The benefits of stabilising geogrids on the railway
by Andrew Lees, on January 23, 2020
Trackbed maintenance and line speed restrictions significantly affect schedules and are expensive and disruptive to the public, industry, operators and asset owners.
Supporting railway tracks
Traditionally, railway tracks comprise of rails and sleepers supported by ballast and sub-ballast above the subgrade. An alternative is ‘slab track’, used for example on bridges and in tunnels and for some high speed lines, where the rails sit on pads supported by a concrete base.
When ballast and sub-ballast are used, they work together to provide bearing capacity to support dynamic loads from trains. The ballast layer is typically 300mm thick, with sub-ballast thickness determined by the strength of the subgrade.
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Typically, railway maintenance aims to rectify poor track geometry and the loss of vertical and horizontal alignment of the rails, caused by subgrade, ballast and sub-ballast deformation, which often leads to speed restrictions. Ballast has to be tamped and additional aggregate placed on top as it settles over time.
Mechanical stabilisation of sub-ballast reduces maintenance
A technique that has been used since the 1980s, mechanical stabilisation of sub-ballast layers using geogrids is a tried and tested approach that can reduce maintenance, by reducing traffic-induced degradation.
Site workers roll out Tensar geogird on railway track
When ballast or sub-ballast is compacted over a geogrid, it partially penetrates and projects through the geogrid’s apertures to create a strong and positive interlock. Granular material is confined and restrained from lateral spread, maintaining horizontal and vertical rail alignment.
Mechanically-stabilised sub-ballast is better able to distribute loads, by increasing the area of influence on the subgrade, so bearing pressures are lower. This improvement in bearing capacity means the rate of track settlement is slower, compared with non-stabilised sub-ballast layers, with track geometry maintained for longer and ballast life extended ten-fold.
Track profiles can also be smoothed at interfaces between structures, for example where embankments meet slab track beneath overbridges and in tunnels, by reducing the effects of differential settlement.
Mechanically-stabilised sub-ballast layers can also be up to 50% thinner, while maintaining bearing capacity, reducing excavation and replacement of weak subgrades, cutting construction time and costs.
Lower quality materials stabilised with geogrid can exceed those of non-stabilised, higher quality materials. This could have a major positive impact on costs and sustainability, as it enables re-use of fouled ballast materials as sub-ballast, in both new track construction and rehabilitation.
Delivering savings over the lifetime of the railway
With an ever-increasing burden being placed on rail infrastructure, there is clearly a need to deliver safe permanent way efficiently, with designs offering value for money with minimal environmental impact. Incorporating stabilising geogrids into sub-ballast can help, reducing costs, in terms of both cash and carbon, over the operational life of the railway.