## 4 Numerical Simulation Results Analysis
### Impact of Excavation in Individual Stages on the Surface Subsidence above the Pipe-Rof Structure  
The whole project was roughly divided into three stages. The first stage was the completion of the working well (and the impact caused by deep excavation), the second stage was the completion of the construction of the pipe roof, and the third stage was the completion of the excavation of the underpass (and the impact caused by the box culvert excavation). There were many factors affecting the deformation around the base due to deep excavation and box culvert excavation in the project; for example, the geological conditions, excavation methods, support types, construction methods, etc., were highly correlated with the size and distribution of deformation. In the general PPM construction process, there is a complex interaction between the inner connection pipes, which in turn affects the closure of the pipe roof and surface settlement [9]. The pipes used in this project were designed with mortise and tenon joints, which allows the pipes to be tightly fixed to each other, thus resisting the surrounding soil and reducing ground subsidence. There are several geometrical cross-section design options for box culvert constructions of rectangular cross-section. Under the condition of the same sectional area, the influence of different aspect ratios on ground surface subsidence is uncertain, and there is a lack of past cases for reference. Therefore, although the actual section aspect ratio used in this case was 0.36, three other different aspect ratios (0.30, 0.40, and 0.45) of the box culvert were used to compare their effects on the surface settlement. Table 13 shows the surface subsidence values above the pipe-roof structure with four different aspect ratios during excavation in individual stages. The maximum surface subsidence caused by the construction of the working well accounted for about 35-40% of the total subsidence. The maximum surface subsidence caused by the completion of the pipe-roof construction accounted for about 10-20% of the overall total subsidence. Compared with other stages, the amount of subsidence caused by this stage was mainly due to the filling of concrete and backfill grouting immediately after the construction of the  
Figure 23: Lateral displacement curves for the diaphragm wall at monitoring point 3: (a) numerical analysis results in different excavation stages; (b) comparison of the lateral displacement and warning value. pipework on site, so as to avoid the occurrence of large surface subsidence due to the rapid drop of the upper water level and soil loss in this stage. In software analysis, the surface of the pipe-roof structure was provided with interface properties, just like the pavement of an impermeable layer. The maximum surface subsidence caused by the construction of the underpass accounted for 30-50% of the total subsidence. The surface subsidence in this stage was caused by the excavation of the box culvert. Before the excavation of the box culvert, the interior of the soil was in a state of original stress. After excavation, this stress state was broken, the surrounding soil deformed towards the inside of the box culvert, the original stress of the surrounding soil gradually decreased, and the box culvert and the stratum deformed to a certain extent. After the support was applied, the interaction between the surrounding soil and the support reached a state of deformation harmony, and the internal stress also reached a state of equilibrium, resulting in the final amount of surface subsidence. The analysis results showed that the pipe-roof structure was suitable for underground excavation with shallow-buried depth in the soft soil of the Taipei Basin. On the other hand, the subsidence curves of the ground above the box culverts with different aspect ratios excavated in different stages are shown in Figures 24-27.