# Numerical Simulation on the Leakage-Induced Collapse of Segmental Tunnels
## 5 Conclusions  
In this paper, a coupled fluid-solid strata model and a nonlinear FEM tunnel model were established to simulate the tunnel collapse process based on test results. After experimental validation, the strata response and the process of the tunnel collapse were analyzed. The following conclusions were drawn from the study:  
1. A DEM-based coupled fluid-solid strata model combined with a nonlinear FEM tunnel model can be used to effectively simulate the complete process of tunnel leakage-induced collapse. By comparing with the tests, it was proved that the coupled fluid-solid model can be used to reproduce the phenomena of seepage erosion. By extracting the external loads of the tunnel obtained from the strata model and applying them to the FEM tunnel model, the tunnel collapse process can be simulated in a refined way, and the collapse mechanism of the tunnel structure can be obtained.
2. Based on the coupled fluid-solid analysis, it was pointed out that the microscopic mechanism of tunnel seepage-induced strata response was the continuous destabilization and reproduction of the soil arching effect formed by the soil particles. During the development of the soil arch, the height and width of the soil arch increased continuously, which led to the degradation of its stability and ultimately triggered the subsidence of the strata surface. On the one hand, the soil arching effect hinders the development of seepage erosion, so the erosion of the soil is limited to the interior of the soil arch, which is manifested as the generation of soil caves in the macroscopic phenomena; on the other hand, the soil arching effect temporarily supports the soil above, and there is no obvious subsidence of the strata surface in the process of erosion, which results in a sudden destabilization of the strata and impact loads on the tunnel.  
Figure 16: Comparison of damage phenomena of tunnel in tests and simulation: (**a**) Test results; (**b**) Simulation results.  
Figure 15: Collapse process of tunnel structure.  
3. The collapse process of the tunnel is as follows: firstly, due to the soil arching effect, the load outside the tunnel structure is redistributed, the loads at the feet of the arch rise significantly, the loads inside the soil arch dissipate, and the tunnel undergoes large deformation as a result; subsequently, due to the impact loads from the destabilization of the soil, the tunnel is damaged at several joints and generates plastic hinges, and ultimately the tunnel is destabilized and collapses.  
Given the unpredictability of tunnel collapse accidents, acquiring insights from engineering accidents is difficult. In this study, the strata response and tunnel collapse mechanism were investigated through numerical simulation. The conclusions drawn from the study can deepen the understanding of the mechanism of tunnel leakage accidents so that effective prevention measures can be put forward in the future.  
Conceptualization, methodology, Q.S. and X.L.; validation, formal analysis, investigation, writing--original draft preparation, Q.S.; writing--review and editing, supervision, X.L., W.D.C. and L.T.; project administration, funding acquisition, X.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number 52078376.  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
The first author would like to express his appreciation for the scholarship from the China Scholarship Council (No. 202106260163) for his study at Ghent University.  
The authors declare no conflicts of interest.