Abstract The un-resolved CFD-DEM method has been applied to the modelling of suffusion in gap-graded soil columns recently. However, a fundamental question that remains unanswered is whether the un-resolved CFD-DEM simulations can reproduce the reasonable critical hydraulic gradient and seepage failure pattern of suffusion in gap-graded soil columns packed under gravity. This research reveals that the un-resolved CFD-DEM models predict unreasonably greater critical hydraulic gradients for suffusion in the gap-graded soil columns packed under gravity. Meanwhile, the seepage failure presents an unreasonable pattern, i.e., the overall instability of fine and coarse particles, which is confirmed by variations of fine and coarse particle energies. It is distinct from the experimental observations. Further investigations with the aid of the authors' recently developed semi-resolved CFD-DEM method [Cheng et al., Compt Geotech, 100: 30–51(2018)] show that the above discrepancies are caused by the excessive locally-averaging and the relatively low resolution of the fluid flow in the un-resolved CFD-DEM method.