Once complete, the 2.1-mile (3.4km) bridge will carry the new rail line across a series of lakes and waterways near Hillingdon on the northwest outskirts of London.
It will be 100-metres longer than Dundee’s Tay Bridge, which currently holds the title of the UK’s longest railway bridge.
Construction of the foundations, the 56 piers and the deck on top, have all been taking place in parallel – with separate teams working from north to south. Deck assembly began last year, with more than 500 metres of the viaduct structure now complete.
The viaduct is being built by HS2 Ltd’s main works contractor Align JV (comprising Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick) working with its foundations contractor KVJV – a joint venture of Keller and Bouygues subsidiary VSL International – which has spent almost two years on the piling.
To install the 66 bored piles in the lakebed, the team first had to construct more than 1,000 metres of temporary jetties, with cofferdams to hold back the water around each set of foundations. The jetties are also used to transport materials and equipment during the construction of the piers and viaduct deck, keeping vehicles off local roads.
“With pier construction and deck assembly also well underway, the viaduct is fast becoming one of the most impressive and recognisable parts of the project,” said HS2 Ltd’s project client, David Emms.
On top of each group of piles – some of which are to a depth of 60 metres into the ground – a concrete pile cap will support the pier, which will in turn support the weight of the bridge structure above.
The main deck of the viaduct is being built in 1,000 separate segments at a temporary factory on the site compound and assembled using a bridge-building machine that lifts each piece into position before shifting itself forward to the next pier.
Align’s surface operations director, Derek van Rensburg said: “Completing the piling is a great achievement for both Align and the team at KVJV, working over the lakes and waterways across the Colne Valley. We were able to complete the piling with no significant impact on the environment, a great result for the whole team.”
Before work began, an extensive programme of test piling was completed with engineers sinking 12 piles at two locations with geological and structural data from these tests fed back into the design of the viaduct. This resulted in a 10-15% reduction in the depth of the piles and associated time and cost savings.