## 2 Overview of the Mine and Workings
### Factors Affecting Crack Initiation and Expansion  
#### 4.2.1 Effect of Different Angles on Crack Initiation and Expansion  
1. \(0^{\circ}\) prefabricated crack  
When the angle of the prefabricated crack was \(0^{\circ}\), high stress concentration occurred at the crack end during the stress accumulation stage (1-6.5 MPa). This is analogous to mining a thin protective layer in the rock, which disrupts the stress concentration zone surrounding the borehole and transfers the high stress to the crack tip. This mechanism effectively fractures the intact rock mass, leading to a \"corkscrew effect\" that enhances the rock's permeability, increases the unloading range of a single borehole, and facilitates complete unloading of the surrounding coal body. During the stage of local crack expansion (6.5-8.5 MPa), with the increase in drilling pressure, at step 5 (the water pressure in the drilling hole was 6.5 MPa), fine cracks formed at the end of the cracks. By step 10, a macroscopic microcrack appeared at the crack tip, and the crack progressively extended and expanded as the water pressure in the borehole increased. At this moment, the cracks were distributed on both sides of the borehole and penetrated with the borehole. During the crack extension and penetration phase (8.5-27 MPa), once the borehole stress exceeded \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & **Mean Value of** & **Degree of** & **Friction Angle/(\({}^{\circ}\))** & **Average** & **Poisson’s Ratio** \\  & **Modulus of** & **Homogeneity/m** & & **Strength/MPa** & \\ \hline Parameter value & 9600 & 3 & 37 & 30 & 0.25 \\ \multirow{2}{*}{**Parameters**} & **Permeability** & **Pore Water** & **Permeability** & **Pore Water** & **Pressure** & **Loading Method** \\  & **Coefficient/m-d\({}^{-1}\)** & **Pressure/MPa** & **Rupture** & **Coefficient** & \\ \multirow{2}{*}{Parameter value} & 0.001 & 1.0 & 10 & 0.1 & Stress loading \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Numerical simulation of mechanical parameters. Figure 10: Numerical modeling of hydraulic fracturing of coal rock bodies. 8.5 MPa, the cracks connected with the borehole and expanded outward, forming additional crack groups, as depicted in Figure 11a. Figure 11: Cont. Figure 11: Plot of the evolution of shear stress in the crack slit. 2. 30\({}^{\circ}\) prefabricated crack  
When the angle of the prefabricated crack was 30\({}^{\circ}\), in the stress accumulation stage (1-5 MPa), the water pressure in the borehole was 3 MPa, and a high stress concentration phenomenon appeared at the end of the 30\({}^{\circ}\) crack in the borehole. With the increase in applied pressure in the borehole, at step 5 (water pressure in the borehole was 5 MPa), fine cracks formed at the crack end. At the stage of localized crack extension (5-8.5 MPa), with the increase in the applied pressure in the borehole, by step 10 (the water pressure in the borehole was 8.5 MPa), a macroscopic tiny crack formed at the crack end, and the crack gradually extended and expanded. At this point, the cracks were distributed on both sides of the borehole, and the direction of crack expansion was upward and downward, centered on the end of the crack. At the stage of crack expansion and penetration (8.5-27 MPa), when the stress in the borehole increased to 8.5 MPa (after 12 steps), the formed cracks were connected with the borehole, penetrated with the cracks, and further expanded outward to form a larger number of crack clusters, as shown in Figure 11b. 3. 60\({}^{\circ}\) prefabricated crack  
When the angle of the prefabricated crack was 60\({}^{\circ}\), high stress concentration was observed at the crack end during the stress accumulation stage (1-5 MPa). At the stage of local crack extension (5-8 MPa), with the increase in pressure applied to the borehole, at step 10 (the water pressure in the borehole was 8 MPa), a macroscopic tiny crack was formed at the end of the crack, and the crack gradually extended and expanded.