Role of initial particle deposition in collapse behaviors of stratified particle column

Mountain materials and submarine deposits are composed of stratified particles with different physical properties. The interface between these stratified particles is a key uncertainty in landslides, while the separation characteristics between particles and flow are not well known. In this study, the typical collapse characteristics of stratified particles are numerically investigated using a coupled computational fluid dynamics-discrete element method (CFD-DEM). A CFD-DEM model, which incorporates the effect of interstitial fluid on both normal contact forces and frictional behaviors in the tangential direction of particles, is developed and validated through experiments involving single particle contact and particle collapse. The interstitial fluid model precisely captures the entire process of particles from stable settling velocity to rest and is particularly important in viscous fluids. The results show that environmental fluid slows down the overall collapse process and prolongs the duration of the deceleration stage. A distinct interface between coarse and fine particles is observed throughout the collapse in immersed conditions, while the interface disappears at the leading edge in dry conditions. More than 60% of potential energy is dissipated due to fluid viscosity and particle interactions. Full no-segregation cases with all coarse particles at the bottom are associated with reduced particle kinetic energy and increased energy dissipation. The final cumulative dissipation energy in full no-segregation cases is approximately 0.5% of potential energy higher than that in full segregation cases with all fine particles at bottom. These insights provide a deeper understanding of the control mechanism of landslides and emphasize the crucial role of initial depositions.