Abstract: Leakage at segment joints during long-term operation of shield tunnels, leading to soil erosion behind segments, serves as a critical trigger for ground void formation and surface subsidence. However, the influence mechanisms of water pressure remain incomprehensively understood. Through self-developed testing equipment, this study analyzes water pressure effects on erosion patterns, soil loss, tunnel stresses, and soil strength. It reveals the coupled development of suffusion and soil gushing and the associated ground failure mechanisms. Experimental results demonstrate that under low water pressure, a "ground void-localized suffusion" mode develops, whereas high water pressure accelerate "surface subsidence-concentrated suffusion" formation. This drives stratified strength-degradation zones comprising: mobile zone (54% reduction in penetration resistance), coarse-particle zone (69% reduction) and void zone. Seepage erosion evolves through three phases: "suffusion-soil gushing-clogging". As water pressure increases, intermittent clogging of leakage points causes water hammer effects, abruptly increasing tunnel surface pressure (35% above pre-erosion levels). Erosion-induced pressure expands radially from leakage points at the tunnel crown toward the springline and invert. These findings offer a necessary guidance for preventing water-soil loss disasters caused by joint leakage in shield tunnels.