## 4 Prevention Measures and Engineering Practice
### Prevention Practices of SCWID
#### 4.3.2 Prevention of SCWID in 7\({}_{1}\)31 Working Face  
Predicating Dangerous Area of SCWID  
According to the prevention and control countermeasures of SCWID under the UCA, when the distance between the primary key stratum and the coal seam is less than 10 times the mining height, the overlying strata are easy to behave as a composite single key stratum structure, and the broken beam structure is easy to slide and lose stability after the breakage of the strata, causing the water inrush accident of the working face. Therefore, the target horizon of presplitting blasting is the primary key stratum within 10 times the coal seam height. It was found that within the 7-10 times mining height in overlying strata of 7\({}_{1}\)31 working face, the thickness of the hard rock stratum is greater than 10 m, or even reaches 20 m. Therefore, targeted measures should be taken to avoid the disasters. The identification results are shown in Table 4.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Thickness/m** & **Depth/m** & **Lithology** & **Before blasting** & **After blasting** \\ \hline
338.70 & 0.00 & Unconsolidated & - & - \\
4.40 & 338.70 & Claystone & - & - \\
18.42 & 343.10 & Mudstone & - & - \\
0.30 & 361.52 & Coal seam & - & - \\
4.12 & 361.82 & Mudstone & - & - \\
0.19 & 365.94 & Coal seam & - & - \\
2.79 & 366.13 & Mudstone & - & - \\
0.80 & 368.92 & 6\({}_{0}\) Coal seam & - & - \\
6.84 & 369.72 & Mudstone & - & - \\
1.79 & 376.56 & 6\({}_{1}\) Coal seam & - & - \\
5.14 & 378.35 & Mudstone & - & - \\
12.31 & 383.49 & Medium & Hard rock-KS2 & Hard rock-KS \\
0.70 & 395.80 & Mudstone & - & - \\
0.41 & 396.50 & Carbon & - & - \\
1.62 & 396.91 & 6\({}_{2}\) Coal seam & - & - \\
2.03 & 398.53 & Mudstone & - & - \\
0.33 & 400.56 & Coal seam & - & - \\
2.10 & 400.89 & Mudstone & - & - \\  & & & & Hard rock- \\
15.07 & 402.99 & Medium & Hard rock- & \\  & & sandstone & KS1(Pre-split blasting) & Broken rock \\
0.47 & 418.06 & Mudstone & - & - \\
3.36 & 418.53 & 7\({}_{1}\) Coal seam & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 4: Results of the key stratum location of borehole No.29-302 near 7\({}_{1}\)31 working face.  
Figure 13: Location of pre-split blasting holes in 7\({}_{1}\)31 working face.  
As shown in Table 4, according to the judgment result of the key stratum, in the original rock strata, the layer of rock stratum with a thickness of 15.07 m, which is 0.47 m above the coal seam, is the primary key stratum, which plays a major controlling role in the movement of overlying rock strata. After its breakage, the overlying 12.31 m thick hard rock stratum is broken synchronously with it. At the same time, the thickness of the broken rock layer reaches the bottom of the thick loose layer with a thickness of 338.7 m. If the artificial presplitting blasting is carried out, the original 15.07 m thick hard key stratum close to the coal seam will be artificially weakened before the mining of the working face and become a damaged rock stratum, and the broken length will follow the distance length set by the artificial presplitting blasting, which will be less than the original broken length, and also less than the damaged length of the upper 12.31 m thick sandstone. After the roof presplitting blasting, the undamaged 12.31 m thick sandstone layer becomes the key layer which plays a major controlling role in the whole overlying strata movement. After blasting, the lower rock strata fully collapse, the length of the block is reduced, and the bulking coefficient is increased. When the upper key stratum is broken, the lower hanging space will be reduced, and the overall breakage of the key stratum will not occur again, thus avoiding the occurrence of water inrush accidents.