## 1 Introduction  
Recently, urban subway construction in China has gradually expanded and an increasing number of cities have formed operational networks of subway lines. When a net of subway lines is built, many existing subway stations are turned into new transfer stations. During transfer station construction, the surrounding environment is complex. Construction tolerances at ultra-small distances and under-through construction are frequent, increasing the construction difficulty. Open excavation is a common construction method in station expansion, but the new structure problematically affects the existing structure. Many studies have been conducted on the interaction between foundation pit excavations and existing underground structures. [PERSON] et al. [1] studied the deformation of a tunnel near the foundation pit through finite element simulation of the foundation pit excavation. Subsequently, they found that the horizontal displacement of the tunnel was affected, whereas the vertical displacement did not change significantly. [PERSON] et al. [2] studied the deformation mechanism of an adjacent tunnel caused by foundation pit excavation using a calculation model and numerical simulation. The results showed that the stress concentration caused deformation of the tunnel lining. [PERSON] et al. [3] studied the force and deformation of adjacent tunnels after foundation pit excavation under soft-soil conditions. The results indicate that changing the excavation sequence can alleviate the internal forcesexperienced by existing tunnels. [PERSON] et al. [4] analyzed the deformation of a tunnel near the foundation pit using a computational model and found that the foundation pit excavation causes longitudinal deformation of the tunnel; the degree of deformation in the retaining structure directly affects the longitudinal deformation of the tunnel. [PERSON] et al. [5] used the Winkler model to calculate the additional stress due to foundation pit excavation under different strata compared to the existing tunnel and obtained a calculation theory that can predict the tunnel deformation. [PERSON] et al. [6] analyzed the mechanical behavior of a pipeline near a foundation pit and determined the influence of soil and pipeline parameters on the pipeline force. [PERSON] et al. [7] analyzed the reasons behind the damage inflicted on existing buildings caused by foundation pits and found that the settlement of existing buildings is related to the precipitation in the pits. [PERSON] et al. [8] analyzed the effect of a deep foundation pit on existing high-rise buildings by field investigation and numerical simulation and proposed to use retaining walls and additional piles to strengthen the foundation pit to control its lateral displacement. [PERSON] and [PERSON] [9] analyzed the displacement mode of high-rise buildings near foundation pits using three interaction models. The results indicated that excavation near the foundation pit may lead to an incline in buildings. [PERSON] et al. [10] analyzed the settlement law of existing buildings near the corner of the foundation pit. The results demonstrated that the deflection and deformation of existing buildings were limited under the pit-corner effect and a partition wall could reduce the settlement of existing buildings. [PERSON] and [PERSON] [11] analyzed the foundation pit under four working conditions using field monitoring data. The geometry of the foundation pit plays a key role in its stability. [PERSON] et al. [12] used specific cases to analyze the interaction between the foundation pit support system and adjacent high-speed railway line. Subsequently, they found that foundation pit excavation caused large settlement deformation to the nearby railway. [PERSON] et al. [13] proposed a method to analyze the effect of adjacent excavations on piles and verified the accuracy of the method by numerical and case analyses. Only a few scholars have investigated the interaction between foundation pit excavation and adjacent subway stations. [PERSON] et al.