Modular sections will form the deck of the UK’s longest rail viaduct

HS2 has started production of 1,000 giant concrete segments for what will become the UK’s longest rail bridge.

The 3.4km-long Colne Valley viaduct will carry passengers across a series of lakes and waterways on the outskirts of London at up to 200km an hour.

The scheme is being built by Align JV, a partnership between Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick. 

It will form a key part of the high speed line linking the route’s central London tunnels with the southern end of the 16km-long Chiltern tunnels in Hertfordshire.

The viaduct’s modular deck segments, which weigh up to 140 tonnes, are being cast at a huge 100m-long temporary factory close to the M25 which is larger than the Royal Albert Hall.

At the peak of construction, around 12 segments will be cast every week using a ‘match-casting’ technique. This approach, where each segment is poured against the previous one, is being used so that linking pieces fit perfectly together when assembled on site.

The bridge will consist of a series of arches up to 80m long, carrying passengers about 10m above the surface of lakes, the River Colne and North London’s Grand Union Canal.

Work has also begun on 56 giant piers that will support the viaduct, each weighing around 370 tonnes.

Each pier is designed to support the full weight of the deck segments above and sits on a set of concrete piles going up to 55m into the ground. 

Foundation work began earlier this year and will see the construction of 292 piles and 56 pile caps across the whole length of the viaduct.

Once complete, Align JV will then use a specialised ‘launching girder’ resting on top of the piers to lift the deck segments into position.

Teams have also completed the construction of the first two of four jetties across the lakes to get equipment into position to support the construction.

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.