Grain-shape characterisation of railway ballast for various levels of simulation fidelity

Grain-shape characterisation analyses have been used to study particulate matter for the best part of a century now (Wentworth, 1919; Wadell, 1932). Traditionally, a set of shape indices are employed to describe the main morphological aspects of a number of grain shapes, either in 2-D or 3-D, followed by a statistical manipulation of the results, to achieve an average characterisation of the bulk material (Fonseca et al., 2012). Regarding available computational methods, the discrete element method (DEM) has been proven ideal for the simulation of granular assemblies. During the past decade, the modelling of non-spherical particles within the DEM has become computationally more affordable. In particular, new particle formulations (e.g. Boon et al., 2012) have allowed for the modelling of thousands of angular polyhedra within reasonable timeframes. Two questions remain: What particle shapes should be considered? How detailed should these shapes be? What is the appropriate scale to describe particles of a granular assembly? In this study, the particle shapes of 50 real ballast grains were analysed using a novel grain-shape characterisation code, which generates variations of each particle shape for different fidelity levels. These ballast grains were scanned by Xiao et al. (2017) using a hand-held laser scanner and in their initial resolution, they consist of some hundred thousand faces each. The main goal is to simplify these particle shapes to levels affordable by our numerical tools and the DEM in particular. During this simplification process, various shape indices are calculated for each fidelity level, so that one can calculate the induced error due to the reduction in resolution and thus manage to strike a balance between accuracy of shape representation and computational efficiency. The various indices are then plotted against each other for each different resolution level, allowing for a comparative quantification of the shape alteration for two different morphological aspects, namely form and roundness.