# Numerical Simulation on the Leakage-Induced Collapse of Segmental Tunnels
## 1 Introduction  
When leakage occurs in underground structures during the construction process, it often leads to structural damage and ground surface settlement, and in severe cases, it may even induce large-scale ground subsidence, resulting in great economic losses [1]. Among the various construction methods, shield tunnels are mostly used to pass through urban areas, which are densely populated with buildings, and in the event of a leakage accident, this may be extremely hazardous [2; 3; 4; 5]. For example, water leakage caused structural damage to the tunnel of Line 1 of the St. Petersburg Metro in Russia [6]. In 2003, the Shanghai Metro Line 4 project was faced with water and sand inrush during construction, which ultimately led to the collapse of the tunnel [7]. In 2018, leakage accidents happened in Foshan Subway Line 2, which led to the collapse of the tunnel [2]. Therefore, it is necessary to carry out research on the mechanism of structural collapse accidents induced by tunnel leakage. Since accidents of tunnel leakage-induced structural collapse are unpredictable and destructive, it is difficult to obtain accident information using on-site measurements or full-scale model tests [8; 9]. Therefore, studies for this type of accident are often carried out using reduced-scale model tests. To investigate the problem of shield tail leakage during construction, leading to structural collapse, [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [2] conducted an experimental study on the failure mode of the shield tail sealing system based on the Foshan tunnel collapse accident. [PERSON] et al. [10] discussed the sealing performance of the shield tail in that accident under different environmentalconditions by means of model tests. To investigate the tunnel collapse accidents caused by leakage in connecting passages, [PERSON] et al. [11] simulated the occurrence of water and sand inrush by removing a segment at the waist of a model tunnel in a reduced-scale model test and then investigated the change in the external load in the process, and pointed out that the tunnel collapse was due to the redistribution of the external loads. Due to the limitation of the scale, reduced-scale model tests are mostly used to obtain a qualitative process and observe phenomena during tunnel accidents, which makes it difficult to study the tunnel collapse mechanism, so it is necessary to carry out numerical simulations for the tunnel collapse process. From the physical phenomena of the tunnel collapse process, it can be seen that the response of the strata contains seepage erosion at the microscopic level [12] as well as discontinuous large deformation of the soil at the macroscopic level, while the tunnel structure exhibits large deformation and structural collapse, which is a relatively complex physical process. Among the simulation methods for soil, it is difficult to reproduce the physical process of large deformation in soil by continuous medium methods (e.g., FEM), so researchers mostly use discontinuous medium analysis methods such as SPH, DEM, and MPM to carry out the simulation. Among them, the DEM method is the most widely used method for the simulation of soil. For the soil seepage erosion problem, the DEM-based fluid-solid coupling model has been chosen by many researchers to carry out micro-mechanism studies, and to discuss the influence of different factors on the erosion process, such as particle shape [13; 14], particle size distribution [15; 16; 17], soil stress [18; 19; 20], clogging effect [21], and so on. For the problem of discontinuous large deformation induced by seepage erosion of soil, the DEM-based coupled fluid-solid analysis method also has relevant applications. For example, [PERSON] et al. [22] simulated tunnel leakage in a saturated sandy soil using the CFD-DEM method. The process of seepage erosion development after tunnel leakage was discussed and parameters such as particle size distribution were analyzed. [PERSON] et al.