## 3 Numerical Simulation of Open Excavation and Added-Stories Construction
### Deformation of the Retaining Structure  
For the existing station, part of the foundation pit supporting piles are end-suspended piles on the top of the station, and part of those are embedded piles of the adjacent station. The horizontal displacements of the embedded piles and end-suspended piles were analyzed via numerical simulation to explore the deformation mode of the retaining structure affected by the existing structure.  
Pile Z1 was a longitudinal midspan embedded pile in the foundation pit. Figure 9a shows the horizontal displacement curve during the excavation of pile Z1. As shown in the figure, the overall deformation of the embedded pile causes bulging in the medium and there is no significant change in the horizontal displacement of the top of the embedded pile under the three-step excavation, indicating that the first steel and concrete supports are effective. When the excavation depths are 4, 8.7, and 12.2 m, the maximum horizontal displacements are 1.06, 2.14, and 2.67 mm, respectively. Accordingly, the maximum horizontal displacements at excavation depths of 4 and 8.7 m increase significantly, and the embedded pile descends to the pit bottom at the maximum displacement. Therefore, monitoring and measurement are necessary during the third excavation stage to ensure the stability of embedded piles.  
Pile Z2 was a horizontal midspan end-suspended pile in the foundation pit. Figure 9b shows the horizontal displacement curve during the excavation of pile Z2, which indicates that the horizontal displacement of the bottom pile can be controlled when the excavation reaches 4 and 8.7 m. When the excavation reaches 8.7 m, the maximum horizontal displacement occurred in the upper part of the pit bottom, and horizontal displacement occurred at the pile bottom. The unexcavated soil at the pit bottom poses certain restrictions on the horizontal displacement of the bottom of pile Z2. When the excavation depth reaches 12.2 m, no soil was embedded in the bottom of the end-suspended pile, which led to a displacement of the pile bottom toward the inner side of the foundation pit, with a maximum horizontal displacement of 4.85 mm. Figure 9c shows a comparison curve for the horizontal displacement during the excavation of pile Z3. Pile Z3 was an end-suspended pile horizontally located at 1/4 of the foundation pit. Unlike pile Z2, pile Z3 was located between two steel diagonal braces. Observe from the figure that when the excavation depth reached 8.7 m, the maximum horizontal displacement of the pile was 2.72 mm. When excavation depth reached the pit bottom, the horizontal displacement of the pile bottom was 6.48 mm. Based on the horizontal displacement of the end-suspended piles Z2 and Z3, we found that the steel diagonal brace restricted the horizontal displacement of the middle and upper ends of the end-suspended pile; however, the displacement of the pile bottom was not effectively controlled, evident by the existing slip deformation. The horizontal displacement of the end-suspended pile bottom between the two diagonal steel braces was significant. Therefore, necessary security controls need to be implemented.  
Figure 8: Comparison diagram of effective plastic strain before and after excavation: (**a**) before excavation; (**b**) after excavation.