At the end of last month, HS2 Ltd announced the start of construction on the railway’s ‘first and longest’ viaduct with the launch of a 700-tonne bridge-building machine at an event just outside London. Stretching for more than two miles across a series of lakes and waterways between Hillingdon and the M25, the Colne Valley Viaduct will also be the longest railway bridge in the UK.

Speaking at the launch event, HS2 Minister, Andrew Stephenson, said: “Today, HS2 began construction on what is set to be Britain’s longest railway viaduct, a landmark moment for HS2 and a feat of British engineering, taking the HS2 line from London, and into Hertfordshire and Buckinghamshire. Infrastructure is the backbone of HS2, and this viaduct will be integral to delivering faster journeys and an increased capacity rail network.”

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 1,000 deck segments will be needed, with each one weighing up to 140 tonnes; and to accommodate the gentle curves of the viaduct as it crosses the valley, all the segments are slightly different in shape — and made on site at a purpose-built temporary factory close to the north abutment. The huge viaduct project is being led by HS2’s main works contractor, Align JV — a team made up of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.

Built in 2004, the launching girder was first used during the construction of the Hong Kong East Tsing Yi Viaduct and was 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).

A total of 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.

At the peak of construction, the viaduct pre-cast factory where the segments are made will cast around 12 segments every week using a ‘match-casting’ technique with which each segment is poured against the previous one to ensure the whole deck fits perfectly when assembled on the piers.

Daniel Altier, Align’s project director, said: “The start of the erection of the deck segments marks the latest important milestone in the construction of the Colne Valley Viaduct. There has been a lot of hard work involved to get us to this point, and the achievements so far are a great example of what collaboration and excellent teamwork can deliver.

“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 around 10m above the surface of the lakes, the River Colne and the Grand Union Canal.”