Track Bed Structures

Abstract

A track bed composite structural element issued in railway track engineering, and has an upper water permeable layer, a lower water permeable layer and a filter material layer provided between the upper and lower water permeable layers. The upper and lower layers may comprise geotextile layers and the filter material may be sand or glass, for example.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to track bed structures and in particular to an improved track bed structure for use in railway track engineering, ameliorating undesirable effects including sub-grade erosion.[0003]2. State of the Art[0004]Cohesive and fine soils have behaviour characteristics that are dominated by their fine particulate content, particularly clay and silt content. Such soils are prone to sub-grade erosion beneath a railway track bed. Mudstones and shale although appearing relatively hard compared with soils, also abrade quickly under ballast comprising the track bed, and are also prone to sub-grade erosion.SUMMARY OF THE INVENTION[0005]The process of sub-grade erosion begins when permeable ballast allows rainwater to percolate downwards and wet the soil below. Alternatively water may flow upwards from the soil beneath, where the sub-grade is lower than the adjacent ground water surface profile (water tabl...

AI Technical Summary

Benefits of technology

[0006]Silt structures will readily disintegrate in water, as will clay structures, but more slowly. The vibration from a train on the track above causes agitation and fluidisation and accelerates this disintegration process for silts, clay and mudrock structures. Additionally, under the effect of a passing train, pressure and deflection can cause the slurry formed by the silt and clay suspended in water to be forced by hydraulic pressure or ‘pumped’ up into the ballast. The ballast becomes contaminated with fines which eventually can result in failure of the track bed. The undesirable consequences of sub-grade erosion are ground loss from the sub-grade, loss of vertical alignment of the track and contamination of ballast leading to a detrimental effect on functional properties, such as a loss of ability for the ballast structure to hold its shape and provide the structural stability required. As a result track maintenance becomes necessary more frequently which is disruptive to transport schedules and expensive.

[0007]Previously, sub-grade erosion has been ameliorated by use of a suitable solution to the problem using blanketing sand in accordance with a UK Standard RT / CE / S / 033. The sand blanket prevents rainwater from ‘ponding’ directly on the sub-grade soil. The fine sand component of the blanketing sand acts as a fine soil filter, inhibiting the passage of silts and clays. Excavation into blanketing sand has shown that the silt and clay sub-grade soil penetrates no more than a few millimetres into the blanketing sand before forming a stable soil filter. A liquid-permeable geo-textile is laid over the blanketing sand in order to prevent the ballast above penetrating into the blanketing sand. Geo-textiles are well known for use in this application and such materials which are permeable, are usually of plastics material, and commercially available from a number of manufacturers such as the material available under the trade name Terram.

Problems solved by technology

Additionally, under the effect of a passing train, pressure and deflection can cause the slurry formed by the silt and clay suspended in water to be forced by hydraulic pressure or ‘pumped’ up into the ballast.

The ballast becomes contaminated with fines which eventually can result in failure of the track bed.

The undesirable consequences of sub-grade erosion are ground loss from the sub-grade, loss of vertical alignment of the track and contamination of ballast leading to a detrimental effect on functional properties, such as a loss of ability for the ballast structure to hold its shape and provide the structural stability required.

As a result track maintenance becomes necessary more frequently which is disruptive to transport schedules and expensive.

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