## 1 Introduction  
With the development of subway construction in China, the construction of shallow underground space is becoming increasingly congested, leading to frequent overlapping between multiple tunnel lines [1; 2; 3; 4]. One of the more prevalent overlapping cases is the orthogonal crossing of new subway tunnels beneath existing tunnels [5; 6; 7]. Especially in the case of an orthogonal crossing during shield tunneling, the change in the relative position of the two tunnels and the coupling effect of the support and unloading of existing tunnels induce complex forces and deformation mechanisms in the soil of the excavation face. This presents great challenges to the shield excavation of urban subway tunnels. Numerous academics have conducted relevant finite element simulation and model test assessments to address the structural deformation and the safety and stability of new tunnels crossing under/above existing tunnels. Scholars have combined 3D finite element analysis with model testing to determine the impact zones of new tunnel construction on existing tunnels [8; 9]. Relevant finite element studies also show that if the impact of the construction joints on the existing tunnel is taken into account, then its shading effect on the new tunnels is weakened [10]. Furthermore, relevant studies have found through numerical simulation that when the new tunnel is inclined to pass under the existing tunnel in close proximity, the settlement and torsional deformations of the existing tunnel are greatest in the area of the intersection [11]. Some scholars have explored the impact of different construction sequences and digging speeds on the internal force variation of existing tunnels and on surface deformation during overlapping shield tunneling with multiple lines using physical modeling experiments [12; 13]. The studies showed that the construction sequence of excavating the lower part of a new tunnel first and then excavating the upper part was better, and that the acceleration of the shield excavation speed results in an increase in force on and deformation of the existing tunnel. Another study has shown that different types of existing tunnels are affected differently by shield tunneling [14]. Studies based on centrifugal testing further revealed that the displacement of and stress on existing tunnels vary approximately linearly with the growth in the ground loss rate and grouting rate of the new tunnel [15]. Furthermore, the effect of the underpass method on the additional internal forces and deformation of the existing tunnel is more significant, with vertical tensile deformation of the existing tunnel occurring [16]. Model tests are a useful tool for validating numerical models. However, the lack of transparency in the soil samples used in these tests makes it challenging to reveal the internal deformation law of unstable strata, and the insertion of sensors into the model tests will likely cause damage to the original structure of the soil body, which may result in a large test error. Therefore, in response to the shortcomings of the current model test setup, transparent clays with a high degree of visualization become an alternative to natural soils [17; 18; 19; 20]. Currently, the usefulness of model tests based on transparent soils has been recognized in many disciplines [21; 22; 23; 24; 25]. [PERSON] [26] carried out excavation surface stability studies by conducting six transparent soil modeling tests using a self-designed modeling device. The results revealed that in clay-gravel composite strata, the shape of the soil destabilization damage mode was a combination of a wedge and an inverted truncated cone at 0.5 times the tunnel diameter and 1.0 times the tunnel diameter burial depths, as well as a combination of a wedge and a prism at 2.0 times the tunnel diameter burial depth. [PERSON] [27] validated the accuracy of the transparent soil model test using a coupled DEM-CFD approach.