## 5 Conclusions  
This study diverges from past research on seepage erosion in underground structures that predominantly focused on sand strata. Instead, the research extends to various strata, presenting three types of erosion modes for the first time through a combination of experimental and simulation methods. Additionally, consideration for clay seepage erosion degradation was introduced into the coupled fluid-solid model, thus surpassing the limitations of previous studies that predominantly focused on simulating sand soil. This approach enabled the successful application of the method to the numerical simulation of multiple different strata, revealing three distinct erosion failure modes for the first time. The following conclusions were drawn from this study:  
(1) Based on the differences in the seepage erosion modes after tunnel leakage, the soil can be classified into three categories: no soil cave (NSC), unstable soil cave (USC), and stable soil cave (SSC). (2) The macroscopic phenomena of the three erosion modes are different: In NSC strata, there is no obvious soil cave phenomenon in the process of seepage erosion, and the multi-layer strata settle synchronously; in USC strata, there are unstable soil caves formed along with the erosion, and there is almost no deformation above the soil cave until the strata surface subside; in SSC strata, the soil stabilizes spontaneously and there is almost no settlement of the strata above the leakage point. (3) The coupled fluid-solid numerical model, which considers the degradation of clay, can effectively simulate the development process of soils with different seepage erosion modes. The DEM-based fluid-solid coupling model can be used to analyze the particle  
Figure 23: Contact force chain of SSC soil: (**a**) soil arch generation; (**b**) contact force chain after 16 million steps. water interaction phenomenon in the process of soil seepage erosion; the introduction of the clay degradation design can further solve the problem that the clay particles are too small to be simulated, and, at the same time, it can reflect the influence of clay particle loss. (4) The difference between the three seepage erosion modes comes from the strength of the soil arching effect: the soil arch of the NSC strata is weak, which cannot prevent the development of seepage erosion, so the seepage erosion range is large, there is no soil cave observed, and the strata show overall settlement; the soil arch of the USC strata is stronger than that of the NSC strata, which limit the seepage erosion to the inner area of the soil arch, and the strata over the arch are temporarily stable, but in the continued action of the seepage force, the soil arch destabilizes and reappears until strata surface subside; SSC strata form the strongest soil arch, which prevents the seepage erosion, and leads to stabilization of the strata above the arch. For ground subsidence accidents caused by tunnel leakage, different damage modes were observed in different strata. This study explored the causes and mechanisms of different erosion modes caused by tunnel leakage in different strata and enhanced the understanding of the relationship between strata and erosion modes. At the same time, the research provided practical insights and tools for engineers to effectively manage and prevent the associated risks in infrastructure projects. Conceptualization and methodology, Q.S. and X.L. ; validation, formal analysis, investigation, and writing--original draft preparation, Q.S. ; writing--review and editing and supervision, X.L., W.D.C. and L.T. ; project administration and 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.