Scour resistance mechanisms of cemented rockfill: A numerical investigation

The resilience of coastal and offshore infrastructure against wave-induced scouring is a critical concern. Cemented granular materials (CGMs), which strengthen mass stability through interparticle bonds, present a potential solution. However, their design is limited by a lack of understanding of microscale fluid–solid interactions under dynamic hydraulic loading. This study aims to create a high-fidelity numerical framework to investigate these anti-scouring mechanisms, focusing on cementation distribution and two-phase flow.

A coupled computational fluid dynamics and discrete element method (CFD-DEM) model was developed to simulate the complexities of two-phase flow (water and air) and irregular particle shapes. The model was based on a validated virtual domain/immersed boundary method. Realistic rubble shapes were modeled using clumped particles, where each irregular grain was represented by a rigid cluster of sub-spheres. The pore structure was analyzed via Voronoi tessellation, and a mesoscale bond model was implemented to define the spatial distribution of cementation. Bonds, generated as cylindrical elements connecting particle centers, were created if the Voronoi faces met a criterion based on a critical interparticle distance related to the cementation ratio (CR), which was the volume of cement over the total volume. These bonds followed a linear elastic model with tensile and shear strength limits; they broke once these limits were exceeded. To account for flow obstruction caused by cement within pores, cemented zones were modeled as an isotropic homogeneous porous medium using a Darcy–Forchheimer approach, with momentum sink terms added to the fluid equations. The DEM module (using LIGGGHTS) solved the translational and rotational motions of each clumped particle, considering contact, bond, and fluid forces. The CFD module (using OpenFOAM) solved the Navier–Stokes equations for the two-phase fluid. Coupling was achieved through momentum exchange, where fluid forces acted on particles, and particle motion constrained the fluid velocity via porosity-weighted correction. A dam-break wave scenario was simulated in a 3D numerical flume to scour a triangular rockfill mound composed of 292 clumped particles, representing a transient high-impact loading condition. Simulations were carried out for noncemented and cemented cases with CR values of 10%.

The model successfully replicated the fluid and contact force dynamics observed in noncemented rockfill. For the cemented cases, key findings included: the following: (1) Permeability control: cementation significantly reduced water infiltration. The noncemented mound exhibited deep seepage, while at CR = 5%, penetration was greatly decreased. At CR = 10% and 20%, water was mainly confined to surface flow, with minimal internal seepage. The effect saturated, with little difference between CR = 10% and 20%. (2) Micromechanical response: cementation bonds changed how internal forces were transmitted. They supported substantial tangential forces during wave action, and their progressive failure (via tensile/shear rupture) dissipated energy. This led to a decrease in the average normal contact force and an increase in tangential force, indicating dynamic rearrangement of force chains. (3) Structural integrity: despite similar peak fluid impact forces, cemented structures retained greater stability by stabilizing the granular skeleton and redistributing internal stresses through the bond network.

An advanced CFD-DEM model is developed to reveal the micromechanisms behind CGMs' resistance to scour. Cementation improves performance not only by strengthening contacts but also by reducing permeability and enabling energy dissipation through bond failure. The effect of cement content is nonlinear; beyond around 10% CR, additional cement shows diminishing returns, which is important for cost-effective design. Future work will include more complex material models and realistic wave conditions to support engineering applications.