## 2 Project Overview
### In-Situ Stress  
The deep rock mass of the Datai coal mine is in an in-situ high stress environment, which leads to serious deformation of and damage to the roadway. In-situ stress is tested using the stress relief method [30]. The in-situ stress is measured from a distant point prior to the construction of the roadway. Two in-situ stress measuring points are set at the level of \(-510\) m. The results of the in-situ stress test are illustrated in Table 1.  
According to the results of the stress measurements performed on the Datai coal mine roadway at \(-510\) m, the following conclusions can be obtained.  
1. The azimuth angles of the principal stress \(\sigma_{1}\) of the two measuring points are \(154.91^{\circ}\) (borehole 1) and \(156.79^{\circ}\) (borehole 2), respectively, values which are basically consistent with the principal stress direction in North China. Near east-west (the Datai coal mine is east of Taihang Mountain), the azimuth angle of borehole 2 is \(224^{\circ}\), and the included angle with the azimuth angle of \(\sigma_{1}\) is \(67.21^{\circ}\). The core discing phenomenon appeared in borehole 2, as shown in Figure 4. This is a main feature of rock masses under high ground stress.
2. At the two measuring points, \(\sigma_{1}\) had dip angles of \(-2.11^{\circ}\) and \(-2.40^{\circ}\), respectively. \(\sigma_{1}\) is oriented in a near-horizontal direction, suggesting that the in-situ stress in this area is mainly characterized by horizontal compressive stress. Therefore, it is necessary to study the effect of horizontal ground stress on the safety of the rock mass, which can be determined by changing the lateral pressure coefficient.
3. The principal stress values of the two measuring points in the near vertical direction are \(24.44\) MPa and \(25.63\) MPa, respectively, values which are basically equal to the pressure of the overlying strata per unit area at the \(-510\) m level.
4. The differences between the dip angles of \(\sigma_{1}\) and \(\sigma_{3}\) at the two measuring points are \(11.06^{\circ}\) and \(12.35^{\circ}\), respectively. The differences between the dip angles are small and similar to the horizontal direction. At the same time, \(\sigma_{1}\) is \(2.39\)-\(2.67\) times the minimum principal stress \(\sigma_{3}\), and the difference is larger. Shear failure in the rock mass of a sidewall is serious.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Number of Measuring Points**} & \multirow{2}{*}{**Principal Stress**} & \multirow{2}{*}{**Principal Stress Value (MPa)**} & \multicolumn{2}{c}{**Principal Stress Direction (\({}^{\circ}\))**} \\ \cline{3-5}  & & & **Azimuth Angle** & **Dip Angle** \\ \hline \multirow{3}{*}{1} & \(\sigma_{1}\) & 45.69 & 154.91 & \(-2.11\) \\  & \(\sigma_{2}\) & 24.44 & 80.21 & 77.62 \\  & \(\sigma_{3}\) & 19.10 & 244.85 & 8.95 \\ \hline \multirow{3}{*}{2} & \(\sigma_{1}\) & 49.30 & 156.79 & \(-2.40\) \\  & \(\sigma_{2}\) & 25.63 & 80.21 & 79.76 \\ \cline{1-1}  & \(\sigma_{3}\) & 18.46 & 246.37 & 9.95 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The magnitude and direction of principal stress on each measuring location.  
Figure 4: Core discing phenomenon in the borehole 2 of the roadway sidewall.