Extending track-bed life using geosynthetics

With ballast placed directly over a clay or silty subgrade there is the possibility of a slurry being formed at the ballast/subgrade interface; particularly if there are depressions or pockets at formation level.

Regular traffic causes the ballast to oscillate at the interface which disturbs the clay/silt, and the presence of water in the pockets causes the particles to form a slurry. As the ballast dilates, the slurry moves into the void. The slurry is pumped upwards as the ballast contracts. This rapid, cyclical effect causes the mobile clay/silt particles to be forced progressively up into the ballast.

An alternative to using a graded sub-ballast in this situation is described in Railtrack’s Line Code of Practice - Track Substructure Treatments 2 and 3 which describes a blanketing sand of specified grading is laid on the sub-grade to act as a fine soil filter/separator to prevent ‘pumping’. 

One of the purposes for using a sub-ballast or a sand blanket is to filter any ground-water so that the ballast does not become contaminated with soil particles as a result of sub-grade erosion. 

Contamination would cause loss of ballast friction and deformation due to reduced load-bearing capacity in the subgrade. The resulting effect on track alignment would mean a reduction in track speed and, ultimately, track renewal.

A 300mm deep layer of sand was originally introduced in the UK to prevent ballast contamination but it was later found that this could be reduced to 100mm if a geotextile filter/separator was used at the sand/ballast interface – TERRAM PW1. Although this geotextile allows downward movement of water coming via the ballast the relative change in permeability at this interface means this water drains laterally to track-side drains.

Sand does not contain the graded-gravel fraction of a sub-ballast and is thus prone to intermixing with the ballast. PW1 also acts to prevent this. Unlike sand, TERRAM PW1 is compact to transport and is rapidly laid ready for placement of the ballast. It has factory-controlled properties which do not rely on the need, unlike sand, for the correct thickness to be laid consistently across and along the track. In addition, excavation and the attendant disposal of fill is reduced when a geosynthetic is used to reduce the sand-blanket depth.

PW1’s ability to act as a filter over the design life is proven in countless projects around the world over the last twenty five years. Localised excavations along 8 to 10 year-old installations have revealed the PW1 to be in good condition and visual inspection provided confidence that it would continue to do its job. One of the reasons for its longevity is thought to be that the sand provides a cushioned support to the PW1 and this prevents puncture and abrasion by the ballast.

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