## 2 Engineering Simplification and Safety Factor Calculation  
### Engineering Simplification  
In the actual excavation conditions of the Guiqiao Road subway station, the corresponding weak soil needs to be excavated inside the maintenance structure of the pipe curtain, and the stability of the soil in front of the excavation has an important influence on the construction safety, so it is important to analyze and ensure its stability for the construction safety. During the construction of this project, the excavation plan of the station is shown in Figure 1.  
Under the excavation condition of this project, because the surrounding pipe curtain has a better supporting effect on the soil, the surrounding pipe curtain is simplified as a fixed boundary; that is, the soil at the palm face will not be deformed on both sides of the tunnel but only in the tunnel longitudinal deformation, because in the excavation process, the soil at the upper part of the palm face also needs to play a supporting role for the upper part of the pipe curtain, so the role of the pipe curtain on the palm face is simplified. Therefore, the effect of the pipe screen on the palm face is simplified to a uniform load acting on the soil in front of the palm face. In summary, the excavated soil in front of the palm face can be simplified and analyzed as a horizontally reinforced slope system with the upper part acting as a load, and the corresponding form is shown in Figure 2.  
As shown in Figure 2, the slope reinforcement is carried out by the MJS (metro jet system) method, which is also known as the omni-directional, high-pressure jetting method. Based on the traditional high-pressure jet grouting process, the MJS method adopts a unique multi-hole pipe and front-end causing device, which realizes the mandatory slurry discharge in the holes and the monitoring of the in-ground pressure and adjusts the amount of mandatory slurry discharge to control the in-ground pressure so as to make the deep slurry discharge and so that the in-ground pressure is reasonably controlled. The MJS method is widely used in soft soil reinforcement, and the stabilization of internal pressure during construction reduces the impact of the MJS rotary pile construction on  
Figure 1: Guiqiao Road Station general plan. the surrounding strata and structures, which is especially suitable for excavation and reinforcement in urban underground space. After sampling of the MJS reinforcement, it can be seen that the extracted core samples are in the form of columns, the overall mixing of core samples is more uniform, the cement cement cementation degree with the soil is better, the cement content of the core samples is high, the pile body is complete and its own strength is higher, and the internal friction angle and cohesion of the soil body are obviously improved after reinforcement, which shows that the MJS method of reinforcement has achieved a good result in the soft soil stratum. The MJS method hydraulic soil core samples are shown in Figure 3.  
In the actual engineering excavation process, it is necessary to set a suitable and economic solid arrangement scheme on the excavation face inside the tunnel to ensure the stability of the project excavation. During the construction of Guiqiao Road Station, the size of the corresponding excavation section is 22.4 m \(\times\) 7.8 m, which is expanded compared with the outsourcing size of the structure considering the deformation of the upper part of the pipe jacking and construction errors. In order to ensure the stability of the excavation face during the construction process, the corresponding reinforcement was constructed inside. In the actual construction process, the diameter of the MJS (metro jet system) zone is usually above 1m, and when the diameter of the MJS zone is 1m, different geometric arrangement parameters and different geometric parameters will have a certain influence on the safety and stability of the slope.