Macro–microscopic mechanism of suffusion in calcareous sand under tidal fluctuations by coupled CFD-DEM

Numerous offshore transportation infrastructures built with delicate calcareous sand (CS) experience intricate seepage conditions caused by tidal fluctuations. Traffic loads can cause substantial particle crushing, leading to mass loss and subsequent settlement. This paper presents a numerical analysis of suffusion mechanisms in three representative volume elements (RVEs) under cyclic seepage using the coupled computational fluid dynamics-discrete element method (CFD-DEM). The microscopic parameters of CS are calibrated by comparing the results with direct shear experiments. The effect of particle breakage on cyclic suffusion can be assessed by varying breakage ratios of crushed soil. Several suffusion tests under periodic seepage are conducted to investigate the fines migration behavior and examine the contact characteristics, with a particular focus on the middle element of the three RVEs. Test results confirm that mass loss of the sample is intensified by particle breakage but is effectively inhibited under high-stress conditions. Fine particles exhibit different migration behaviors in this study compared to those observed under unidirectional seepage, including reverse migration. The change in connectivity of the particle assembly during cyclic seepage indicates a gradual weakening of its overall stability. Lastly, the anisotropy of the normal contact force distribution exhibits a significant increase as more particles are eroded.