Development and role of structure in railway ballast

英国对铁路基础设施的性能要求现在比以往任何时候都要高，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摆式列车在弯道时的运行速度比以往任何时候都要快，因此在轨道上施加的载荷比传统列车更大。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。在其他地方，作为英吉利海峡隧道高铁的一部分，从Folkstone到伦敦的高铁将把TGV的速度带到从东南部进入伦敦的通勤列车。由于这些增加的需求，可用的维护窗口逐渐变得更少和更窄。随着国家铁路朝着24小时运营的目标迈进，这一趋势将继续下去。在这种情况下，高效和成本效益的铁路基础设施设计是至关重要的，以实现最大的性能和最小的维护。铁路基础设施主要由有碴轨道组成，在成本和易维护性方面具有许多优势。然而，尽管铁路道碴得到了广泛的应用，但人们对它的力学仍不完全了解。因此，道碴规范仍然主要是经验的，在某些情况下是由手头的材料驱动的。此外，对道碴的机械性能以及交通和维护操作如何影响这一特性缺乏科学的了解。提高对压载力学的认识，将能够更好地设计、维护和更新压载基础，以便更密集地运载更重的货物和更快的客运服务。这项拟议的研究将通过调查两个显著影响道碴的力学行为及其作为轨道基础的性能的因素来促进这一点：(1)道碴的结构结构，以及(2)其离散性质。(1)像道碴这样的颗粒材料中的结构结构是颗粒互锁的结果，众所周知，它对其力学行为有显著的影响。然而，目前对于道碴的织物结构是什么，或者在道碴生命周期的不同阶段，例如由于交通负荷或轨道维护，它是如何发展的，科学了解很少。这项拟议的研究将通过从运营的铁路轨道上恢复保存下来的道碴样本，并详细研究道碴结构在现场条件下如何随时间发展来研究道碴织物结构。这些研究将得到受控实验室实验的补充，以调查道床在不同应力条件和应力路径下的结构发展和由此产生的力学行为，具有代表性的是现场道碴所经历的那些。(2)相对于枕木占地面积和道碴深度，道碴颗粒相对较大，这意味着轨枕通过相对较少的接触点与道碴相互作用。此外，众所周知，在颗粒介质中，少数颗粒接触对集料的行为产生了不成比例的影响，导致道碴/枕木界面上的接触压力分布非常不规则，不同枕木之间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集料，以调查接触压力的变化及其对枕木下道床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.