<p>Internal erosion is a major factor in the failure of water-retaining structures, with the initial state of the soil critically influencing its progression. This study investigates the development of internal erosion in gap-graded cohesionless soils through a series of upward seepage tests using an improved triaxial permeameter. Specifically, the influence of initial relative density and hydraulic gradient loading patterns on internal erosion behavior is explored. The findings indicate that under constant hydraulic loading, both the erosion ratio and volumetric deformation exhibit a rapid initial increase followed by stabilization. Moreover, the ultimate erosion ratio initially increases with the hydraulic gradient and then stabilizes, while it decreases with increasing relative density. Conversely, multi-stage hydraulic gradient loading reveals a progression from stabilization to erosion development and eventual failure, characterized by a stepwise increase in the erosion ratio and specimen deformation. It is noteworthy that loose specimens exhibit large volumetric deformation due to erosion, indicative of suffosion, whereas dense specimens exhibit suffusion behavior. Additionally, specimens with higher pre-erosion relative density exhibit greater critical hydraulic gradients, indicating improved erosion resistance. These results underscore the practical significance of pre-erosion density in assessing internal erosion evolution. Finally, building on the continuity hypothesis and incorporating critical state theory with a nonlinear incremental stress–strain relationship, an erosion model accounting for the pre-erosion relative density was developed. The model was calibrated using constant hydraulic gradient tests and validated against multi-stage test results, demonstrating its feasibility in accurately predicting the development of internal erosion in cohesionless soils.</p>