The Colne Valley Viaduct, under construction near Hillingdon, will carry HS2 trains 3.4km across the Grand Union Canal, River Colne, local roads and a series of lakes between the end of the London tunnels and the start of the Chiltern tunnels.
For the last year, a 700-tonne launch girder – the only one of its kind in the UK – has been assembling pre-cast concrete segments to form the first 1km of the viaduct deck along the edge of the valley.
Now the first V-shaped piers have been completed in the lake, to allow the viaduct to curve eastward over the water. The piers, each weighing 1,800 tonnes, will support a row of arches. To allow for the gentle curve of the viaduct, each of the 1,000 segments that form the arches and deck are slightly different – and all are manufactured at a temporary factory set up nearby.
The 160-metre-long launching girder – effectively a bridge-building machine – is used to lift the concrete deck segments that form the viaduct’s arches into position. Once each section is complete, the machine inches itself forward to build the next stage.
The launching girder was originally built for Bouygues in 2004 for the construction of the Hong Kong East Tsing Yi viaduct. In the last year, it has installed more than 300 out of 1,000 segments required to complete the viaduct deck, installing up to three pairs of segments each day.
Construction of this section of the project is led by Align JV – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick, working with suppliers including Kilnbridge, Keller, Bouygues subsidiary VSL, and CRH subsidiary Tarmac.
Align project director Daniel Altier said: “I am delighted that by working as an integrated team with our supply chain partners, we have been able to erect nearly a third of the deck and complete the first V pier, just a year since Dominique starting operating.
“What has been achieved would not have been possible without the support of the different teams across the project including Quality, Lifting, Safety, Consents, Environment, Surveying and also Earthworks, for maintaining the haul road on which the 60-140 tonne segments are transported from our factory to meet the girder.”
Designed to bear the weight of the 80-metre wide arches over the lakes, the ‘v-piers’ are twice as large as simpler piers that carry the viaduct over land.
Cast in place using a series of moulds, each pier has a separate tower crane, with a temporary access bridge linking them with the main construction site. Cofferdams were used to hold back the water while the 60-metre deep foundations were built into the bed of the lakes.
Each pier weighs around 1,800 tonnes and took nine months to complete. To help the engineers master the complex shape of the pier, a mock-up was built off site before work began on the real piers. In total, 11 ‘v-piers’ will support the viaduct over water with a further 45 piers on land.
Once the piers are ready, deck segments will be placed alternately on each side, using a cantilever approach to balance the structure, as two half-arches are constructed simultaneously. Steel tensioning cables are then threaded through the segments to strengthen the bridge.