HS2 begins constructing the UK’s longest railway bridge
Image credit: hs2
Construction has started on the UK’s longest railway bridge, which will stretch for two miles across a series of lakes and waterways between Hillingdon and the M25.
The Colne Valley Viaduct, which is part of the HS2 project, is being built with the help of an enormous 700-tonne bridge-building machine.
Known as a ‘launching girder’, the 160m long bridge-building machine is the only one of its kind in the UK and will be used to lift the giant concrete deck segments that form the viaduct’s arches into position. Once each section is complete, the machine will inch itself forward into position to build the next stage.
A total of one thousand deck segments will be needed, with each one weighing up to 140 tonnes. To allow for the gentle curves of the viaduct as it crosses the valley, all the segments are slightly different shapes and made on site at a purpose-built temporary factory.
HS2 minister Andrew Stephenson described the start of construction as a “landmark moment” for the UK’s largest rail project.
“Infrastructure is the backbone of HS2 and this viaduct will be integral to delivering faster journeys and an increased capacity rail network,” he said.
Originally built in 2004, the launching girder was first used during the construction of the Hong Kong East Tsing Yi Viaduct. Specially designed to handle complex viaduct construction, the machine is named ‘Dominique’ in memory of Bouygues engineer Dominique Droniou, who played a leading role in its design and development.
56 piers each weighing around 370 tonnes are being constructed along the Colne Valley ahead of the girder, with the girder moving from one pier to the next, installing the deck segments as it goes.
One segment is put in place each side of the central pier, using a cantilever approach to balance the structure, as two half-arches either side of each pier are constructed simultaneously. Steel tensioning cables will be threaded through the segments to strengthen the bridge.
The viaduct pre-cast factory where the segments are made at the peak of construction will cast around 12 segments every week using a ‘match-casting’ technique. This approach - where each segment is poured against the previous one - will ensure the whole deck fits together when assembled on the piers.
Once construction is complete, the factory and surrounding buildings will be removed and the whole area between the viaduct and the Chiltern tunnel will be transformed into an area of chalk grassland and woodland as part of HS2’s ‘green corridor’ project.
The design of the Colne Valley Viaduct was inspired by the flight of a stone skipping across the water, with a series of elegant spans - some up to 80m long - carrying the railway approximately 10m above the surface of lakes, the River Colne and the Grand Union Canal.
In January this year, the government introduced a bill to parliament with the aim of extending HS2 northwards to Manchester.
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