An investigation of fabric and of particle shape in railway ballast using X-ray CT and the discrete element method

Abstract

The mechanical behaviour of uncemented granular matter is influenced by the grain shape (i.e. form, angularity and concavity) and the material’s fabric (i.e. the spatial arrangement of particles and contacts). Discrete element modelling has been used in the past to compare different shaped particles (e.g. spheres, ellipsoids, etc.) but without any clear quantitative link to the shape of real materials. Existing methods of direct observation of fabric are restricted to sands (i.e. in the sub-millimetre particle scale) that bear little relation to gravel sized material and above, such as railway ballast, which is typically an order-of- magnitude larger and much different in shape.

Non-destructive, micro-focus X-ray Computed Tomography (XCT) imaging in 3D is used to visualise, quantify and assess the fabric of intact large-scale ballast bed sections and complete 150 mm diameter laboratory element test specimens for the first time. Shape information obtained by detailed 3D characterisation of real ballast particles has been used to create a library of representative ‘DEM ballast particles’ that are employed (in a new triaxial shear model based upon the principles of potential particles) to investigate, isolate and elucidate the impact of particle shape on mechanical and fabric response observed in railway ballast.

Fabric in railway ballast experiences load induced changes (primarily) through the breaking and/or forming of (new) contacts points and increased anisotropic intensity of contact orientations in favour of the load path. This is accompanied by some loss in anisotropy of the particles orientation but this is not significant indicating that confinement of surrounding particles (in the crib and shoulder) inhibit particle movements. A 90° stress rotation appears to alter the fabric in such a way that reverting to the original loading path does not recover the original fabric. This is true for particle and contact orientation fabric. In the volumetric response domain, form (i.e. the 3D proportions of the particle) changes the packing characteristics such that greater compression is seen during initial stages of shear. While angularity and concavity increase the rate of dilation, the latter has the greatest rate change. With greater shape complexity (form?angularity?concavity), stronger mobilised strength characteristics are also seen. In the domain of fabric, particle rotation is the principal parameter controlling how the structure in granular materials behaves under loading. As the shapes tends towards realistic concave objects, stronger more resilient fabric develops that impedes rotation through a combination of improved packing, higher contact coordination number and increased intensity of contact orientation anisotropy

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Identifiers

ORCID for S. Ahmed: orcid.org/0000-0002-3290-3592

ORCID for W. Powrie: orcid.org/0000-0002-2271-0826

ORCID for A. Zervos: orcid.org/0000-0002-2662-9320

ORCID for J. Harkness: orcid.org/0000-0003-0908-0791

ORCID for L. Le Pen: orcid.org/0000-0002-4362-3895

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