## 5 Conclusions  
In this research, a super-large diameter shield twin tunnel was reported. The shield twin tunnel under-crossed a deep foundation pit which contained an underground structural with a raft-pile foundation.  
Influences on the ground subsidence curve from different construction methods were studied in this paper. Eight numerical models were established to explore these influences. One of these models was consistent with the practical project, aiming to validate the correctness of these numerical models. Through these eight numerical models, we drew the following conclusions:  
(1) For the super-large diameter shield tunnel, the effect on controlling ground subsidence of synchronous grouting was better than backwall grouting, because synchronous grouting is timelier for soil support. Backwall grouting can grant the soil a higher Young's modulus, which can provide more horizontal resistance force in the horizontal direction. Therefore, combining the two grouting methods can obtain more ideal results on controlling pit-bottom ground subsidence.  
(2) The excavation of super-large diameter shield tunnels may cause uplift of the surface soil, and proper backfilling can control the uplift value.  
First, when an extra-large diameter shield tunnel passes beneath a foundation pit or a lightweight underground structure, if the weight of the underground structure is insufficient to counterbalance the unloading effects caused by the excavation of the foundation pit and the tunnel this may result in upward movement of the surface soil at the bottom of the foundation pit.  
Second, in the construction of an extra-large diameter shield tunnel the construction parties often tend to be conservative, and an excessive amount of grouting can cause soil expansion and subsequent uplift of the superficial soil.  
Third, due to the continuous increase in the groundwater level in Beijing, after the completion of the engineering project a persistent rise in the water level may lead to the uplift of the tunnel, thereby introducing uplift of the superficial soil. This aspect requires close attention.  
(3) During tunnel construction, the closer the two shield machines are, the greater the settlement at the bottom of the foundation pit will be; however, the difference in the final total subsidence is small.  
In addition, when combining our eight models we found that the studied project has very unique load characteristics. These load characteristics have a greater impact on the shape of the surface settlement curve. To solve this problem, two correction coefficients were proposed to correct the surface settlement curve. Following the inverse analysis method, the modified settlement curves of single- and double-track tunnel excavation were obtained. These two formulas can well reflect the settlement law of the soil at the foundation pit bottom under-crossed by the super-large diameter shield tunnel.  
Furthermore, this soil settlement prediction provides a possible way for rapid calculation of the negative skin friction distribution, which enables engineers to take targeted protective measures for areas that may be heavily affected by tunnel excavation.  
Validation, Y.L. and H.L.; Investigation, Y.J., X.G. and H.L.; Resources, Y.J.; Data curation, Y.L. and X.G.; Writing--original draft, Y.J. and Y.L.; Writing--review & editing, Y.L. and A.Y.; Funding acquisition, A.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by [National Natural Science Foundation of China] grant number [51578023] and the APC was funded by [[PERSON]].  
Not applicable.  
The data used to support the findings of this study are available from the corresponding author upon reasonable request.  
The authors declare that there are no conflict of interest regarding the publication of this paper.