# Stability Analysis of Box Tunnel Working Face under Non-Uniform Support Pressure
## 2 Materials and Methods  
### Pressure Distribution Characteristics in Rectangular Tunnels  
A rectangular tunnel was bored through the ground, as shown in Figure 1a. The working face of the rectangular tunnel had a height of \(D\), a width of \(L\), and a burial depth of \(C\). The overload on the ground is expressed as \(\sigma_{8}\) and is assumed to be \(\sigma_{8}\) = 0 kPa considering that, in practice, road traffic above the working face will be prohibited when clearing. The rectangular tunnel section was large, the boring machine used multiple cutters, the opening rate was high, and the pressure in the earth bin was prone to wave movement and uneven distribution. Based on the characteristics of the cutter disc and screw outlet, the cut soil was discharged through the bottom outlet. Figure 1b shows that, as the soil was discharged from the area near the outlet of the double screw excavator, assuming that the soil in the silo had been modified to have better fluidity, the upper part and the surrounding soil then collapsed so that the central region was more affected, and the remaining two parts were much less affected, the residual soil served as a support for the working surface, and the pressure was stabilized at \(s_{1}\); the air pressure in the central region was low and unstable. The pressure fluctuation zone was formed, and the pressure \(s_{2}\) was smaller than \(s_{1}\). Therefore, the working face was supported non-uniformly, and the support pressure had a distribution that was large on both sides and small in the middle. Based on this distribution, the working face was divided into three sections, and to simplify the calculation, the central irregular empty area was simplified into a rectangle, as depicted in Figure 1c. The width of the central fluctuation zone was B, while the support pressure in the stable zone was s1. This paper assumes that the geology was homogeneous and obeyed the linear Moore-Coulomb criterion. The effective internal friction of the soil body was \(\varphi^{\prime}\). The effective cohesion was \(c^{\prime}\) and the soil weight was \(\gamma\).  
Tunneling for the T221 project in Singapore [35] was suspended multiple times due to encountered obstacles, resulting in a reduction in support pressure. As illustrated in Figure 2, rectangular pipe jacking machines were equipped with man bins to open the bins under pressure to remove obstacles, such as steel fragments, from the soil bins. As a result, the pressure increased, and settlement was better controlled during the opening process. However, due to cost constraints, the majority of rectangular boring machines were not equipped with man bins; consequently, bin opening was usually performed under atmospheric pressure, which is dangerous. In the case of an underground project connected to a station on Suzhou Line 5, the pipe jacking machine was dug into the soft soil and encountered block-filling during construction, resulting in poor soil discharge. The jacking machine was not equipped with a human silo, so the spiral earth-exit was eliminated, and the obstacle was removed through the bottom silo door. This process led to a stability study for the most unfavorable case of opening the bunker (\(s_{2}\) = 0, \(s_{1}\
eq 0\)).