The movement of particles caused by erosion is one of the main reasons for the destruction of projects, such as dams, tunnels, and foundation pits. This study highlights a theoretical model to assess the occurrence of erosion in gap-graded, sand-gravel soils under variable seepage direction based on the critical hydraulic conditions of particle initiation. The model introduced the effects of relative exposure degree, relative hidden degree, and seepage direction by considering the difference in particle initiation conditions. On the basis of the variable-section capillary tube model formed by the skeletal pores, the mechanical analysis of the movable particles in the pores was performed, and the formulas for the critical hydraulic conditions were obtained according to the moment balance equation. Subsequently, the coupled CFD-DEM method and the available experimental data were used for validation. The comparison revealed a deviation of 0.0268 for the mean of the ratio between the calculated and simulated values compared to 1, with a covariance (COV) of 0.0344. Further, the mean value of the ratio between the calculated and test values compared to 1 had a maximum deviation of 0.095 and a covariance (COV) of 0.0143. The high degree of agreement between the data proved that the theoretical model can assess the occurrence of erosion more accurately. Finally, based on the theoretical model, the study further explored the effects of seepage direction and relative particle position on the variability in particle initiation conditions, thus finding that, unlike in other studies, the effect of seepage direction was not linear.

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