## 2 Similar Model Test Design
### Sample mold  
The tunnel is buried at a depth of approximately 20.0 m, with a diameter of 6.0 m. Based on the geometric similarity relationship between the engineering prototype and the indoor similar model, the scaled experimental tunnel has a burial depth of 1.0 m and a diameter of 0.3 m. To minimize the impact of the test model's boundary conditions on the experimental results and to accommodate the size constraints of the testing equipment, the sample size for the scaled test was determined to be 1.5 m in length, 1.5 m in height, and 0.5 m in thickness. The scaled test sample was created by filling soil into the mold depicted in Figure 2, which has an internal dimension of 1.5 m in length, 1.5 m in height,  
Figure 1: Scale test equipment diagram. and 0.5 m in thickness. The tunnel scaling test mold, as illustrated in Figure 3, comprises a tunnel mold and a sample box. The tunnel mold consists of a pressure plate, a force transfer rod, and a supporting structure, while the sample box is made up of a mold enclosure and high-strength plexiglass. Pins and stiffeners are incorporated to ensure that the plexiglass meets the testing requirements. Thanks to the transparency of the plexiglass, the deformation of the tunnel face during the shield propulsion process can be clearly observed. By taking into account the symmetry of the tunnel, this study considers the vertical plane passing through the center of the tunnel as the plane of symmetry. Consequently, only half of the tunnel is selected as the research object for the experimental investigation. (3) Rail model  
According to engineering practice and the feasibility of the research process, the overlying strata of the tunnel can be simplified into a 0.06 m thick gravel cushion, a 0.06 m thick cement-soil layer, a 0.78 m thick yellow soil layer, a 0.45 m thick silty clay layer, and a 0.15 m thick fine sand layer. In addition, there was Longhai Railway and Xulan high-speed railway on the ground surface. In order to consider the influence of vibration load during train passage on the formation-deformation process of the tunnel face during shield propulsion, a rail model similar to the train track structure was made (as shown in Figures 4 and 5), and the rail model was placed above the similar model of the tunnel. The effect of train vibration load on the overlying strata of the tunnel can be simulated by applying a simplified load to the rail model through the slope stability system. Figure 2: Tunnel scale test specimen mold (physical drawing). Figure 4: Rail model diagram. Figure 3: Tunnel scale test mold structure (three views). (4) Monitoring equipment  
During the scale test of the shield tunnel, the YC-VWE-type vibrating wire earth pressure gauge provided by Changsha Yunchao Information Technology Co., Ltd. (Changsha, China) was used to monitor the pressure change process and the final pressure distribution characteristics in front of the tunnel face and inside the overlying strata. The YC-VWE-type vibrating string earth pressure gauge has a measuring range of 0-3000 kPa and a resolution of \(\leq\)0.025%; it is composed of a backplane, an inductor, an observation cable, a vibrating wire, and an exciting electromagnetic coil. It is suitable for long-term monitoring of compressive stress inside geotechnical materials and is an effective piece of monitoring equipment to grasp the changes in soil pressure inside geotechnical materials in real time, which can meet the needs of this test. The layout of the earth pressure box in the mold is shown in Figure 6, which mainly focuses on the shield direction of the tunnel face and the soil layer above in order to monitor the change in soil pressure, which is mainly affected during the test.