Nonlinear buckling instabilities of interspersed railway tracks

Highlights

• Unprecedented effects of unconstrained length of railway tracks are highlighted.

• Physical nature of track buckling is studied using nonlinear FEM models.

• Interspersed approach can shift failure from progressive to snap-through buckling.

• Buckling of timber sleepered tracks can be prevented by interspersed approach.

• Track lateral resistance plays a significant role in track buckling prevention.

Abstract

In a conventional railway system, timber sleepers have been widely used for ballasted railway tracks to carry passengers and transport goods. However, due to the limited availability of reliable and high-quality timbers, and restrictions on deforestation, the “interspersed” approach is adopted to replace ageing timbers with concrete sleepers. The replacement of ageing timber sleepers is frequently done over old and soft existing formations, which have been in service for so long, by installing new stiff concrete sleepers in their place. This method provides a cost-effective and quick solution for the second and third track classes to maintain track quality. Presently, railway track buckling, caused by extreme temperature, is a serious issue that causes a huge loss of assets in railway systems. The increase in rail temperature can induce a compression force in the continuous welded rail (CWR) and this may cause track buckling when the compression force reaches the buckling strength. According to the buckling evidences seen around the world, buckling usually occurs in ballasted track with timber sleepers and thus there is a clear need to improve the buckling resistance of railway tracks. However, the buckling of interspersed tracks has not been fully studied. This unprecedented study highlights 3D finite element modelling of interspersed railway tracks subjected to temperature change. The effect of the boundary conditions on the buckling shape is investigated. The results show that the interspersed approach may reduce the likelihood of track buckling. The results can be used to predict the buckling temperature and to inspect the conditions of interspersed railway tracks. The new findings highlight the buckling phenomena of interspersed railway tracks, which are usually adopted during railway transformations from timber to concrete sleepered tracks in real-life practices globally. The insight into interspersed railway tracks derived from this study will underpin the life cycle design, maintenance, and construction strategies related to the use of concrete sleepers as spot replacement sleepers in ageing railway track systems.

Introduction

At present, due to the increase in global temperature, track buckling is a serious issue [1], [2], [3]. Hence, railway infrastructure developments related to adaptation to future extreme temperature are expected. In railways, high temperature can possibly induce rail buckling, catenary dilatation, signaling and the heating of rolling stock components. As for railway tracks, summer heat can significantly increase the rail temperature and cause the rail to expand, leading to a build-up of axial compression force in Continuous Welded Rail (CWR). Although CWR provides a smooth ride and has a lower maintenance cost, it still suffers from drawbacks as the track tends to be buckled easily when the rail temperature reaches a certain limit [4], [5], [6], [7]. Based on the evidence [8], [9], [10], track buckling can cause derailment leading to a huge loss of assets passenger lives. Track buckling around the world usually occurs in conventional railway ballasted tracks with timber sleepers.

Timber railway sleepers have been widely used and are expected to serve for about 15–20 years. Due to the limited availability of reliable and high-quality timbers, and restrictions on deforestation, most countries have adopted alternatives to replace ageing timber sleepers [11], [12], [13]. Concrete railway sleepers have been widely adopted as the replacement. It is also important to note that the main function of “spot replacement” railway tracks (also called “interspersed railway tracks”) is to enhance the performance of the lower-class tracks with low operational speed. These can be found in various countries such as Australia, Japan, the United Kingdom, and the United States [11], [12], [14], [55]. Although a partial replacement of aged and rotten sleepers is obviously more economical than complete track renewal or reconstruction, interspersed tracks have some drawbacks. According to open literature and industry knowledge, this practice could consequently undermine the existing ground foundations and also induce inconsistent local stiffness problems in the rail track system [13], [15], [16], [17] and in addition to different track decay rates [18], [19], [20], [21], [22], [23], [24], [25]. Studies of interspersed tracks have been carried out [26], [27], [28], [29] and it has been found that the replacement of timber sleepers by concrete possibly increase the deterioration rate of the railway track, as uplift behaviour occurs [29].

Previous studies have shown that the buckling strength of ballasted railway tracks depends on the track conditions, track layer geometries, types of elements (fasteners, sleepers, ballast) etc. It is interesting to note that timber and concrete sleepers have different material properties and geometry that lead to the inconsistency in resistance when it comes to interspersed tracks. According to previous studies on track buckling analysis, the major factor, that influences buckling strength, is track lateral resistance. The lateral resistance of tracks consists of sleeper base-ballast friction, sleeper side-ballast friction and ballast shoulder end force. Importantly, different types of sleepers provide different values of lateral resistance and the contribution of lateral resistance of each part is due to their properties [30], [31], [32], [33], [34], [35], [36]. It is found that the lateral resistance of timber sleepers is about 60–80% of that of concrete sleepers. Moreover, maintenance activities, such as ballast tamping, stone blowing, and sleeper replacement, can significantly reduce the compaction of ballast, leading to a reduction in the lateral resistance [32], [33]. Furthermore, torsional resistance also depends on the fasteners and sleeper types. Importantly, the replacement of sleepers should be done carefully since it may severely reduce lateral track stiffness. As seen in many studies on lateral resistance of ballasted tracks, the displacement limit of the lateral force-displacement obtained by STPTs is usually lower than those used in the previous buckling analysis. This implies that previous studies have slightly overestimated the buckling temperature of ballasted tracks. Although track buckling has been widely investigated [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], interspersed tracks and their inconsistency have never been fully analysed. The previous study of buckling of interspersed tracks has been preliminary studied using linear analysis and found that the interspersed track can improve the buckling strength of the ageing railway track [47]. However, there is still a need to fully address the benefits of interspersed tracks in buckling prevention.

In this study, the advanced three-dimensional finite element modelling of interspersed railway tracks under various conditions exposed to extreme temperature are presented using LS-DYNA. The simulations are divided into two parts: linear analysis and nonlinear analysis. This paper firstly studies the effects of boundary conditions on buckling temperature and buckling shapes using linear eigenvalue buckling analysis. Secondly, the nonlinear buckling analysis is used considering various parameters that influence the buckling strength. This study applies the values of lateral resistance within the range that possibly buckles the track. The paper thus provides the buckling temperature of interspersed railway tracks under different track conditions. The insights will help track engineers to improve track buckling mitigation methods for conventional ballasted and interspersed railway tracks.