## 1 Introduction
It was concluded that the damage pattern of the tunnel excavation surface in sandy soil was chimney-shaped, while in clay soil, it was funnel-shaped. [PERSON] et al. [26] investigated the \"greenfield model\" and found the displacement pattern of different strata and soil layers during the gradual advancement of shield tunnels. A number of centrifugal modeling tests have been carried out by [PERSON] et al. [27] based on the passage of tunnels under an existing pipeline, and a method for calculating the internal forces in the existing pipeline has been developed. As for the conventional model tests, [PERSON] et al. [28] used solid cylinders to simulate tunnels in shield tunnel tests in sandy soil to investigate the formation loss formed by shield advancement. For the field observation method, [PERSON] et al. [29] studied MRT tunnels under extensive excavation and used a motorized total station monitoring system to monitor the MRT tunnels during the excavation of the foundation pit. [PERSON] [30] analyzed two parallel shield tunnels in soft soil. It was also noted that volume changes due to the expansion or compression of the soil are the main factors affecting ground movement. The large physical model tests mentioned above are costly, and the opaqueness of the geotechnical body makes it challenging to monitor internal deformations, requiring numerous sensors and additional complexity. To facilitate the clear and convenient observation of deformations within the soil body, the properties and applications of synthetic transparent soil have been studied. [PERSON] [31] invented transparent clay in 1990 and used it to model the non-Newtonian flow problem of clay. This was the earliest prototype of transparent synthetic clays. After this, [PERSON] et al. [32] investigated the physical, mechanical properties of consolidated specimens of transparent mud in their subsequent studies. The results showed that such transparent mud could be used to simulate natural soil. This result laid the foundation for future artificial transparent synthetic soils. Then, [PERSON] et al. [33; 34; 35; 36; 37] synthesized transparent clays using amorphous silica powders and pore-flowing liquids. The existing types of transparent clays and areas of application were also summarized. [PERSON] et al. [38] and [PERSON] et al. [39] developed transparent clay materials using LaponiteRD material and Gelita material, respectively. [PERSON] [40] studied the infiltration of water in clay soil via the target tracking technique and used transparent synthetic soil to study the deformation of the soil around the pile during its subsidence. [PERSON] et al. [41; 42] synthesized a transparent material using amorphous silica gel and pore fluids with the same refractive coefficient. [PERSON] et al. [43] developed a complete transparent soil shield tunnel model test system. For the first time, transparent materials were used to study shield tunnel excavation. [PERSON] [44; 45] selected a mixed solution of silica gel powder and mineral oil to formulate transparent rock. The deformation, damage mechanism, and rupture evolution law of the surrounding rock of deeply buried tunnels were analyzed. Most studies mentioned above have focused on research in situations where a new tunnel crosses two existing tunnels simultaneously. Additionally, most scholars use transparent sand and clay for tunnel excavation tests, while the use of transparent rock to simulate tunnel excavation has been adopted by only a few researchers. In addition, the range of conditions studied is relatively limited. To address these gaps, this study combines transparent rock testing with the scenario of tunneling through two existing tunnels simultaneously, providing a detailed investigation into the impact of new tunnel excavation on both the upper and lower existing tunnels.