## 4 Analysis of Influencing Factors of Long-Distance Drilling Blasting
### Overburden Rock Migration and Stress Variation Characteristics  
In order to more clearly and intuitively evaluate the subsidence of overlying strata in coal seam mining before and after blasting, one of the schemes was selected to compare the cloud maps of displacement and stress changes. For example, Figures 5-7 show the subsidence, stress change characteristics, and fractures of overlying strata in long-distance drilling blasting for Scheme 1.  
Figure 6 is a cloud map of overburden subsidence before and after long-distance borehole blasting, and Figure 7 is a cloud map of maximum principal stress before and after long-distance borehole blasting. By comparing the subsidence cloud map of overburden rock before and after long-distance drilling blasting, it can be seen that after long-distance drilling blasting, the subsidence of overburden rock was V-shaped, and the subsidence  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Level**} & \multicolumn{5}{c}{**Factors**} \\ \cline{2-7}  & \multicolumn{2}{c}{**Distance from Coal**} & \multicolumn{2}{c}{**Lithology of blasting**} & \multicolumn{2}{c}{**Blasting Layer**} \\  & \multicolumn{2}{c}{**Seam/m**} & \multicolumn{2}{c}{**Layer**} & \multicolumn{2}{c}{**Thickness/m**} \\ \hline
1 & 50 & Silstone & 20 & 3.02 & 2.62 \(\times\) 10\({}^{7}\) & 1827 & 162 \\
2 & 50 & Kern stone & 30 & 2.86 & 2.23 \(\times\) 10\({}^{7}\) & 1727 & 145 \\
3 & 80 & Silstone & 30 & 3.05 & 2.67 \(\times\) 10\({}^{7}\) & 1869 & 173 \\
4 & 80 & Kern stone & 20 & 3.25 & 2.82 \(\times\) 10\({}^{7}\) & 1935 & 196 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Orthogonal experimental design table.  
Figure 5: Cloud map of overlying rock subsidence before and after blasting. **(a)** No blasting. (b) After blasting.  
Figure 6: Cloud map of maximum principal stress before and after blasting. **(a)** No blasting. (b) After blasting.  
value of overburden rock in the middle of working face was the largest. Compared with the mining conditions without blasting, the subsidence range of overburden rock after blasting increased, which indicated that blasting pre-splitting rock stratum can make the rock stratum collapse more fully during the mining process of the working face. Additionally, the roof rock stratum was more broken, so as to minimize the rock burst risk caused by the large-area suspended roof. By comparing the maximum principal stress cloud diagram before and after long-distance drilling blasting, it can be seen that after long-distance drilling blasting, the stress distribution in the roof area was more dispersed, the stress concentration area before blasting was larger, and the stress concentration degree above and below the pre-splitting layer after blasting was reduced.  
Figure 8 shows the change of roof rock cracks before and after long-distance borehole blasting. According to the figure, after long-distance borehole blasting, the roof rock was more broken and the fracture range was larger. At the same time, according to the simulation results, the maximum shear stress value before blasting reached 286.88 MPa, and the maximum shear stress value after blasting reached 291.75 MPa. After blasting, the total number of cracks increased from 1232 to 1727, and the total length increased from 119 m to 145 m, which increased by 28.66% and 17.93% respectively. This shows that blasting pre-splitting rock strata can avoid the roof above the working face due to the existence of a large-area suspended roof, an increase in the number of cracks, and avoidance of the accumulation of a large amount of elastic energy, thereby reducing rock burst risk.  
Figure 8: Curve of subsidence and stress value of overlying rock before and after blasting in Scheme 1. (**a**) The curve of overlying rock subsidence value. (**b**) Stress value curve.  
Figure 7: Diagram of changes in rock fissures before and after blasting. (**a**) No blasting. (**b**) After blasting.