Assessing the potential for suffusion in sands using x-ray micro-CT images

Abstract

Internal erosion is a major safety concern for embankment dams and flood embankments and is the focus of much research internationally. Suffusion is a mechanism of internal erosion which affects gap-graded or broadly graded cohesionless soils and is characterised by selective removal of fine material, leaving behind a coarse material with increased hydraulic conductivity. Early studies on suffusion proposed design criteria based on laboratory testing, and presented conceptual models to explain the results in terms of grain-scale behaviour. The study by Kenney & Lau (1985) identified three criteria for suffusion: 1 – Fine particles must be free to move (mechanical criterion); 2 – Fine particles must be small enough to fit through the void space between coarse particles (geometric criterion); 3 – Fluid flowing through the void space must have sufficient velocity to transport the fine particles (hydraulic criterion). Recent studies have examined the first two criteria using grain-scale models with idealised particles, including analytical models and discrete element models (DEM) with circular or spherical particles. This thesis presents a new methodology, using non-destructive 3D imaging (micro-CT) to characterise the internal microstructure in physical specimens of sands and glass beads. This methodology involved the development of innovative image processing and numerical techniques to quantify unstable particle assemblies and to measure particle size distributions and void constriction size distributions. The new method was validated and was shown to produce good agreement with existing methods for idealised particle configurations, however the results for real sand specimens provided new insights into the effects of particle shape, particle size distribution and density on void constriction sizes. Furthermore, the 3D images of real specimens have provided new insights into the appropriateness of existing conceptual models for gap-graded particle structures. These results were used to critically examine and evaluate existing mechanical and geometric criteria for suffusion. The 3D images showed, qualitatively, that the void structures in sands varied significantly from those in porous rocks – which had been the basis for the majority of existing grain-scale fluid flow models. To examine this issue quantitatively, computational fluid dynamics (CFD) simulations were performed within the 3D images of sands and glass beads, in parallel to laboratory permeameter tests on the same materials. The results presented in this thesis provided entirely new insights into the patterns of fluid flow in sands, they allowed correlations to be made between fluid flow and void constriction sizes and also showed how local velocities varied from volume-average discharge and seepage velocities. This study provides new information to support, clarify and improve upon the current understanding of suffusion, filtration and seepage flows in sands. The detailed methodology and results also highlight issues of great importance to future micro-scale modelling of these phenomena.