Estimation of the critical hydraulic gradient of coarse soil using the DEM-CFD coupling method

The critical hydraulic gradient of the soil is a significant parameter to investigate the seepage behavior of soils in geotechnical engineering. In this research, the flow velocity-based index and the porosity-based index are proposed to identify the critical hydraulic gradient of the soil sample. For this purpose, based on the discrete element method-computational fluid dynamics (DEM-CFD) coupling model, a synthetic soil sample constructed in three-dimensional particle flow code (PFC3D) is used to simulate the seepage processes under various hydraulic gradient conditions. Parametric studies are also carried out to investigate the effects of particle diameter of the soil sample on the determination of critical hydraulic gradient using the proposed two indexes. The results show that the value of critical hydraulic gradient decreases dramatically with the increase of particle diameter when d < 3.0 mm. The values of critical hydraulic gradients obtained from the numerical model were also compared with those calculated from theoretical Terzaghi model. It is found that comparing with the numerical model, Terzaghi model overestimates the critical hydraulic gradient of soil samples when d >2.0 mm, which may result in an unsafe engineering design.