## 4 Discussion  
### Influence of Load Characteristics on Settlement Curve  
In Section 3, results of the eight cases have been plotted and thoroughly discussed. Combining the characteristics of this practical engineering and super-large diameter shield tunnels, there are a number of issues worth discussing. In this research, the influence of a super-large diameter shield tunnel under-crossing the foundation pit on the settlement curve of the soil at the bottom of the foundation pit was studied. Therefore, the unique load characteristics of the project have a great impact on the settlement curve, mainly in the following three aspects. (1) When the shield tunnel under-crosses the foundation pit, the asymmetry of the supporting structures on both sides of the foundation pit lead to deviation of the settlement curve. The supporting structure of the foundation pit in this practical engineering is the diaphragm wall, which is a cantilevered retaining structure. According to [PERSON] [30], the actual earth pressure distribution of the soil under foundation pit can be seen in Figure 9.  
Figure 8: Pit bottom subsidence with different distances between shield machines. When a cantilevered retaining structure exists, the peak value of the passive earth pressure of the soil under the foundation pit is located near the ground surface of the pit bottom. Therefore, when the twin tunnel is excavated, the asymmetry of the supporting structures on both sides of the foundation pit can lead to non-negligible horizontal displacement of the ground surface at the bottom of foundation pit. This explains the reason for the deviation of the settlement curve in Section 3 from the perspective of soil mechanics. (2) When the grouting effect is ideal, because the soil loss is small, the super-large diameter shield tunnel may cause upward displacement of the soil due to unloading. In practical engineering, ground subsidence is always induced by soil losses. The soil losses are induced by the gap in the tail of shield machines. From the perspective of soil mechanics, tunnel excavation causes soil unloading. When the rebound effect caused by soil unloading is less than the total weight of the segments, the deformation law of the soil above the tunnel results in settlement. Therefore, when the tunnel diameter is small, this unloading rebound effect is often ignored by researchers. However, in terms of the super-large diameter shield tunnel in this project, this unloading rebound effect cannot be ignored. When the tunnel grouting support effect is good, the soil mass floating caused by this unloading rebound will be more obvious. (3) When the shield tunnel under-crosses the foundation pit, the piles in the raft-piles foundation can strengthen the soil and reduce the actual loss rate of the soil. In this project, the piles in the pile raft foundation are friction piles. Therefore, there is a strong interaction between the pile and the soil. When the tunnel under-crosses the pile, the friction between the pile and the soil can strengthen the soil, which can reduce the actual settlement of the soil on the surface of the bottom of the foundation pit. According to [PERSON] (1969), the actual ground settlement caused by the excavation of a single shield tunnel under the non-grouting situation in this project can be expressed as in Equations (1)-(3). \[S_{(x)}=S_{max}\cdot\exp\{-\frac{x^{2}}{2i^{2}}\} \tag{1}\]  
\[\mathrm{i}=\frac{Z}{\sqrt{2\pi\tan\bigl{(}45^{\circ}-\frac{\varphi}{2}\bigr{)}}} \tag{2}\]  
\[S_{max}=\frac{V_{s}}{i\cdot\sqrt{2\pi}} \tag{3}\]  
In Equations (1)-(3), \(V_{s}\) is the soil volume loss during the excavation of the shield tunnel, i is the distance between the tunnel center line and the bending point of the settlement curve, \(\varphi\) is the internal friction angle of the stratum where the tunnel is located, and \(Z\) is the distance from the surface to the center of the tunnel.