## 4 Conclusions  
This study aims to discuss the effects of the destabilization of the excavation surface when the shield is underpassing an existing tunnel. The study adopts laboratory model tests and finite element simulation to analyze the impact of various parameters on the deformation of the existing tunnel and the surrounding soil. The parameters included in the study are horizontal spacing, vertical spacing, and tunnel diameter ratios between the two tunnels. The main conclusions of this study are as follows:  
(1) For horizontal spacings of \(L/D=1\) and \(L/D=0\), compared to the ultimate support pressure in the case without the existing tunnel, the ultimate support pressure is shown to be lower by 17.6% and 8.7%, respectively. As the horizontal spacing between the two tunnels is reduced, the impact of the existing tunnel on the excavation face gradually weakens. For a constant horizontal spacing between the two tunnels, the ultimate support pressure of the excavation face decreases with decreasing vertical spacing between the two tunnels. Conversely, the ultimate support pressure of the excavation face increases with a decreasing tunnel diameter ratio between the two tunnels. (2) When \(L/D=1\) and \(L/D=0\), the presence of existing tunnels mainly restricts the development of the upper silo in the destabilization mode, which narrows the influence range in the horizontal and vertical directions. The reduction in the horizontal spacing between the two tunnels, the reduction in the tunnel-to-diameter ratio, and the increase in the vertical spacing weaken this restrictive effect. (3) For the soil located between the two tunnels (\(Z=-0.22\) m), the existing tunnels prevent the transfer of stresses from the upper soil, which reduces the vertical and average horizontal stresses in the soil below the existing tunnels, with the initial average vertical stress in the longitudinal direction shrinking by about 10.8% for \(L/D=1\), and by about 20.8% for \(L/D=0=\) in the transverse direction. When the horizontal spacing between the two tunnels is constant, the reduction tendency becomes more and more obvious as the vertical spacing between the tunnels decreases and the tunnel-to-diameter ratio increases. (4) When \(L/D=1\) and \(L/D=0\), the deformation of the existing tunnel is mainly characterized by an elliptical deformation inclining towards the soil stress release zone. However, the deformation direction of the two tunnels varies considerably when the relative positions of the tunnels are changed. The deformation of the existing tunnel is extensive when \(L/D=1\), and increases with the reduction in the vertical spacing between the two tunnels. As the tunnel-to-diameter ratio is decreased, the deformation of the existing tunnel mainly manifests as an overall settlement towards the stress-release zone. Therefore, in each working condition of the shield orthogonal underpass, although when \(L/D=1\) the stability of the excavation surface plays a certain support role, at this time the existing tunnel subject to the excavation surface destabilization zone, which has the greatest impact; thus, at this point, measures to strengthen the existing tunnel protection should be implemented. **Author Contributions:** Conceptualization, S.M. and Z.H. ; validation, Z.D. and Z.Z. ; investigation, Z.H. ; writing--original draft preparation, Z.Z. ; writing--review and editing, P.A., B.H. and J.L. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the National Natural Science Foundation of China (No. 52268062 and No. 52278337), the Guangxi Key Project of Nature Science Foundation (No. 2020 GXNSFDA238024), and the Systematic Project of Guangxi Key Laboratory of Disaster Prevention and Engineering Safety (No. 2022 ZDK025). **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.