Development and role of structure in railway ballast

英国对铁路基础设施的性能要求比以往任何时候都要高，预计在近期到中期还会增加。例如，最近在西海岸干线（WCML）上引入的Pendolino倾斜列车在弯曲时的运行速度比以往任何时候都要快，因此比传统列车在轨道上施加了更大的负荷。此外，这条线路本身的设计是为了适应货运列车的最大轴重从目前的25吨增加到30吨。此外，作为英吉利海峡隧道高速连接的一部分，从福克斯通到伦敦的高速连接将使TGV从东南部进入伦敦的通勤列车提速。由于这些增加的需求，可用的维护窗口逐渐变得越来越少和狭窄。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，高效和经济的铁路基础设施设计是必不可少的，以最少的维护实现最大的性能。铁路基础设施主要由有碴轨道组成，在成本和易于维护方面具有许多优势。然而，尽管它被广泛使用，铁路镇流器的力学仍然没有完全了解。因此，镇流器规格在很大程度上仍然是经验的，在某些情况下是由手头的材料驱动的。此外，对压载物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。对压舱物力学的进一步了解将有助于更好地设计、维护和更新压舱物地基，以便更密集地运送更重的货物和更快的客运服务。建议的研究将通过调查两个显著影响镇流器力学行为及其作为轨道基础性能的因素来对此做出贡献：(1)镇流器的织物结构，(2)其离散性。(1)颗粒状材料（如镇流器）中的织物结构是颗粒互锁的结果，并且已知它对它们的力学行为有显著影响。然而，对于压载物的结构结构是什么，以及它在压载物生命周期的不同阶段（例如由于交通负荷或轨道维护）是如何发展的，目前的科学认识很少。拟议的研究将通过从运营铁路轨道上恢复保存的镇流器样本来研究镇流器织物结构，并详细检查镇流器结构在现场条件下随着时间的推移如何发展。这些调查将辅以受控的实验室实验，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压载物的机械行为，代表现场压载物所经历的那些。(2)相对于轨枕占地面积和轨枕深度而言，道砟颗粒尺寸较大，这意味着轨枕与道砟的接触点相对较少。此外，在颗粒介质中，我们知道，少数颗粒接触对骨料的行为产生不成比例的影响，导致压舱物/轨枕界面的接触压力分布非常不规则，每个轨枕的接触压力分布可能有很大差异。拟议的研究将使用一种数值方法，允许在颗粒尺度上模拟颗粒聚集体，以研究接触压力的变化及其对轨枕下方压载物整体机械行为的影响。从这项研究中获得的知识和见解将有助于制定更好的设计标准和压载基础维护程序，从而为新一代列车提供更好的价值和更好的铁路轨道系统。

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.