## 5 Conclusions  
FLAC\({}^{\text{3D}}\) software was used to carry out a numerical simulation analysis of the interaction between the USCSP supporting structure and the adjacent existing gravity type retaining wall during dredging excavation, and the following conclusions were obtained:  
* It is necessary to comprehensively consider the overall structural safety in line with the internal force and deformation of supporting piles and retaining walls. The increase in sheet piles is able to significantly reduce the horizontal displacement of the pile body and the existing retaining wall after excavation but increases the pile body stress and the retaining wall back soil pressure at the same time. Therefore, the pile length design should be a comprehensive process to combine the pile body material strength with existing retaining wall stability.
* When the cross-sectional size of the USCSP increases, the bending resistance of its cross-section is improved, and the horizontal displacement at the pile's top and the stress in the pile is reduced. In addition, the horizontal displacement of the retaining wall gradually decreases, and the tendency of rotation gradually slows down. The type I plate is not suitable for application in this project and cannot meet the tensile strength requirements of C60 concrete.
* With the pile-wall spacing expanding, the interaction effect between the supporting structure and the retaining wall caused by excavation unloading is gradually weakened, the horizontal displacement of the pile body, pile stress, and horizontal displacement of the retaining wall are gradually reduced, and the soil pressure of the retaining wall's back is increased.
* As the retaining wall grows, the passive resistance of the soil in front of the wall grows when the retaining wall retains its stability, which results in a constant increase in the horizontal displacement of the pile and pile stress after excavation. In this study, a new type of support structure, a U-shaped concrete sheet pile, was used for numerical simulation to provide guidance for the future application of U-shaped concrete sheet piles in various support projects. Due to the limitations of the numerical simulation software, it is impossible to fully consider the various influencing factors in real engineering. Thus, this study focuses on analyzing the effects of overall structural safety.  
J.Y. participated in the writing of the manuscript, the calculations, and data analysis; Z.Q. participated in the development of the calculation plans and the manuscript revisions and provided funding support; N.J. participated in the scheme discussion, research, basic data collection, manuscript revision, and provided financial support; [PERSON] and Z.C. participated in the scheme discussion, on-site investigation, basic data collection, and hydrological data organization and analysis. Y.N. participated in the scheme discussion, software calculation guidance, manuscript revision, and data proofreading; and Y.Z. participated in the establishment of numerical models, software operation, and the manuscript format review. All authors have read and agreed to the published version of the manuscript.  
This study was supported by the Key R&D projects in Zhejiang Province (2021C03019).  
This study did not involve humans or animals.  
Not applicable.  
The authors declare that they have no conflict of interest in this work.