Internal erosion poses a significant threat to the stability of earth and rock-fill dams. Suffusion is one mechanism of internal erosion. Gap-graded, cohesionless and underfilled soils are susceptible to suffusion as the fine particles in this type of material are under relatively low stress. Previous research such as Skempton & Brogan (1994) has revealed that fine particles within this kind of soil are washed out due to seepage flow at much lower hydraulic gradient than critical hydraulic

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... ent calculated from Terzaghi's classic theory for heave, and they defined a stress reduction factor α to quantify this phenomenon. This contribution develops these concepts by using particle-scale simulation. Hitherto, there has been a lot of attention paid to the material susceptibility, which is assessed by considering the material's particle size distribution. However, there has been little research considering the hydro-mechanical aspects of susceptibility. The hydro-mechanical aspects of susceptibility are intimately related to the particle-fluid interaction. The contribution of this thesis is to fill the gap in understanding by using the coupled discrete element method (DEM) with computational fluid dynamics (CFD) to observe the fundamental mechanisms that govern the onset of suffusion. In this research, virtual permeameter tests with a constant head were simulated with a coupled DEM-CFD code. The permeameter simulations were validated using the work of Suzuki et al. (2007) and were compared with the results obtained from Shire (2014). Four gap-graded soils with two fines contents and two densities were investigated to consider the influence of coordination number, seepage velocity and normalised particle stress on particle displacement under varying hydraulic gradients.