## 3 Research Methods
### Numerical Simulation Modeling
#### 3.2.2 Design of Working Conditions  
Two working conditions were designed in this study to comparatively analyze the effects of different reinforcement measures on surface settlement:  
1. No reinforcement working condition: no soil reinforcement measures are taken during tunnel excavation.
2. Flower tube grouting reinforcement condition: A single row of flower tube grouting reinforcement is used within 30 m above and on both sides of the centerline of the tunnel, with a grouting depth of 24 m and a grouting interval of 9 m. The tunnel is reinforced with a single row of flower tube grouting reinforcement.  
Through numerical simulation, this study compares, in detail, the change rules of tunnel force and surface settlement under the two conditions to evaluate the effect of flower tube grouting reinforcement.  
Based on the stratigraphic information of the study area, numerical modelling of the actual soil conditions was carried out in this study using PLAXIS 3D software. The soil body was modelled using the H-S small-strain principal model, and the soil body was initially modelled as a rectangular body with dimensions of 84.5 m (length) \(\times\) 50 m (width) \(\times\) 50 m (height). In the numerical simulation, an elastic model is used for the pipe sheet, grouting material and shield shell, and a Mohr-Coulomb model is used for the soil body. According to the above parameters, the soil layer was set up in this study using PLAXIS 3D software. The specific steps are as follows: firstly, the distribution of each soil layer is set according to the information of borehole location and soil burial depth; secondly, the data of borehole location is inputted in combination with the function of the software to accurately reflect the actual geological conditions. Through the above steps, the distribution of underground soil layers was successfully simulated in this study.  
A shield tunnel with a diameter of 6.2 m is arranged in the model, and the tunnel boring length is set to 66 m, with 1.5 m boring each time, totaling 44 rings. In order to combine with the actual project, the tunnel modeling direction is set as downward-sloping, and the angle with the x-axis horizontal direction is 4\({}^{\circ}\). Considering the symmetry of surface settlement, only half of the tunnel structure is modeled in this study to simplify the calculation and improve efficiency. Finally, the numerical simulation structure built in PLAXIS 3D software is shown in Figure 5. The depths of the boreholes corresponding to the soil layers are shown in Table 3.  
According to the actual construction process of the tunnel, the numerical simulation mainly includes two parts: surface grouting reinforcement and shield tunneling. In the simulation process, a 1.5 m wide ring of tubes is used as a step, from excavation to complete penetration. The specific steps are as follows: firstly, grouting reinforcement is carried out, and the parameters of the reinforced soil body are used in the numerical simulation; secondly, the soil layer distribution, cell structure, and grid division are defined; and finally, the construction process is simulated in phases, including the steps of soil excavation, pipe sheet installation, and grouting reinforcement.