Particle form and its impact on packing and shear behaviour of particulate materials

Abstract

A small but important body of geomechanics literature now exists to show qualitatively that whether a specimen is composed of particles that are bulky or platy can have a significant effect on its behaviour during compression and shear. This particle shape characteristic has been termed 'form', which has sometimes been estimated on the basis of 'sphericity', a measure of how close a given particle form comes to that of a sphere. Before the effects of form can be established in an objective manner it will be necessary to develop and evaluate simple, practical, and robust three-dimensional measures of this aspect of particle shape, and to assess the importance of these in controlling the behaviour of particulate materials. As a contribution to this fundamental study this thesis reviews experimental evidence on the importance of particle shape, the nature of clastic and crushed rock particles, and examines previous definitions of sphericity. It then considers potential methods of obtaining data to define the three-dimensional form of particles, and proposes two practical methods (one based on a volumetric equivalent scalene ellipsoid for coarse particulates and the other based on laser diffraction and turbidity measurement for fine particulates) by which this can be done. The proposed methods are applied to specimens with a number of particle shapes, and the results are assessed by comparison with other possible measures of form, and against the visual estimates obtained from either the static imaging in 3-orthogonal directions or the SEM images. Subsequently, the effects of form on packing arrangement of particles during deposition and on drained shear behaviour (in triaxial compression) of particulate materials are examined by carrying out laboratory tests on a variety of particle shapes and DEM simulations on wide range of particle forms. Novel techniques of determining the limiting void ratios and preparing the samples over a range of densities (with more or less the same fabric) are developed for DEM simulations. The results indicate that the limiting void ratios, inherent anisotropy, stiffness, peak strength and the strain to reach peak state, ultimate strength and the strain to reach ultimate state, the strain and the moblised stress ratio at the onset of dilation, the maximum dilatancy rate and the amount of dilation, particle breakage, and the existence of critical state, are all highly dependent on the form of constituent particles.

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