<p>This study proposes a Boltzmann-based model that comprehensively incorporates the combined effects of hydraulic gradient, mean effective stress, and deviatoric stress ratio to characterize the stiffness degradation of soil resulting from seepage-induced erosion. The model was validated through comparisons with experimental data. Coupled with a localized equivalent permeability method, it establishes a numerical framework for simulating localized seepage erosion in tunnels. This framework was applied to analyze the process of localized seepage erosion under various conditions, including leakage location, range, pattern, groundwater level, and leakage intensity. The influence of these factors on ground deformation and the mechanical responses of the tunnel lining was systematically investigated. Results show that a lower leakage location, larger leakage area, higher initial pore pressure, and greater leakage intensity lead to a broader and more significant reduction in pore pressure, resulting in a more extensive pressure-drop funnel. This in turn induces increased asymmetric ground surface settlement and greater deformation of the tunnel segments. Seepage erosion markedly alters the stress state of the lining, causing redistribution of bending moments and axial forces. The most critical condition occurs when seepage erosion takes place at the sidewall. The proposed soil stiffness reduction model and localized leakage simulation method provide an effective tool for assessing the impact of seepage erosion on tunnel safety and stratum stability. This study offers valuable insights for risk assessment and maintenance of shield tunnels operating in water-rich environments.</p>