## 6 Conclusions  
1. The rock mass in the Guobo station project area is mainly mudstone, and sandstone is only found at borehole 4-4. The UCS of mudstone in the natural state is 4.43-42.43 MPa, with an average value of 18.60 MPa. The BTS is 0.66-4.30 MPa, and the average value is 1.51 MPa. The strength of the mudstone specimens differs greatly from different boreholes, but both the UCS and BTS obey lognormal distributions. The average UCS of sandstone in the natural state is 65.31 MPa, and the average BTS is 3.06 MPa.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Section Mileage** & **Slope Angle \(\alpha\)(\({}^{\circ}\))** & \(e_{1}\)**(kPa)** & \(e_{1}\)**(kPa)** & \(e_{2}\)**(kPa)** & \(e_{2}\)**(kPa)** \\ \hline HMDK15 + 388 & 22.42 & 22.96 & 59.26 & 45.14 & 98.26 \\ HMDK15 + 520 & 12.70 & 19.65 & 32.86 & 43.33 & 64.43 \\ HMDK15 + 560 & 19.22 & 29.19 & 58.66 & 51.92 & 93.16 \\ \hline \end{tabular}
\end{table}
Table 6: Bias load at some typical sections of the Guobo station.  
2. The softening coefficients of mudstone and sandstone are 0.33 and 0.73, respectively, which indicates that the water content has an important impact on the strength of both mudstone and sandstone but has a greater effect on mudstone.
3. The in situ stress field in the project area is affected by horizontal tectonism. The relationship of the principal in situ stresses is: \(\sigma_{\rm H}>\sigma_{\rm h}>\sigma_{\rm V}\). In the area with a cover depth of approximately 60 m, the maximum horizontal principal stress ranges between 4.25\(\sim\)7.36 MPa. The azimuth is distributed between N45\({}^{\circ}\)W\(-\)65\({}^{\circ}\)N\({}^{\circ}\)W, and the lateral pressure coefficient is 2.11\(\sim\)5.71, with an average value of 3.39. A large lateral pressure coefficient may cause a tensile stress concentration on the side walls of the station after excavation.
4. The uniaxial compressive strength of the rock mass at Guobo station does not exceed 1 MPa. The strength-stress ratio of the surrounding rock is only 0.12\(\sim\)0.21 when the GSI is 50, which indicates that the tunnel is prone to large deformation during excavation.  
Based on the present experimental results and risk analysis results, we propose the following suggestions for the excavation and support methods of the station: (1) in terms of the excavation method, the double-side heading method with double-side supports may be the best excavation method; (2) in terms of the support methods, the pre-reinforcement method of pipe shed grouting should be adopted to improve the unfavorable stress state of the lining under bias pressure. Aside from this, it is also advisable to reserve enough deformation space, to improve the strength of the initial support and to perform waterproofing during the whole life cycle of the station.  
Conceptualization, D.L.; Investigation, D.L., A.Z. and Y.C.; Methodology, D.L., L.W. and C.M.; Project administration, L.W.; Resources, C.M.; Supervision, D.L.; Validation, A.Z.; Writing--original draft, A.Z.; Writing--review and editing, D.L. and A.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number U1934211.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.