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

[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.

[0010]Track renewals and repairs are frequently carried out at night and the length of track being improved has to be closed to traffic. This is a major cost. The more quickly that repairs can be effected, then the less disruption and expense.

[0017]It is preferred that the upper, and / or the lower water permeable layers comprise geo-textile materials. Such materials are robust and suited to civil engineering situations.

[0026]It certain embodiments it will be desirable that the sealing flap is water impermeable. In other situations it may be preferable that the flap is permeable. In this case the function is more protective against foreign bodies intruding into the joint rather than being liquid sealing. Preferably, the sealing flap extends along substantially an entire edge of the structural element. Also, this protects against loss of filter material from the joint through pumping or movement.

[0033]It is preferred that where sealing flaps are provided on the upper layer of the structural element, that an overlaying anchor is provided upon which the ballast can be laid. The overlaying anchor can be a permeable geo-textile and / or a geo-grid layer to weight down the flaps and prevent them ‘lifting’ when the ballast is applied on top. A geo-grid adds additional strength to the arrangement.

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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