摘要
铁路路堤与桥、涵等结构物之间的过渡段处易出现明显的差异沉降,形成"跳车",地基土体的固结和蠕变并不能完全解释这个现象。以往研究表明,过渡段处轨道顶面的不平顺和轨道基础刚度差异会导致轮轨动力荷载增大,进而引起轨道基础的附加压缩。首先确定了过渡段地基刚度的变化规律;然后采用动力计算模型分析了过渡段处的轮轨动力荷载,结果表明列车通过该过渡段时会产生很明显的动力荷载,最大可达列车静轮载的1.5倍。此外,计算还显示动力荷载主要由轨道沉降差和结构物刚度所控制;最后,采用压密模型对动力荷载下轨道基础的附加压缩量进行了预测。
Railway transition zones between the embankment and fixed structures like bridges and culverts are one of the main concerns of the railway administrations. Differential settlements usually occur at these places, forming dips at the end of structures and leading to bad driving conditions （i. e. low safety and comfort）. More importantly they cause a lot of additional maintenance, which cost a lot of money and reduce the availability of railway infrastructures. Even after regular maintenance, the problem occurs periodically. The differential settlements due to subsoil consolidation and creep cannot fully explain the excessive maintenance. Therefore, a better understanding of the behaviour of transition zones is required to obtain knowledge on mechanisms governing the high maintenance requirement of railway transition zones. Based on a review of available literature and relevant theories, it is expected that an uneven track surface and subgrade stiffness difference at transitions result in high vertical accelerations and large dynamic wheel-loads, and these effects have influences on the degree of rail foundation compaction. It is widely believed these effects might be responsible for the excessive maintenance requirement of railway transition zones. Therefore, the purpose of this study is to investigate the behaviour of a railway transition zone under dynamic loading of a train passage, determine the amplitude of dynamic axle forces and estimate the additional densification of ballast induced by the dynamic loads for the studied transition zone. This study is based on a real case of an existing railway transition zone between normal embankment and a concrete culvert. The transition is situated along the Utrecht-Rotterdam/the Hague railway line in the western part of the Netherlands. As a prerequisite of dynamic simulations, and one of the commonly believed major causes of high wheel-rail force and track degradation at transition zones, the subgrade stiffness variation of the transition zone was determined first, with the finite element code PLAXIS and Mtrack 3D/2D model. It turns out that there is a big stiffness difference between two-dimensional and three-dimensional calculations. A relation between them was found and the stiffness variation at the transition was determined. It was found that the stiffness ratio between the culvert and the embankment is about a factor of three and the stiffne.ss variation length is about 15m for the studied transition case. The dynamic wheel-rail forces at the transition were determined with software developed by Cambridge University, which takes into account both a varying subgrade stiffness and track settlement, and incorporates the dynamic characteristics of railway vehicles. Significant dynamic forces up to one and a half of the static wheel load occur at the transition under normal circumstances. In addition, parameter studies were performed. The studies reveal that the stiffness variation at the transition is of minor influence, and the overall dynamic behaviour is dominated by the track settlement difference at the transition and the stiffness of the fixed structure. Finally, the additional densification of track foundation due to the calculated dynamic loads was predicted with a compaction model proposed by Sawicki and Swidzinski. The deformation of ballast and sand layer due to cyclic loading can be estimated with an acceptable accuracy for the first maintenance cycle, by the proposed compaction model. The deformation difference between the embankment and the structure is in the order of 10mm after around 2 month＇s operation. The study also shows that the differential setdements caused by the load difference itself are relatively small （only in the order of lmm）, and the settlement difference is mainly caused by the thickness difference of compactable ballast between the embankment and the structure. While in the long run, the proposed compaction model can not take into account the influence of regular maintenance, thus it is not capable of giving a reasonable explanation to the excessive maintenance at railway transition zones, further research is necessary for finding out the real leading mechanism.
出处
《水利水电快报》
2008年第8期43-50,共8页
Express Water Resources & Hydropower Information
关键词
过渡段
动力荷载
刚度
压密
路堤
结构物
transition zone
dynamic load
stiffness
densification
embankment
structure
