## 3 Results and Discussions
### Discussions  
In order to study the working conditions of the new tunnel passing under existing tunnel a and crossing over existing tunnel b simultaneously, when the clear distance between the new tunnel and the existing tunnel, that is, the thickness of the interlayer layer changes, the impact on the existing tunnel can be determined. And it can be compared and analyzed with the transparent rock mass test mentioned above. Combined with the engineering background, we have three working conditions that conform to the similarity ratio of the test. Among them, the thicknesses of the interlayer layers were set at 0.3\(D\), 0.5\(D\), and 0.7\(D\), respectively. Figures 13 and 14 show a comparison between the finite element results and the results of the transparent rock mass model test. Existing tunnel a and existing tunnel b can be seen under the conditions of three interlayer thicknesses \(h=0.3D\), \(h=0.5D\), and \(h=0.7D\), respectively. The results show similar patterns, and the deformation curves all conform to the Gaussian curve distribution. And the peak values are not very different. But it can be seen that in the simulation results of the finite element method, the newly built tunnel passing under the existing tunnel causes existing tunnel a and existing tunnel b to have a deformation range that is larger than that of the indoor model test. This might be due to the fact that the finite element software simulates the actual working conditions in a similar way, but the similarity between the finite element software and the model test is relatively large, at 1/200. So, during the excavation process, there are the existing tunnels in the limiting element software that are affected at a larger range compared to those in the experiment. On the other hand, it can be seen in the model test that settlement occurred on both sides of the uplift deformation, which might be due to the middle part of the existing tunnel being subjected to new forces, the effect of excavation,  
Figure 12: Vertical displacement of existing tunnel b with varying soil layer thickness.  
and unloading during tunnel construction, which reduces the surrounding rock pressure above the existing tunnel arch, causing it to bulge and deform the rocks on both sides, which are squeezed towards the middle, thus causing a small-scale settlement phenomenon.  
Figure 13: Cont.  
Figure 14: _Cont._  
Figure 13: Numerical simulation of existing tunnel a compared to the transparent rock tests.  
The technology of transparent rock and soil mass still needs to be improved. Currently, the obtained transparent rock and soil mass materials can only simulate some soft rocks, and both the transparency and model size are subject to certain limitations. Increasing the consolidation pressure is a feasible method to enhance strength; however, it will greatly reduce its transparency. Therefore, the next step of this work will consider finding more reasonable ways and methods to enhance the material strength on the premise of ensuring transparency, so as to simulate more types of rock masses and better serve the research work of geotechnical engineering.  
Figure 14: Numerical simulation of existing tunnel b compared to the transparent rock tests.