 4 m per day and the spacing between roof blasting holes is 10 m, the 501 working face implements roof blasting measures three times weekly  
The triangular coal pillar exhibits significant stress concentration at a distance of 0-50 m from the fault, and it is necessary to carry out blasting measures on the coal seam. Implementing coal seam blasting holes at a distance of 50 m from the fault in air-return roadway maximizes stress relief on the triangular coal pillar. The step distance is 10 m. Each group comprises 2 blasting holes, with elevation angles of 36\({}^{\circ}\) and 26\({}^{\circ}\), and lengths of 26.5 m and 23.5 m, respectively. The length of explosives are 15 m and 13 m, and the amounts of explosives were 36 kg and 31 kg, respectively. Similarly, given the average mining speed of 4 m per day of working and the step distance of 10 m between coal blasting holes in the 501 working face, coal blasting measures are implemented three times weekly.