Fillmaster Rail Embankments Case Study
Bridge 193 Replacement, Irlam, Manchester
Fillmaster was specified for this innovative project because its low density would limit the weight of the overall embankment and consequently minimise long-term ground settlement, even under repeated train loadings. Its light weight and ease of handling would also allow rapid construction without disruption to rail traffic. Although Irlam is the highest railway embankment to use EPS, similar projects have been undertaken outside the UK, most notably in Scandinavia and Japan.
Built in the 19th Century as a railway bridge to cross the River Irwell, near Irlam on the outskirts of Manchester, the bridge became redundant after 1899 when the river was diverted into the Manchester Ship Canal and the old riverbed infilled. Due to the deteriorating condition and capacity of the bridge, Railtrack had to impose a 20km/hr speed limit on trains passing over it.
To reduce maintenance liabilities and to raise line speeds, Railtrack decided to replace the bridge with a solid embankment. After thorough evaluation, Fillmaster was specified for the embankment because it would reduce overall settlement and have a reduced environmental impact. The lightweight nature of EPS, typically 1-3% the weight of traditional fill material, also meant that it was extremely easy to handle on site whilst avoiding the excessive settlement expected of conventional materials.
Designed by Mott MacDonald of Altrincham for Railtrack, the embankment, is 95m long and 14.5m high – believed to be the highest of its kind in the world and comprises 13,000m3 of Fillmaster.
Construction started by preloading the site for nine months with 4m of granular fill to surcharge the ground and form a firm foundation. This was then removed and a gas blanket and geogrid layer installed. Fillmaster blocks were then built up 8.5m, interleaved with two concrete regulating layers, underfilling the existing railway bridge and forming a firm base. A final layer of concrete was then cast over the top of the EPS. The old bridge was then lifted out in a 99-hour continuous rail possession. A further 3.5m of Fillmaster was laid to reach the height of the original bridge, surmounted by a concrete trough which was filled with ballast and the track placed.
The Fillmaster blocks, each measuring 2440 x 1220 x 610mm were stacked 18 high to form the embankment, with a 1:1 slope. As work proceeded, the slope was protected with a cover of fill. Computer modelling was used to optimise the configuration with different EPS grades which allowed less dense Fillmaster 110 to be used in areas of lower stress with Fillmaster 500, an exceptionally strong grade, being specially developed for use at the top of the embankment, where the imposed loads of passing trains would be the most intense.
The embankment was constructed in under three months and, apart from the 99-hour possession period, the line was operational throughout. This was largely achieved due to the lightweight nature of the standard-sized Fillmaster blocks which are easy to handle and permit rapid construction.