## 4 Prevention Measures and Engineering Practice
### Artificial Pre-Split Blasting in 7131 Working Face  
The artificial pre-split blasting method can be adopted to prevent the simultaneous breakage and large-scale overhang of the overlying strata. The pre-split blasting scheme has been designed based on the mechanism of the SCWID. In literature [30], the roof presplitting blasting in 7121 working face of Qidong coal mine was carried out, but due to the limited condition in the roadway, only one roadway was blasted. According to the conditions of 7131 working face, the previous technology is now improved and the roof blasting is carried out in both roadways of the working face to achieve a better result. Along the advancing direction, 65-mm-diameter drill holes were constructed from the roof of haulage and ventilating roadway to the goaf of working face at an interval of 16 m. Each group included four boreholes, as shown in Figure 14a, A1, A2, B1, B2 for the haulage roadway, and C1, C2, D1, D2 for the ventilating roadway. The boreholes with smaller inclined angle with the roadway strike direction were mainly used to cut off the rock strata, such as A1, A2, C1, and C2. A drill hole with a large inclined angle was mainly use for controlling that broken length of a rock stratum, such as B1, B2, D1, and D2. The angle between group A/C and the two vertical roadways was 70\({}^{\circ}\) and the angles between the two boreholes in the group and the horizontal plane were 30\({}^{\circ}\) and 52\({}^{\circ}\) respectively. The angle between group B/D and the two vertical roadways was 10\({}^{\circ}\). The angle between group B and the horizontal plane was 40\({}^{\circ}\) or 60\({}^{\circ}\), and that between group D and the horizontal plane was 30\({}^{\circ}\) or 40\({}^{\circ}\). In each group of boreholes, the length of the boreholes was 20 m, the controlled vertical height was 15.9 m, and the depth of the boreholes was about 17.8 m in the vertical direction of the two roadways of the working face. The design scheme of roof pre-split blasting of 7131 working face is shown in Figure 14b. At each construction site, the hole is drilled from the top of the roadway to the direction of the goaf, and the locations of the holes are shown in Figure 14c.  
Effect of Preventing Practice  
According to the previous analysis, 7131 working face had the risk of water irnush and support crushing. When the blasting was not carried out at the initial stage, the roof pressure was large and the distance of pressure behavior was long (Figure 15). After that, artificial pre-split blasting was implemented according to the mining conditions.  
Figure 14: Design scheme of roof pre-split blasting: (a) plan position of pre-split blasting; (b) scheme of borehole; (c) hole location of pre-split blasting.  
From the support resistance curve in Figure 15, it can be seen that when the working face is in the area of pre-split blasting, the support resistance is reduced. The roof weighting step and persistence length, the continue length of pressure reduced by 6.6 m, and the roof weighting step reduced by 6.7 m, indicating that the overall pressure strength decreases. Table 5 presents the pressure characteristics of the face before and after the presplit blasting. Practice has proven that artificial pre-split blasting can significantly reduce the roof pressure, change the fracture characteristics of the overlying strata and avoid the key strata breaking simultaneously.  
Based on the mining conditions of 7131 working face in Qidong mine, the safe mining was achieved through the prediction of dangerous areas of SCWID and the implementation of roof pre-splitting blasting. No SCWID occurred in the operation of 7131 working face, and the safety production of 1.277 million tons' coal was reached, which ensures the balance of exploitation, thereby creating remarkable economic benefits.