# Stability Analysis of Box Tunnel Working Face under Non-Uniform Support Pressure
## 5 Conclusions  
This paper presents a theoretical study of the stability of rectangular tunnel boring faces, and by employing parameter analysis, the influence of various factors on the stability was investigated. The theoretical method was then applied to the Suzhou project for verification, and the model's reliability was evaluated. The following conclusions are reached:  
1. A silo-wedge model is used to analyze the instability zone under low pressure or no support conditions for the problem of uneven pressure fluctuations at the palm face. This is a new method for calculating the safety factor of the excavated face. In the analysis, the unstable wedge is horizontally striped, and the concept of strength reduction is introduced into the limit equilibrium analysis to derive the safety factor \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{3}{c}{**Computed Results**} & \multicolumn{3}{c}{**Field Monitoring Data**} \\ \hline \begin{tabular}{c} **Security** \\ **Coefficient** \\ \end{tabular} & \begin{tabular}{c} **Theoretical** \\ **Earth** \\ **Pressure/kPa** \\ \end{tabular} & \begin{tabular}{c} **Shutdown Clearance Phase** \\ **\(s_{1}\)/kPa** \\ \end{tabular} & \
\begin{tabular}{c} **Normal Driving Phase** \\ **\(s_{1}\), \(s_{2}\)/kPa** \\ \end{tabular} &
\begin{tabular}{c} **Maximum** \\ **Surface** \\ **Settlement/mm** \\ \end{tabular} \\ \hline
2.1 & 68.7 & 58–67 & 65–78 & 21 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of calculated and monitored values. Figure 11: Boulder removal. Figure 12: Surface settlement during opening. for the unstable zone. The implicit expression of the safety factor is then solved using the mathematical software MATHEMATICA. 2. A parametric analysis of the soil strength parameters and tunnel geometry parameters was performed using the solutions of this paper, showing that \(f_{\rm s}\) decreases as the open area increases because the larger that the open area is, the more soil that loses support, and the more likely that it is to slide out of stability. It is important to note that, when the opening ratio is greater than 0.75 and the friction angle within the soil is less than 20\({}^{\circ}\), the safety factor is less than 1, and the risk is greater. \(f_{\rm s}\) increases as \(\varphi^{\prime}\) increases because the shear strength of the tunnel envelope increases, and the more self-stabilizing that it is, the better that it is for maintaining stability. \(f_{\rm s}\) decreases slightly with increasing depth of burial because the greater that the depth of burial is, the greater that the overburden pressure is, and therefore, the greater that the susceptibility to instability and slumping is. Such a variation law is also consistent with [PERSON]'s solution and [PERSON]'s formula. 3. The solution of this paper is applied to the actual project, and the safety coefficient of the stability of the working face is calculated under the specific project-opening situation, verifying the reliability of this paper's solution. The paper proves that the solution can serve as a theoretical reference for additional projects of a similar type. Conceptualization, X.Z. and W.L. ; methodology, M.Y. and J.L. ; validation, M.Y., C.Y. and X.Z. ; investigation, J.L. ; writing--original draft preparation, X.Z. ; writing--review and editing, W.L., C.Y. and X.Z. ; supervision, W.L. 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 under grant number 51978430; Power China Huadong Engineering Corporation Limited (KY2019-ZD-02); the Key R&D Program of Zhejiang Province (2019C03111). Not applicable. The data presented in this study are available upon request from the corresponding authors. The authors declare no conflict of interest.