A 700t launching girder has begun its work on the HS2 high-speed railway’s first and longest viaduct. The Colne Valley Viaduct will stretch for more than 3.4km across a series of lakes and waterways between Hillingdon and the M25 just outside London.

The 160m-long bridge-building machine is the only one of its kind in the UK and will be used to lift the concrete deck segments that form the viaduct’s arches into position. Once each section is complete, the machine will move itself forward into position to build the next stage.

HS2 launching girder

The HS2 project is designed to improve rail links between London, Birmingham and north, help level-up the economy and provide a low-carbon alternative to car and air travel.

The UK government’s HS2 minister, Andrew Stephenson, said: “Today, HS2 began construction on what’s 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.”

A total of one thousand deck segments will be needed, with each one weighing up to 140t. 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 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.

The launching girder was originally built in 2004 and was first used during the construction of the Hong Kong East Tsing Yi Viaduct. The machine is named ‘Dominique’ after Bouygues engineer Dominique Droniou, who played a leading role in its design and development.

Fifty-six piers each weighing around 370t 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 precasting factory where the segments are made will cast around 12 segments every week at the peak of construction, using a ‘match-casting’ technique. The 100m-long factory, which is has an internal volume of 105,000m3.

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.

Align Project Director, Daniel Altier said: “The start of the erection of the deck segments marks the latest important milestone for Align, relating to 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. I would specifically like to acknowledge the contribution from our supply chain partners - VSL, Kilnbridge, KVJV, VolkerStevin, Tarmac, Sendin and Vaughan Plant Haulage.”

The design of the Colne Valley Viaduct was inspired by the flight of a stone skipping across the water, with a series of spans, some up to 80m long, carrying the railway about 10m above the surface of the lakes, River Colne and Grand Union Canal.

Set low into the landscape, wider spans will carry the viaduct crosses the lakes, with narrower spans for the approaches. This design was chosen to enable views across the landscape, minimise the viaduct’s footprint on the lakes and help complement views across the natural surroundings.

The team has also completed the construction of three of four jetties across the lakes to get equipment into position to support the construction and help take construction vehicles off local roads. Where the viaduct crosses the lakes, the piles are being bored directly into the lakebed, using a cofferdam to hold back the water while the pier is constructed.