## 4 Comparative Analysis of Top-Down Method and Cut-and-Cover Method
### Comparative Analysis of Simulation Results  
#### 4.2.1 Foundation Pit Deformation  
Figure 10 illustrates the horizontal displacement of the diaphragm wall and the ground surface settlement outside the pit for both the cut-and-cover method and the top-down method. As depicted in Figure 9(a), the deformation patterns of the diaphragm wall resulting from both methods exhibited similarities, characterized by a \"bow-shaped\" pattern with pronounced deformation in the middle and small deformations at both sides. The maximum deformation of the diaphragm wall associated with the cut-and-cover method reached 49.35 mm, which was generally larger than that observed with the top-down method. This difference can be attributed to the fact that the top-down method uses the floor slab of the main structure as the support framework for the foundation pit. This configuration provided enhanced support stiffness compared to the cut-and-cover foundation pit, which relied on concrete and steel struts. Consequently, this increased stiffness exerted a damping effect on the inward movement of the diaphragm wall towards the pit. Therefore, in the context of this project, the top-down method demonstrated superior performance in controlling the deformation of the retaining structure compared to the cut-and-cover method. Further analysis of Figure 9(b) shows that the ground surface settlement associated with both construction methods tends to follow a typical \"concave groove\" shape. The maximum ground surface settlement caused by the cut-and-cover method reaches \(-\)48.19 mm, which is slightly smaller than the deformation produced by the top-down method. Analysis of the settlement curve indicated that, on the left side of the \"concave groove\" shape, the settlement resulting from the top-down method was larger than that of the cut-and-cover method. Conversely, on the right side of the \"concave  
Figure 9: Numerical model of Top-Down Method foundation pit.  
groove\" shape, the settlement from the top-down method was smaller than that from the cut-and-cover method. This observation further supports the conclusion that the top-down method is superior to the cut-and-cover method in controlling the deformation of the surrounding environment.  
In conclusion, the comparative analysis indicated that the top-down method is superior in controlling the deformation of the retaining structure and mitigating the effects of construction on the surrounding environment. However, the deformation caused by the top-down method was recorded at 0.23% H (where H is the excavation depth of the foundation pit), which far exceeded the specified limit of 36.0 mm, as outlined in the  
Figure 10: Deformation of foundation pit for both construction methods.  
relevant regulations (where the maximum lateral displacement of the diaphragm wall for a first-class foundation pit is 0.18% H [24]). It can be known that the cover excavation method also cannot meet the safety standards. Therefore, in regions with stringent deformation control requirements, it is recommended to implement servo steel supports or to adopt a construction scheme that integrates multiple construction methods.