## 1 Introduction  
The subway has become an important and indispensable vehicle in many big cities with the promotion of large-scale urbanization in China, and there is a trend of developing and constructing multilevel subway superstructures such as commercial, office, and residential buildings in the upper space of the subway to make full use of the land resources in the core of the city. With the construction of the subway superstructure, surcharge/unloading will inevitably occur on the left and right sides above the existing shield tunnel, adversely affecting the tunnel and the surrounding soil. Therefore, it is important to study the impacts of surcharge/unloading on both sides of the existing tunnel below.  
To date, several domestic and foreign scholars have conducted studies on the influence of surcharge/unloading above existing shield tunnels on the tunnel and the surrounding soil, including field measurement methods [1; 2; 3; 4; 5], model test methods [6; 7; 8; 9; 10; 11; 12], limit element analysis methods [13; 14; 15; 16; 17; 18; 19; 20; 21], theoretical calculation methods [22; 23; 24; 25; 26; 27; 28; 29; 30; 31], etc., among which the model test methods can simulate the actual engineering situation to the greatest extent, and the measured results are relatively accurate and reliable. [PERSON] et al. [6] studied the impacts of surcharge and excavation directly above a tunnel in soft clay on the ground settlement and the existing shield tunnel below via model tests. [PERSON] et al. [7] studied the influence of the surcharge above a tunnel in sand on the lateral and vertical deformation of the shield tunnel through model tests. [PERSON] [8] studied the influence of surcharge above a tunnel in sand on the additional confining pressure of the shield tunnel through model tests. [PERSON] et al. [9] studied the effect of surcharge position on the lateral deformation of shield tunnels in different soft and hard strata by means of comparative tests. [PERSON] et al. [10] studied the deformation mechanism of shield tunnels under different surcharge sizes, surcharge areas, and tunnel depths by means of indoor model testing, and also verified the reliability of the model test by means of finite element simulation. [PERSON] et al. [11] studied the deformation mechanism of shield tunnels by means of indoor model tests, with emphasis on the buried depth and location of the tunnel. [PERSON] et al. [12] investigated the stability changes of a tunnel under different surcharge and support conditions by means of model tests and theoretical calculations. The existing model test studies were all conducted for surcharge/unloading on one side of the existing shield tunnel, while there have been few studies of surcharge/unloading on both sides above existing shield tunnels, and the location of the surcharge/unloading and soil determine the adverse effects of surcharge/unloading on the tunnel and the surrounding soil. Therefore, it is necessary to adopt the model test method to consider the effects of two-sided surcharge/unloading, surcharge/unloading position, and soil changes on the existing shield tunnel and the surrounding soil.  
A large-scale indoor model test was conducted in this study to determine the influence of surcharge/unloading position and soil changes on the lateral convergence, additional surrounding pressure, additional soil pressure, and ground shift of an existing shield tunnel in the event of surcharge/unloading on two sides of the shield tunnel, in order to obtain a better understanding of the possible adverse effects of surcharge/unloading on two-sided subway superstructure construction.