Development and role of structure in railway ballast

英国铁路基础设施的性能要求比以往任何时候都高，预计在中短期内还会增加。例如，最近引入西海岸干线（WCML）的Pendolino摆式列车在转弯时的运行速度比以往任何时候都高，因此比传统列车在轨道上施加更大的载荷。此外，该线路本身的设计可容纳货运列车的最大轴重从目前的25吨增加到30吨。在其他地方，从福克斯通到伦敦的高速线路作为英吉利海峡隧道高速线路的一部分，将把TGV的速度带到从东南部进入伦敦的通勤列车上。由于这些需求的增加，可用的维修窗口逐渐变少和变窄。随着国家铁路公司朝着24小时运营铁路服务的目标迈进，这一趋势将继续下去。在这种情况下，铁路基础设施的高效和成本效益的设计，以最大限度地提高性能和最低限度的维护是至关重要的。铁路基础设施主要包括有碴轨道，这在成本和易于维护方面具有许多优势。然而，尽管其广泛使用的机械铁路道碴仍然没有完全理解。因此，道碴规格在很大程度上仍然是经验性的，在某些情况下由手头的材料决定。此外，对道碴的机械性能以及它如何受到交通和维护操作的影响缺乏科学的理解。提高对压载机械的了解，将有助于更好地设计、维护和更新压载地基，以更密集地运送更重的货物和更快的客运服务。拟议的研究将有助于这一点，调查两个因素，显着影响道碴的力学行为和它的性能作为轨道基础：（1）道碴的织物结构，和（2）其离散的性质。(1)颗粒状材料（如道碴）中的织物结构是颗粒联锁的结果，并且已知对其机械性能具有显著影响。然而，目前对道碴的织物结构是什么或者它如何在道碴生命周期的不同阶段发展（例如，由于交通负荷或轨道维护）的科学理解很少。拟议的研究将调查道碴织物结构，恢复保存的道碴样品从运营的铁路轨道，并详细检查如何在现场条件下随着时间的推移，道碴结构的发展。这些调查将通过受控实验室实验进行补充，以调查织物结构的发展以及在不同应力条件和应力路径下道渣的机械行为，代表现场道渣所经历的情况。(2)相对于轨枕覆盖区和道碴深度的相对大尺寸的道碴颗粒意味着轨枕通过相对小数量的接触点与道碴相互作用。此外，在粒状介质中，已知少数颗粒接触对集料的行为施加不成比例的影响，导致道渣/轨枕界面处的高度不规则的接触压力分布，其可在轨枕与轨枕之间显著变化。拟议的研究将使用数值方法，允许在颗粒尺度上模拟粒状骨料，以研究接触压力的变化及其对轨枕下方道碴整体力学行为的影响。从这项研究中获得的知识和见解将有助于制定更好的道碴基础设计标准和维护程序。从而为新一代列车带来更好的价值和更好的性能。

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.