The Align JV made up of Bouygues, Sir Robert McAlpine and VolkerFitzpatrick has started work on the construction of the UK’s longest railway bridge, with the sinking of 292 piles for the Colne Valley Viaduct.
The viaduct, which will carry the HS2 high-speed rail line for 3.4km across a series of lakes and waterways on the north west outskirts of London. It will be almost a kilometre longer than the Forth Rail Bridge.
Some spans of the viaduct will be up to 80m long, carrying the railway around 10m above the surface of the lakes, River Colne and Grand Union Canal.
The structure will be supported by 56 piers, with the widest spans reserved for where the viaduct crosses the lakes, and narrower spans for the approaches.
On top of each group of piles – some of which will go up to 55m into the ground – a concrete pile cap will support the pier, which will in turn support the 6,000 tonne weight of the bridge structure above. Instead of hammering the piles into the ground, holes will be bored before being backfilled to create the pile.
The main deck of the viaduct will be built in sections at a temporary factory nearby before being assembled from north to south.
A programme of test piling has already been 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 has resulted in a 10%-15% reduction in the depth of the piles and associated time and cost savings.
Align project director Daniel Altier said: “I have no doubt that the viaduct will become one of, if not the most striking element of HS2 phase 1 once complete. The way it will be constructed is going to be equally fascinating for engineers young and old. The sections for the deck will be fabricated at our main construction site to the west of London just inside the M25, and using a huge launching girder, the deck will be formed from north to south, along the line of the route, thereby keeping unnecessary construction traffic off the roads.”
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