## 5 Numerical Modeling
### Results of the Numerical Study and Discussion  
#### 5.3.1 Surface Settlement  
In accordance with the construction sequence of Northeastern Road subway station, the whole construction simulation was divided into 6 stages, as follows:  
(1) Stage 1: Construction of the side pilot tunnels Z5 and Z6;  
(2) Stage 2: Construction of the pilot tunnels Z1 and Z2 and cylindrical steel column C1;  
(3) Stage 3: Construction of the pilot tunnels Z3 and Z4 and cylindrical steel column C2;  
(4) Stage 4: Construction of the top plate at mid-span;  
(5) Stage 5: Construction of the station hall;  
(6) Stage 6: Construction of the platform layer. The soil excavation was simulated in the subway construction stage, which causes stress redistribution of in the surrounding soil and surface settlement. A key point on the centerline of the cross-section, which is 15 m away from the initial excavation section was selected as the typical monitoring point, and the behavior of surface settlement is studied in the construction procedure. Comparison of surface settlements from in-situ monitoring and the numerical simulations is shown in Figure 24. As can be seen from the figure, the maximum ground settlement recorded in the site is around 12 mm occurred at the end of stage 5 of the construction. The ground uplift of 1.5 mm was observed after the platform layer is constructed mainly due to the heave of the soil under the supporting structure. Figure 25 shows a reasonable agreement between the field data and the numerical simulations given the complexity of the nature of the problem. Based on the finite difference model (FDM) of the construction, the ground settlement profile at the end of each construction stage was obtained, as shown in Figure 26. The following observations can be made:  
End of stage 1: The maximum monitored ground settlement is 2.2 mm, which accounts for around 20% of the maximum total ground settlement. Soil disturbance due to the construction of the side pilot tunnels are clearly seen. End of stage 2: The total ground settlement is increased to 6.1 mm accounting for 33% of the maximum ground settlement. The maximum ground settlement is 2.5 mm due to the excavation of pilot tunnel Z2 and is 0.65 mm due to the excavation of pilot tunnel Z1. The maximum ground settlement increases 0.7 mm due to the construction of column C1 and excavation of soils surrounding  
Figure 25: Comparison of surface settlement between in-situ monitoring and numerical simulations. it. The ground settlement due to the construction of pilot tunnel Z2 accounts for 65% of the ground settlement of this stage, indicating that the excavation of lower pilot tunnels results in a considerable portion of the ground settlement when STS pipe-roofing supporting system is used. The initial lining acts as supporting points for pipe-roof structure, which reduces the span of horizontal pipe-roofing system during the excavation of upper pilot tunnels. The large stiffness of the STS support system prevents large ground settlements. End of stage 3: The cumulative ground settlement is 8.3 mm. The ground settlement increases 2.2 mm at this stage which accounts for around 20% of the maximum ground settlement. The contribution to the final ground settlement reduces in this stage compared to stage 2, most probably because the left side of supporting system is already complete and settled during the construction of stage 3, which reduces the effective lateral span of the pipe-roofing system, therefore increasing its rigidity, resulting in less ground settlement. End of stage 4: The cumulative ground settlement is 11 mm. The ground settlement increases 2.7 mm in this stage, which accounts for 23% of the maximum ground settlement. The vertical loading is transferred to the beam and pile system due to the removal of the linings on the pilot tunnels, which is the cause of the settlement in this stage. End of stage 5: The cumulative ground settlement is 11.6 mm, and the ground settlement increases 0.52 mm which accounts for 5.16% of the maximum ground settlement.