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Lightweight fill brings rail line back to speed

LIGHTWEIGHT EXPANDED polystyrene fill is being used on a railway contract for the first time in the UK.

The new pounds2.5M railway embankment for UK railway infrastructure manager Railtrack, built in just under three months, will replace the existing bridge 193 on the Manchester to Liverpool line at Irlam, near Manchester. Here, the railway crosses the former river bed of the River Irwell, diverted to the Manchester Ship Canal around 1900.

The bridge, first built in 1892, has been subject to a speed restriction of 32km/h for some time due to its poor condition and needed replacing. But because the river was backfilled with up to 8m of very soft clayey silts and very loose silty sands, the new structure had to be strong enough to support rail traffic and light enough to sit on the soft material. The river bed is also contaminated with arsenic, hydrocarbons and methane gas.

In October 1997, main contractor Birse Construction placed a 20,000t, 4m thick layer of granular fill to induce settlement. This was left for nine months before being removed to allow embankment construction to start in July this year.

A new sewer was then placed and surface drainage installed and the area levelled. A gas blanket was then laid to prevent methane build up from the former riverbed, with a geogrid reinforced granular mattress placed on top to give even load distribution.

The 14.5m high embankment, designed by consultant Mott MacDonald, consists of 18 layers of manufacturer Vencel Resil's Fillmaster expanded polystyrene blocks, regularly separated by up to 200mm thick layers of concrete, to reduce stress in the ground and the blocks themselves.

In all, 8000 Fillmaster blocks made up of three different sizes and five load bearing grades were used. The rot proof and non-toxic blocks which are insoluble in water range from Fillmaster 110 at the base up to the strongest load bearing grade, Fillmaster 500, specially developed for the contract, at the top. Finally, the outer layer is fire retardent blocks covered in a granular fill to form the 1:1 slopes of the embankment.

The existing bridge was due to be cut into six sections and lifted off by crane at the beginning of this month, allowing placing of the last few layers of blocks and 20 precast concrete troughs that will support the new track.

Once complete, the structure will be able to support rail traffic travelling up to 137km/h, with the potential to increase to 161km/h in the future.

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