Numerical modelling of railway ballast at the particle scale

Ahmed, Sharif; Harkness, J.B.L.; Pen, Louis Le; Powrie, W.; Zervos, A.

doi:10.1002/nag.2424

Cited by 28 publications

(53 citation statements)

References 32 publications

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“…Ahmed et al [2] presented discrete element simulations of railway ballast that showed close agreement with monotonic laboratory triaxial tests carried out by Aingaran [3], on onethird scale ballast. The experimental test rig is shown in Fig.…”

Section: Introductionmentioning

confidence: 58%

“…It was shown not to be possible to match the laboratory behaviour using the standard Hertzian elastic contact law with an elastic-perfectly plastic tangential slip model, presented in [2]. This suggests that some damage must be occurring at particle contacts.…”

Section: Discussionmentioning

confidence: 96%

“…5. These results were obtained using the discrete element model and the specimen of potential particles [11] presented in [2]. Reducing the interparticle friction angle has little effect on the initial response, which remains stiffer than the laboratory test.…”

Section: Introductionmentioning

confidence: 97%

“…Other factors, such as approximations in the modelling of the triaxial test itself are discussed by Ahmed et al [2].…”

Section: Introductionmentioning

confidence: 99%

“…The particles were chosen so that the distribution of particle forms and PSD (shown in Table 2) matched the real ballast [2,18], and the void ratio at the start of the test was the same as the void ratio measured in the laboratory. Furthermore, approximately the same number of particles (2800) were used in the real and simulated tests.…”

Section: Introductionmentioning

confidence: 99%

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Discrete element simulation of railway ballast: modelling cell pressure effects in triaxial tests

et al. 2016

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The paper investigates reproducing the effects of confining pressure on the behaviour of scaled railway ballast in triaxial tests in discrete element models (DEM). Previous DEM work, using a standard Hertzian elastic contact law with an elastic-perfectly plastic tangential slip model, has been unable to replicate the behaviour observed in laboratory tests across a range of confining pressures without altering both the material stiffness and the inter-particle friction. A new contact law modelling damage at the contacts between particles is introduced. Particle contact is via sphericallycapped conical asperities, which reduce in height if overstressed. This introduces plasticity to the behaviour normal to the contact surface. In addition, the inter-particle friction angle is varied as a function of normalized contact normal force. At relatively low normal forces the friction angle must be increased for peak mobilized friction angles to match the laboratory data, an effect that is attributed to interlocking at the scale of surface roughness. Simulation results show close agreement with laboratory data.

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Section: Introductionmentioning

confidence: 58%

Section: Discussionmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 97%

“…Other factors, such as approximations in the modelling of the triaxial test itself are discussed by Ahmed et al [2].…”

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Discrete element simulation of railway ballast: modelling cell pressure effects in triaxial tests

et al. 2016

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Development of a vector‐based 3D grain entrainment model with application to X‐ray computed tomography scanned riverbed sediment

Voepel

Leyland

Hodge

et al. 2019

Earth Surf Processes Landf

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Sediment transport equations typically produce transport rates that are biased by orders of magnitude. A causal component of this inaccuracy is the inability to represent complex grain‐scale interactions controlling entrainment. Grain‐scale incipient motion has long been modelled using geometric relationships based on simplified particle geometry and two‐dimensional (2D) force or moment balances. However, this approach neglects many complexities of real grains, including grain shape, cohesion and the angle of entrainment relative to flow direction. To better represent this complexity, we develop the first vector‐based, fully three‐dimensional (3D) grain rotation entrainment model that can be used to resolve any entrainment formulation in 3D, and which also includes the effect of matrix cohesion. To apply this model we use X‐ray computed tomography to quantify the 3D structure of water‐worked river grains. We compare our 3D model results with those derived from application of a 2D entrainment model. We find that the 2D approach produces estimates of dimensionless critical shear stress ( τcr∗) that are an order of magnitude lower than our 3D model. We demonstrate that it is more appropriate to use the c‐axis when calculating 2D projections, which increases values of τcr∗ to more closely match our 3D estimates. The 3D model reveals that the main controls on critical shear stress in our samples are projection of grains, cohesive effects from a fine‐grained matrix, and bearing angle for the plane of rotation (the lateral angle of departure from downstream flow that, in part, defines the grain's direction of pivot about an axis formed by two contact points in 3D). The structural precision of our 3D model demonstrates sources of geometric error inherent in 2D models. By improving flow properties to better replicate local hydraulics in our 3D model, entrainment modelling of scanned riverbed grains has the potential for benchmarking 2D model enhancements. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.

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Generalized contact model for polyhedra in three‐dimensional discontinuous deformation analysis

Zheng

Jiao

Sitar

2018

Num Anal Meth Geomechanics

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We present a generalized contact computation model for arbitrarily shaped polyhedra to simplify the contact analysis in discontinuous deformation analysis. A list of generalized contact constraints can be established for contacting polyhedra during contact detection. Each contact constraint contains information for 2 contact points, unique contact plane, and related contact modes (open, locked, or sliding). Computational aspects of the generalized contact model include identification of contact positions and contact modes, uniform penalty formulation of generalized contact constraint, and uniform updating of contact modes and contact planes in the open-close iteration. Compared with previous strategies, the generalized contact computation model has a simpler data structure and fewer memory requirements. Meanwhile, it simplifies the penalty formulation and facilitates the open-close iteration check while producing enough accuracy. Illustrative examples show the ability of the method to handle the full range of polyhedral shapes. KEYWORDS contact formulation, contact identification, generalized contact model, open-close iteration, threedimensional discontinuous deformation analysis 1 | INTRODUCTIONCorrectly modeling contact interaction of discrete bodies is the key issue in discontinuous computation methods. 1-5 Focusing on polyhedral bodies, resolving the contact interaction is difficult as the non-smooth change of polyhedron face normal. In DEM and FDEM modeling, the common plane model, 2 the energy-conserving contact interaction model, 6 the triangulated rounded bodies model, 7 the polygon-based description, 8 the potential particle model, 9,10 dilated polyhedra 11,12 and the potential function model 13,14 have been applied to treat contact of polyhedra. These contact models are mostly for granular materials such as ballast and soils, and the contact interaction law is based on the overlap of 2 contact bodies. However, for the 3-dimensional (3-D) discontinuous deformation analysis (DDA) method, 3,4 a strict "no penetration" requirement leads to very small overlap during contact interaction. The resolution algorithms for both convex and concave polyhedral blocks are mostly depending on the classification of the contact types. [15][16][17] The final contact constraint information can be obtained according to the first entrance rule or shortest exist rule. 5 A 4-type identification phase 15-17 and the separate treating of the vertex-to-face model 18,19 and edge-to-edge models [20][21][22] are used, which may complicate the data structures and contact analysis procedure of the program. A more general contact model for polyhedral blocks that strictly fits the contact constraint requirement and the implicit solution approach

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Numerical modelling of railway ballast at the particle scale

Cited by 28 publications

References 32 publications

Discrete element simulation of railway ballast: modelling cell pressure effects in triaxial tests

Discrete element simulation of railway ballast: modelling cell pressure effects in triaxial tests

Development of a vector‐based 3D grain entrainment model with application to X‐ray computed tomography scanned riverbed sediment

Generalized contact model for polyhedra in three‐dimensional discontinuous deformation analysis

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