Simulation of fallpipe rock dumping utilizing a multi-phase particle-in-cell (MPPIC)-Discrete Element Method (DEM) model. Part I: Concepts and formulation

Abstract

Dumping blasted rock fragments into water bodies through vertical fallpipes can be a challenging engineering activity due to the concerns of dispersion of fine contents (suspended sediments). This study aims to provide better understanding of the fallpipe rock dumping processes through numerical simulation approaches. We propose a modified multiphase particle-in-cell (MPPIC) method to simulate the fluid–solid mixture flows, and an advection–diffusion model for handling the fine contents. The modifications to the MPPIC method include: (i) changing the pure Eulerian fluid model to a hybrid Eulerian–Lagrangian model for numerical flexibility in handling large free-surface deformations, and (ii) replacing the empirical interparticle stress model with the Discrete Element Method (DEM) for numerical stability when dealing with large-size solids. The numerical model is first validated via two physical experiments: (i) wave generation by a granular collapse and (ii) sand dumping from a split hopper. The simulations generally agree well with the experimental findings, reflecting both the fluid–solid interaction and propagation of suspended sediments. Fallpipe rock dumping is then simulated with a consideration of the effects of fallpipe size on the dynamic interactions between the DEM solid particles, the fluid, and, hence, the fine contents. The results show that long narrow fallpipes perform better in terms of containing the fine contents and limiting the spreading range of the DEM solid particles on the seabed. The time-varying distribution of the fines concentration and the fluid velocity within the fallpipe are also discussed based on the simulation results.