The University of Southampton
University of Southampton Institutional Repository

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

An investigation of fabric and of particle shape in railway ballast using X-ray CT and the discrete element method
An investigation of fabric and of particle shape in railway ballast using X-ray CT and the discrete element method
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
Ahmed, S.
37570e92-ba6b-4e03-9144-c70fa7722c51
Ahmed, S.
37570e92-ba6b-4e03-9144-c70fa7722c51
Powrie, W.
600c3f02-00f8-4486-ae4b-b4fc8ec77c3c
Zervos, A.
9e60164e-af2c-4776-af7d-dfc9a454c46e
Harkness, J.
026f02e8-41d9-403f-83be-0d880058ecf1
Le Pen, L.
4a38e256-d113-4bba-b0d4-32d41995928a

Ahmed, S. (2014) An investigation of fabric and of particle shape in railway ballast using X-ray CT and the discrete element method. University of Southampton, Faculty of Engineering and the Environment, Doctoral Thesis, 316pp.

Record type: Thesis (Doctoral)

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

Text
An_investigation_of_fabric_and_of_particle_shape_in_railway_ballast_using_X-ray_CT_and_DEM_Sharif_Ahmed.pdf - Accepted Manuscript
Download (57MB)

More information

Published date: 11 November 2014
Organisations: University of Southampton, Infrastructure Group

Identifiers

Local EPrints ID: 379177
URI: http://eprints.soton.ac.uk/id/eprint/379177
PURE UUID: 993205e5-9f61-4a63-92d9-8f82c4214cbd
ORCID for S. Ahmed: ORCID iD orcid.org/0000-0002-3290-3592
ORCID for W. Powrie: ORCID iD orcid.org/0000-0002-2271-0826
ORCID for A. Zervos: ORCID iD orcid.org/0000-0002-2662-9320
ORCID for J. Harkness: ORCID iD orcid.org/0000-0003-0908-0791
ORCID for L. Le Pen: ORCID iD orcid.org/0000-0002-4362-3895

Catalogue record

Date deposited: 28 Jul 2015 13:40
Last modified: 12 Dec 2021 03:36

Export record

Contributors

Author: S. Ahmed ORCID iD
Thesis advisor: W. Powrie ORCID iD
Thesis advisor: A. Zervos ORCID iD
Thesis advisor: J. Harkness ORCID iD
Thesis advisor: L. Le Pen ORCID iD

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×