Polydispersity - implications for granular material behaviour and modelling challenges

Abstract

A lot of studies in granular mechanics have restricted consideration to materials with a relatively narrow range of particle sizes. However engineers have long recognised that the range of sizes in a granular material significantly influences its mechanical behaviour. Quantifying the polydispersity, i.e. determining the particle size distribution, is one of the most basic characterizations we perform on granular materials. However, our understanding of how changes in the particle size distribution influence the mechanical behaviour of granular materials is incomplete. Recently generated DEM data provide a new perspective on how changes in the particle size distribution change the fabric, the distribution of stresses and stress wave propagation in polydisperse granular materials. Gap graded materials (i.e. materials with two distinct size fractions) have attracted a lot of interest in geomechanics over the past decade. Our DEM data indicate that some of the hypotheses that have emerged are not robust; for the concept of a transitional fines content is not supported by our data. Through these studies key challenges associated with using DEM to simulate polydisperse materials have emerged: large numbers of particles must be considered and the accuracy of the coarse grained approach often used in DEM-CFD modelling is compromised.