Deterioration of the track geometry has been recognised to be the main source of the need for track maintenance. This deterioration is mainly caused by the settlement of the track ballast substructure, which tends to depend on the site conditions. Ballast is the most important component of the substructure because it is the only imported material applied to the track in order to restrain it. Ballast is also important for providing a medium of restoring track geometry. As the wheels pass over the track there is a cyclic loading imposed on each sleeper. This causes stresses in the ballast that are sufficient to produce movement and settlement of the ballast.
This track settlement causes deterioration of the track geometry, which needs to be
restored by tamping. However, tamping causes further ballast breakdown due to the
action of the tamping machine. Repeated tamping as part of the maintenance cycle
will eventually clog the ballast with fine material generated from ballast breakdown.
This reaches a critical level when the water fails to drain from the ballast properly. At
this stage, the track needs to be maintained either by ballast cleaning or ballast renewal.
Further problems occur with maintaining track alignment particularly in heavily canted curves where the alignment can be lost due to ballast movement under lateral forcesCan the problems of maintaining ballast be reduced or eliminated?
Two strategies could be adopted to reduce the burden of maintaining ballast:
- Improve the ballast to increase its performance and longevity, or
- Adopt an alternative concrete track form such as slab track, which does not use ballast
An interesting approach using a polymer reinforcement technique to modify the
behaviour of the ballast has been demonstrated in the UK – the XiTRACK®. GeoComposite treatment modifies the behaviour of the ballast to enhance the track support stiffness and strength to a desired level, without altering the other beneficial properties of the ballast such as free drainage.
XiTRACK uses a tailored visco-elastic polymer applied to the exposed top surface
RAILWAY STRATEGIES December-January 2007 91 of the ballast as shown in the picture, which cures as it penetrates into the ballast to form a three-dimensional reinforcing cage down to a specified depth determined by the polymer rheology. The treated track retains its geometry for longer and delivers significant reductions in maintenance. XiTRACK has been used to correct persistent problems with vertical track stability, prevent loss of horizontal alignment on curves and to correct problems with weak formations in isolated problem locations.Slab Track – an alternative ballastless track form
Whilst it is generally accepted that ballastless tracks require less maintenance and hence have a lower whole-life cost, there is a perception that slab track is significantly more costly than ballasted track (i.e. it is too expensive to install). Studies have looked at the initial cost and life cycle cost of slab tracks and give estimates of 1.3 to 1.5 times ballasted track initial cost with maintenance costs reduced by 30 to 80 per cent, but these studies have gathered data for slab tracks. It is easy to appreciate that such a system is time-consuming and complex to construct and fails to eliminate a significant inspection and maintenance activity around the rail fixings. Embedded rail offers a virtually zero maintenance alternative for rail fixing in a slab track.
Embedded rail technology has undergone an evolutionary process, each step improving the efficiency of material use and reducing cost. Some of the more recent
embedded rail solutions can now meet the goal of low first cost and can be considered as a real alternative to conventional track.
Examples of embedded rail can readily be found in urban railways, particularly tramways, where embedment is used to fix grooved rails to create a running surface level with the surrounding structure allowing use by mixed traffic.
The need to prevent earth leakage currents has led to the development of the
ALH factory pre-coated embedded rail which has been used for most of the new
tram systems in the UK and Ireland including Nottingham and Dublin.
More recently, the availability of high performance microcellular polyurethanes has enabled rail support components to be designed with dramatically better performance in dynamic support and noise and vibration control. This technology can equally be applied to tramways, metros or heavy rail to deliver cost-effective groundborne vibration mitigation.
Although slab track has become the preferred track system for new tunnels it is not yet widely installed in plain line. Why is this so when sophisticated railway operators
can readily quantify its benefits? A part of the answer may be inertia – if you have so
much investment in equipment to maintain ballast and so many people employed it is
hard to change. Although initial costs are dominant for maintenance savings greater
than ten years from installation and do not provide any significant discount on the
invested capital, it is possible to make a good financial case for the extra life obtained by installation of the newer designs of embedded rail slab track, which offer much shorter pay-back periods of around three years.
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