Fluidization of Subgrade Soil Under Rail Tracks Through CFD-DEM Coupling

The increasing excess pore pressure under rail tracks especially in coastal soft soil regions has been indicated as one of the major reasons causing numerous track issues such as mud pumping. When the excess pore pressure develops to a certain degree, it causes considerable increase in hydraulic gradient associated with hydraulic forces over the soil depth that results in upward migration of fines, i.e., mud pumping followed by severe loss of soil mass and excessive deformation of soil matrix. This issue is simulated in this paper by using the computational fluid dynamics (CFD) coupled with the discrete element method (DEM). Subgrade soil is built in DEM with respect to realistic soil profiles while an increasing hydraulic gradient is generated to model upward fluid flows in CFD. The study indicates soil fluidization develops from localized scale via soil piping to overall scale where all particles become suspended and migrate upwards (i.e., heave formation). The overall fluidization results in a severe degradation in the contact network of soil particles (i.e., soil fabric) and effective stress. The study also shows reasonable predictions of the CFD-DEM coupling on the hydraulic response of subgrade soils.