## 1 Introduction  
For improvements to China's railway network, new railway sections are connected to existing railway sections. Therefore, construction near existing lines is increasing every year [1, 2]. When a subgrade structure is widened, geosynthetic materials, lightweight fillers, and new retaining structures are required to minimize the impact of the new embankment structures on existing structures [3, 4]. The integration of the new and existing sheet-pile-wall retaining structures is challenging. The new reinforcement structure cannot affect road operations. The new and existing structures should have high structural strength to meet the force and deformation requirements. It is critical to clarify the force mechanisms of the new and existing retaining structures and the synergistic interactions.  
The sheet pile wall is an improvement on the anti-slip pile to retain a slope. Retaining plates are placed between the anti-slip piles. The sheet pile wall retains an embankment by transferring the earth pressure of the cantilevered section to the foundation anchored in the ground. The lateral displacement of the pile causes the resistance of the foundation soil in front of the pile to achieve a mechanical balance [5]. [PERSON] [6] analyzed the influences of the pile length, stiffness, position, and soil parameters on the synergistic effect of a sheet pile wall using numerical simulations and field measurements. An analysis of the earth pressure on both sides of the pile body indicated that the optimal pile length was 14 m. [PERSON] [7] analyzed the influences of the pile stiffness and the anisotropy of soft clay on the lateral earth pressure of the sheet pile wall using numerical calculations. In a cantilever sheet pile wall, the displacement of the pile top commonly exceeded the standards, requiring a reduction in the size of the pile section. [PERSON] [8] analyzed the changes in the pile bending moment, pile displacement, and bolt reaction force of a double-anchor sheet pile wall during the excavation of the surrounding soil and proposed optimization parameters. [PERSON] [9] used numerical analysis to investigate the horizontal displacement and bending moment of sheet pile walls with different pile lengths, optimized the number and length of short piles, and designed a reliable and cost-effective retaining structure. [PERSON] [10] and [PERSON] [11] each proposed a calculation model for a double-row sheet pile wall under horizontal loads and used model tests to obtain the wall deformation and bending moments to verify the reliability of the calculation models. Combined with an actual project, [PERSON] [12] analyzed the function of a stepped pile group and optimized the parameters. [PERSON] [13] used numerical modelling to analyze the mechanical behavior of an anchored sheet pile wall during excavation. Different types of sheet pile walls and structural combinations are used in different terrains. This study proposes a sheet pile wall consisting of new and existing components for the third and fourth lines of the Zhongwei-Lanzhou Passenger Line connected to the Lanzhou-Xinjiang Line, as shown in Figure 1. The purpose of the project is to ensure that the superstructure has sufficient width. Differently from finite element and discrete element analysis methods, laboratory model testing has guiding significance [14; 15].  
A laboratory model test was carried out to simulate the excavation and compaction. The earth pressure distribution of the composite retaining structure was investigated in different construction stages, and the internal force and horizontal displacement of the composite sheet pile wall were analyzed. The earth-pressure ratio between the existing pile and the new pile was determined. The synergistic interaction between the new and existing components was evaluated. The research results can provide technical support for the construction of the new and existing components of sheet pile walls used in railway-engineering applications.