With the rapid advancement of rail transit, shield tunnels have been extensively constructed worldwide. However, leakage at the shield tail can lead to severe consequences, including shield machine subsidence, structural damage to the tunnel, or even catastrophic tunnel collapse. Research on tunnel collapse induced by shield tail leakage remains in its infancy. The mechanisms underlying such accidents are not yet fully understood by researchers and engineers, and effective preventive measures have yet to be developed. In this study, a reducedscale model test was conducted to investigate the processes and mechanisms of tunnel collapse induced by shield tail leakage. The findings reveal that tunnel collapse is primarily triggered by the impact loads generated from the destabilized soil cave. The soil cave, formed due to erosion caused by leakage, propagates upward in a cycle of destabilization and regeneration until the ground surface collapses, resulting in load redistribution around the tunnel. Additionally, the study compares tunnel collapses induced by shield tail leakage and connecting passage leakage, highlighting that while both share similar collapse mechanisms, their boundary conditions differ. The coupling effect between the tunnel structure and surrounding soil is more pronounced in shield tail leakage, leading to more intense load fluctuations and greater structural damage to the tunnel.

The internal structure of sandy cobbles strata is sensitive to disturbances in the urban underground environment, but the structural evolution process under coupling hydraulic and dynamic loads remains unexplored. This paper presents a detailed investigation into the migration patterns and mechanisms of fine particles in sandy cobbles induced by coupled hydraulic and dynamic loading. A sandy cobble specimen with a typical particle size distribution (PSD) was designed and tested using an apparatus that included a constant inlet water head control system and an eccentric-vibrator-based dynamic loading system. Based on physical modeling tests, a numerical model was constructed to reproduce the internal structural evolution under hydraulic and dynamic loading by calibrating the time history of local permeability. The test results indicate that the application of dynamic load can instantly disrupt the stable internal structure of sandy cobbles under static seepage, imparting kinetic energy to fine particles that detach from the skeleton structure and migrate along the seepage direction. Significant fine particle loss occurs near the seepage outlet, but due to energy loss during migration, fine particles far from the seepage outlet are recaptured by the skeleton pore throats and clogged again in the migration path. As the intensity of the dynamic loading increases, the migration path for fine particles becomes longer, and the amount and size of fine particles lost significantly increase. The changes in the internal structure of the soil are reflected in hydraulic parameters as a transient increase in local flow velocity, an increase in local pore water pressure due to clogging, and a decrease in the overall permeability coefficient with the loss of fine particles. These findings enrich the knowledge of internal erosion in urban underground environmentand will be meaningful for future geotechnical engineering design and analysis.