<p>Seepage erosion in sandy soils is a complex process with multiple influencing factors, often triggering disasters that endanger underground structures. This study aims to reveal the failure modes and mechanical mechanisms of seepage erosion in sandy soils under different hydraulic gradients, additional earth pressure, and leakage joint size. Therefore, seven grounds of seepage erosion model tests were conducted in this study using a self-developed test apparatus and gap-graded sand with 35% fine particle content. Real-time data on erosion morphology, cumulative mass loss, pore pressure, and earth pressure were analyzed, elucidating failure mechanisms under varying influencing factors. It is found that three characteristic zones formed in eroded soil: flow zone, coarse-particle zone, and stable zone. The seepage erosion process undergoes three stages: the initial erosion stage, the erosion development stage, and the erosion stabilization stage. A correlation model quantifying the relationship between cumulative mass loss and the relevant influencing factors, as well as a predictive model for the settlement trough height, was developed. High hydraulic gradient intensifies pore pressure rebound, triggering structural failure and causing up to a 72% loss of soil strength near the leakage joint. High earth pressure significantly suppresses the pore pressure response and also reduce the structural load acting on the tunnel after erosion, with a maximum reduction of up to 60%. Larger leakage joints promote pore pressure dissipation and effective stress increase, whereas smaller ones restrict the flow field. This study significantly advances the understanding of seepage erosion evolution processes, providing valuable guidance for ensuring shield tunnel safety and protecting underground structures against suffusion-induced hazards.</p>