## 1 Introduction  
Hazards such as leakage [1] or earthquakes [2, 3] in underground structures may induce severe consequences, including surface subsidence, structure damage, and even casualties, which seriously threaten urban safety. Among them, leakage of underground pipelines and subway tunnels is the major cause of induced ground subsidence [4, 5, 6, 7]. The reason for this is that when leakage occurs in underground structures such as tunnels, the surrounding soil continues rushing into tunnels due to seepage forces, triggering catastrophic consequences such as ground subsidence. Recent research indicates the potential of deep learning techniques in identifying damages within tunnel structures [8] and enhancing the resilience of underground structures by structural health monitoring in this field [9, 10]. However, to prevent the hazards, an investigation into the mechanism is also necessary. The essence of the problem of tunnel leakage-induced disasters is seepage erosion of the strata. To investigate the disasters caused by water leakage in tunnels, research has been conducted through model tests. [PERSON], et al. [11] conducted a reduced-scale model test to investigate the settlement pattern of strata generated by soil erosion after tunnel leakage and proposed a simplified calculation formula for settlement. [PERSON], et al. [12] investigated the effects of four parameters, including soil particle size, pipe crack dimension, water height, and soil depth, on the erosion process of strata around a cracked sewage pipe. [PERSON], et al. [13] selected three kinds of quartz sands with different particle sizes and carried out an experimental study on the erosion development process of the strata outside a damaged sewage pipe. In addition to the reduced-scale model tests, researchers also adopted numerical simulation methods to explore the erosion development process of the external strata after leakage in underground structures. Due to the characteristics of soil as a typical discontinuous granular material, the discrete element method (DEM) is an effective numerical simulation method in this field. Different calculation methods of fluid are coupled with DEM to realize the coupled fluid-solid analysis of the seepage erosion process. [PERSON], et al. [14] simulated the erosion process of soil caused by periodic water level changes using a CFD-DEM model. [PERSON] and [PERSON] [15] adopted a coupled FDM-DEM method to simulate the erosion in water-rich sands due to tunnel leakage, and further analyzed the existence of the soil arching effect in this phenomenon. [PERSON], et al. [16] dealt with the simulation of the tunnel leakage issue by first calculating the distribution of water pressure around the tunnel when leakage occurs using FEM, and then applying the hydraulic forces to the DEM model, to analyze the erosion of the strata induced by the tunnel leakage. From the above discussion, it can be seen that there are already some studies on the seepage erosion problem induced by tunnel leakage. However, the existing research mainly focuses on seepage erosion that occurs in sandy soil strata, and less on the other types of soil such as silt, clay, etc. The seepage erosion modes in different strata have not yet been clarified. Actually, in many accidents induced by tunnel leakage, the erosion and damage modes are different, but the mechanisms and development processes have not been systematically investigated. Therefore, it is necessary to carry out research on the seepage erosion mode of tunnel leakage in different soil types. To investigate the damage modes and mechanisms of seepage erosion induced by tunnel leakage in different types of strata, a series of tests on soil seepage erosion was designed and carried out. From this, the differences in the process of seepage erosion induced by tunnel leakage in different soils were investigated. Furthermore, through establishing a coupled fluid-solid simulation model, the mechanisms that generate different erosion damage modes are explored.