Discrete element modelling of railway ballast

Discrete element modelling has been used to capture the essential mechanical features of railway ballast and gain a better understanding of the mechanical behaviour and mechanisms of degradation under monotonic and cyclic loading. A simple procedure has been developed to generate clumps which resemble real ballast particles. The influence of clump shape on the heterogeneous stresses within an aggregate was investigated in box test simulations. More angular clumps lead to greater homogeneity and the interlocking provides a much more realistic load-deformation response. A simple two-ball clump was used with two additional small balls (asperities) bonded at the surface, to represent a single particle; it is shown that particle abrasion gives the correct settlement response. A clump formed from ten balls in a tetrahedral shape was used in monotonic and cyclic triaxial test simulations and found to produce the correct response. The interlocking and breaking of very small asperities which find their way into the voids and carry no load was modelled using weak parallel bonds. The interlocking and fracture of larger asperities was modelled by bonding eight small balls to the ten-ball clump. Monotonic tests were performed on triaxial samples under different confining pressures and the results compared with existing experimental data. Tests were also simulated using uncrushable clumps to highlight the important role of asperity abrasion. Cyclic triaxial tests were then simulated on the same aggregates under a range of stress conditions and the results compared to existing experimental data for the same simulated ballast. The clumps are able to capture the behaviour of ballast under different conditions, and asperity abrasion plays an important role in governing strength and volumetric strain under monotonic loading, and on permanent strains under cyclic loading. The contribution of this thesis is therefore to show that it is possible to model a real granular material under static and cyclic conditions, providing much micro mechanical insight.EThOS - Electronic Theses Online ServiceGBUnited Kingdo