# Stability Analysis of Box Tunnel Working Face under Non-Uniform Support Pressure
## 4 Discussion  
In this section, the theoretical solution of this paper is verified based on an underground passage attached to a station of Suzhou Line 5 [37]. As shown in Figure 9, the rectangular tunnel was constructed by the jacking method with a cross-sectional dimension of 6.9 m \(\times\) 4.2 m. The tunnel length was about 62.8 m, and the overburden depth was about 4.5 m. A total of 42 rings were jacked by the rectangular jacking method. The rectangular boring machine used six independent spoked cutters for excavation and a double screw conveyor for soil discharge. The main soil layers traversed by the tunnel were: 1 the fill layer and 3 the powder clay layer, and the lower lying soil layer was 3 powder clay with powder sand layer. The physical and mechanical properties of each soil layer are shown in Table 3.  
The rectangular roadheader stops and opens the position when it encounters obstacles such as thrown rocks while advancing 28 and 29 rings, as shown in Figure 10. Three surface settlement monitoring points were set up at 42 m from the start shaft (1.5 m per ring), i.e., above the position between the pipe pieces of rings 28 and 29, at the position of the axis (here referring to monitoring point K2 on the drawing) and 4 m from both sides of the axis (here referring to monitoring points K1 and K3 on the drawing), as shown in Figure 11, and they were monitored once per day. A number of drift stones, up to 50 cm in size, were removed by opening the silo for clearance, as shown in Figure 12. The maximum settlement was about 21 cm at monitoring point K2 on the axis and about 9 cm at monitoring points K1 and K3 on both sides of the axis. According to the engineering conditions, we set the opening ratio as 0.35 and substituted the actual conditions of each project into the formula of this paper. The safety factor of this project can be calculated according to Section 2.1. The results are shown in Table 4.  
During the normal digging phase, the support pressure varied between 65 kPa and 78 kPa. During normal pressure opening during shutdown, the residual support pressure on both sides ranged from 58 kPa to 67 kPa. Due to the small number of openings, the high risks involved, and the limited construction site conditions, ground stability was mainly judged by surface settlement. Although surface settlement occurred during the opening process, the settlement was less than 30 mm, the development of the ground deformation tended to be smooth, and the pressure was sufficient to maintain ground stability. The actual engineering situation fitted the presumed safety factor of 2.1 in this paper, and the comparison verified the reliability of the current model.  
Figure 10: Layout of a project in Suzhou.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Silo Layer**} & **Unit Weight** & **Modulus of Compression** & **Cohesion** & **Frictional Angle** \\  & \(\gamma\)/ kN/m\({}^{3}\) & \(E_{\textbf{S0.1}-\textbf{0.2}}\)/MPa & \(c^{\prime}\)/kPa & \(\varphi^{\prime}\)/\({}^{\circ}\) \\ \hline \hline \⃝_1 Miscellaneous fill & 18.0 & 4.19 & – & 27.0 \\ \⃝_1 clay & 19.0 & 5.44 & 16 & 26.0 \\ \⃝_3 Clay silty soil with silty clay & 18.6 & 6.87 & 8 & 22.5 \\ \⃝_2 silt & 19.1 & 10.40 & 1 & 31.0 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Soil properties.  
Figure 9: Opening and clearing of a rectangular jacking pipe in Suzhou.