HS2 has started construction work on what it claims is the country’s longest railway bridge. 

The Colne Valley Viaduct will stretch for more than two miles across a series of lakes and waterways between Hillingdon and the M25 on the outskirts of London.

The start of construction is the latest major milestone in the delivery of HS2, which is designed to improve rail links between London, Birmingham and the North, help level-up the economy and provide a low carbon alternative to car and air travel.

Work has begun with the launch of a 700-tonne bridge-building machine, the only one of its kind in the UK. It is being 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. 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 chief executive Mark Thurston said: “I’m absolutely delighted that we have started work to assemble the giant deck segments that will form the Colne Valley Viaduct. It is yet another big milestone for HS2 Ltd, as we work to deliver the UK’s new high speed railway. Once complete, this record-breaking structure will form a key part of the HS2 railway – helping to deliver better connections across the UK, free up rail capacity on the train network, and offer passengers a zero carbon travel option.”

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.

In total 56 piers, each weighing around 370 tonnes, are being constructed along the Colne Valley ahead of the bridge building machine. The machine will then move from one pier to the next installing the deck segments as it goes. 

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 perfectly when assembled on the piers. The mammoth 100m long viaduct precast factory, which is visible from the M25, has an internal volume of 105,000 cubic metres – making it larger than the Royal Albert Hall.

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.”

In another visible sign of progress on the project, 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.