A Coupled CFD–DEM Approach to Examine the Hydraulic Critical State of Soil under Increasing Hydraulic Gradient

Increasing hydraulic gradients and associated seepage in a soil foundation accompanied by a reduction in effective stress, degradation of soil stiffness, and diminished internal stability contribute to adverse conditions in engineered earth structures, including dams and transport infrastructure. Although much attention has been drawn into these geotechnical challenges, most previous analytical and experimental studies could not properly capture the detailed response of fluid and soil particles, especially the localized or microscopic fluid–soil perspectives. In this regard, this paper aims to apply a numerical approach to analyze the response of a soil–fluid system under increasing hydraulic gradients. Soils with different gradation properties and porosities are created using the discrete element method (DEM), which is then coupled with computational fluid dynamics (CFD) based on Navier-Stokes equations. This numerical investigation reveals different stages in the development of hydraulic critical state, that is, from localized erosion (e.g., piping) to overall heave and fluidization. The transformation of fluid and particle characteristics, such as particle migration, the erosion rate, and hydraulic conductivity associated with porosity when soil approaches critical state, is discussed in detail. Micromechanical degradation within the contact network and the associated reduction in effective stress of soil due to an increasing hydraulic gradient are also analyzed in this study. A number of key factors that govern the soil response, such as friction, porosity, and grain uniformity, are addressed through numerical investigations. This study demonstrates acceptable numerical predictions for hydraulic behavior and erosion rates that are in good agreement with previous experimental data.