Basic Functioning Of The Ballasted Track Substructure

A ballasted railway track substructure is a layered system; its components include ballast, subballast and subgrade soils combining to provide support to the rail and sleepers. A capping layer (or improvement layer) may be included in the track support system to protect the natural ground or fill from moisture ingress and to form a unified subgrade layer.

This combined track support system serves to keep the rails and sleepers intact at the required position by resisting and dissipating the vertical, transverse and longitudinal forces transmitted through the rails, the fastening system and the sleepers. The track support system distributes the loads to the layers below, protecting the subgrade from excessive stresses, attenuates mechanical shocks and provides efficient drainage. A schematic overview of a ballasted railway track substructure is seen below.

Subballast and capping layers in conjunction with compacted natural ground or imported fill embankment

In the prevention of high maintenance costs the subballast plays an imperative role as a subballast layer helps in protecting the upper surface of the subgrade from the intrusion of ballast stones and acts as an inverted filter in the case of mud pumping (potentially causing ballast fouling) while facilitating rainwater run-off and further distribution of static and dynamic loads – the latter of course being of special importance as the controlling subgrade stress is usually at the top zone of the subgrade.

A suitable subballast layer may even function as a high-performance capping layer in parallel with its features as subballast – introducing a ductile layer of high strength and high stiffness – eliminating the costly need for either replacing weak top soils with soils exhibiting desirable characteristics or, equally costly, chemical stabilization via lime or cement of the existing weak top soils.

Reduced Ballast Height

Special considerations often arise in situations where building height is restricted, e.g. in tunnels, underpasses and on bridges.In such situations, track engineers may be confronted with a demand to limit the ballast height. A suitable sub ballast layer in the form of a resilient mat, providing for a known and constant degree of stiffness and material damping in between the concrete trough and the ballast, is often the most economically and technically viable solution whereby track buckling can be avoided while at the same time maintaining other key functional characteristics of the track.

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Subballast In Conjunction With Bridge Noise Control

The noise generated during the passage of rolling stock on a bridge is usually much higher than when the same rolling stock runs on plain track at grade. The noise increase varies considerably from one bridge to another but it can typically be 10 dB or more. Bridges, therefore, often form areas of special importance in noise maps. Although bridges vary greatly in design and construction, one of the most important factors behind the bridge noise amplification phenomenon, together with the chosen trackform and rail fastening method, is the “loudspeaker membrane” effect of the bridge deck. This means of course that for a ballasted track, resilient subballast mats have great potential as a highly effective means of mitigating the dynamic forces entering the bridge deck.

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