Development and role of structure in railway ballast

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Performance demands on railway infrastructure in the UK are now greater than ever before and are predicted to increase in the near to medium term. For example, the Pendolino tilting trains recently introduced onto the West Coast Main Line (WCML) operate at higher speeds than ever before when curving and consequently place greater loading on the track than conventional trains. In addition, the line itself was designed to accommodate an increase in the maximum axle load of freight trains from the current 25 tonnes to 30 tonnes. Elsewhere, high speed links from Folkstone to London as part of the Channel Tunnel high speed link will bring TGV speeds to commuter trains into London from the South East. As a result of these increased demands, available maintenance windows progressively become fewer and narrower. This trend will continue, as National Rail moves towards a target of operating a 24-hour rail service. In this context, efficient and cost-effective design of rail infrastructure for maximum performance with a minimum of maintenance is essential.Railway infrastructure predominantly consists of ballasted track, which has many advantages in terms of cost and ease of maintenance. However despite its widespread use the mechanics of railway ballast are still not fully understood. As a result, ballast specification continues to be largely empirical and in some cases driven by the materials to hand. Furthermore, there is lack of scientific understanding of the mechanical behaviour of ballast and how this is affected by traffic and by maintenance operations. An improved knowledge of the mechanics of ballast would enable better design, maintenance and renewal of ballast foundations to carry heavier freight and faster passenger services more intensively. The proposed research will contribute to this by investigating two factors that significantly influence the mechanical behaviour of ballast and its performance as the track foundation: (1) the fabric structure of ballast, and (2) its discrete nature.(1) Fabric structure in granular materials like ballast is the result of particle interlocking, and it is known to have a significant effect on their mechanical behaviour. However, there is currently very little scientific understanding of what the fabric structure of ballast is or how it develops at different stages of the ballast lifecycle, e.g. due to traffic loads or track maintenance. The proposed research will investigate ballast fabric structure by recovering preserved ballast samples from operational railway track and examining in detail how ballast structure develops over time in field conditions. These investigations will be complemented by controlled laboratory experiments, to investigate the development of fabric structure and the resulting mechanical behaviour of ballast under different stress conditions and stress paths, representative of those experienced by ballast on site.(2) The relatively large size of ballast particles in relation to sleeper footprint and ballast depth means that sleepers interact with the ballast through a relatively small number of contact points. Furthermore, in granular media it is known that a small minority of particle contacts exert a disproportionate level of influence over the behaviour of the aggregate, resulting in highly irregular contact pressure distributions at the ballast/sleeper interface which can vary significantly from sleeper to sleeper. The proposed research will use a numerical method allowing simulations of granular aggregates at the particle scale, to investigate this variability in contact pressure and its effect in the overall mechanical behaviour of the ballast below a sleeper.The knowledge and insights gained from this research will contribute to the development of better design criteria and maintenance procedures for ballast foundations, leading to better value and better performing railway track systems for a new generation of trains.