## 1 Introduction  
River dredging is an important measure to improve urban flood control systems and enhance flood resilience. When the river channel is adjacent to existing structures, the unloading effect caused by dredging and excavation will have a significant impact on the structural buildings near the river channel. The interaction between excavation and retailing structures is very complex, and excavation unloading will lead to the destruction of the original ground stress equilibrium so that the soil's stress state is constantly changed until a new equilibrium is reached [1; 2; 3; 4]. The reduced column strength caused by the process of re-equilibrium will give rise to the change in soil pressure on the supporting structure, so the development rule of supporting structure deformation needs to be simulated and predicted [5; 6; 7] to ensure the stability of the supporting structure. Moreover, the foundation of the adjacent existing buildings will generate additional stress and lateral displacement because of soil displacement after excavation [8; 9; 10; 11; 12; 13], and the deformation of the adjacent buildings will be most significant when the foundation pit is dug into the bottom area [14]. [PERSON] et al. [15] found that soil erosion between supporting piles is the main element resulting in the uneven settlement of adjacent shallow foundation buildings. [PERSON] et al. [16] found by means of numerical calculation that the excavation of the foundation not only causes additional horizontal displacement of the adjacent existing wall but also makes the retaining wall be lifted unevenly which is caused by the excavation rebound effect. At present, in terms of the influence of soil excavation on adjacent existing structures, studies have been conducted at home and abroad in the field and are described below: the horizontal displacement of the pile foundation of a building will gradually change with time and exert adverse effects on the building [17; 18], the settlement of the roadbed will increase rapidly during the construction period and affect the safety of traffic [19], the tunnel will produce uneven deformation thus induce various kinds of problems [20; 21; 22; 23], and the disturbance of the surrounding strata will produce additional internal forces and deformation in station and then affect its normal operation [24], damage the underground pipeline, and threaten its safety [25; 26]. However, there are few reports that cover the interaction between the supporting structure and the adjacent existing gravity retaining wall during river dredging excavation today.  
During dredging excavation support, compared with traditional pile types, U-shaped concrete sheet piles are widely applied because of their great bending resistance, large retaining width, and easy and fast construction [27]. The cross-section of the USCSP is similar to the I-beam one, and its bearing capacity can be calculated by an equivalent method [28]. This paper uses a large number of numerical simulations to study the interaction effect of the USCSP and an adjacent existing gravity retaining wall during dredging excavation, explores the influence of sheet pile cross-sectional size, pile length, gravity retaining wall height, and pile wall spacing on the bearing performance of the supporting structure and the existing retaining wall, and aims to provide theoretical guidance for both the design and construction of similar projects.