## 4 Test Results and Analysis
### In Situ Stress Test  
To obtain the in situ stress in the project area, the Kaiser points of the samples were obtained by the AE method. The Kaiser point of the samples in the vertical group is its vertical principal stress. The horizontal principal stress and the maximum principal stress direction are calculated from the three Kaiser points in the horizontal group by the principle of elasticity:  
\[\left\{ \begin{array}{c}\sigma_{H}=\frac{1}{2}(\sigma_{I}+\sigma_{II})+\frac {1}{2\alpha\alpha\beta 2}(\sigma_{I}-\sigma_{II})\\ \sigma_{h}=\frac{1}{2}(\sigma_{I}+\sigma_{II})-\frac{1}{2\alpha\alpha\beta}( \sigma_{I}-\sigma_{II})\\ tan2\theta=\frac{2\sigma_{II}-\sigma_{I}-\sigma_{II}}{\sigma_{I}-\sigma_{II}} \end{array} \right. \tag{7}\]  
where \(\sigma_{I}\), \(\sigma_{II}\) and \(\sigma_{II}\) are the Kaiser points in the directions of \(0^{\circ}\), \(45^{\circ}\) and \(90^{\circ}\) of the horizontal group, \(\sigma_{H}\) and \(\sigma_{h}\) are the maximum and minimum principal stresses of the horizontal direction, and \(\theta\) is the direction of the horizontal maximum principal stress. Figure 10 and Table 5 show the in situ stress distribution at each measuring point. Vertical stress is basically equivalent to gravitational stress. The relationship between the three principal stresses is as follows: the maximum horizontal principal stress (\(\sigma_{H}\)) > minimum horizontal principal stress (\(\sigma_{h}\)) > vertical principal stress (\(\sigma_{V}\)). The directions of the maximum horizontal principal stress are consistent; in the WNW direction, the azimuth is distributed in N \(45^{\circ}\) W-N \(65^{\circ}\) W, and \(\sigma_{H}\) ranges from 4.25 to 7.36 MPa. The ratio of the maximum horizontal principal stress to the vertical principal stress (lateral pressure coefficient) is from 2.11 to 5.71, with an average value of 3.39, which varies greatly in the project area. On the one hand, this is caused by the larger or smaller lateral pressure coefficient measured at a few boreholes. The lateral pressure coefficient measured at most boreholes is between 3.17 and 3.72. On the other hand, the lateral pressure coefficient may be related to the distribution of the major joints and local groundwater in the project area. The existence of the joints affects the stress transmission of the rock mass, and seepage of the rock mass also has a certain impact on in situ stress [30], which may lead to a local stress  
Figure 9: Typical stress–strain curves and failure photos of mudstone (**a**) and sandstone specimens (**b**) under dry and saturated conditions; (**c**) softening coefficients. concentration. When the lateral pressure coefficient is large, a tensile stress concentration may occur on the side walls of the roadway (circular section > 3, square section > 1.2) after excavation, and the value and range of tensile stress will increase with the increase of the lateral pressure coefficient [30; 31; 32]. Since the tensile strength of the rock mass in the project area is extremely low, the stress on the side walls of the station shall be mainly considered to avoid large tensile stress concentration zones on the side walls after the excavation of the rock masses in the station. In addition, the absolute value of the in situ stress is not very high; however, compared with the mudstone with a low strength, which is widely distributed in the project area, the in situ stresses are relatively high. Therefore, it is an unfavorable factor to be considered during excavation. \(\sigma_{V}\): vertical principal stress; \(\sigma_{S}\): gravitational stress; \(\sigma_{H}\): maximum horizontal principal stress; \(\sigma_{h}\): minimum horizontal principal stress; \(\theta\): direction of maximum horizontal principal stress. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Borehole No. **} & \multirow{2}{*}{ \begin{tabular}{c} **Buried Depth of** \\ **Measuring Point** \\ **(m)** \\ \end{tabular}