HS2 Ltd’s main works contractor Align JV – made up of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – has begun work on the UK’s longest rail viaduct.
The JV has begun the sinking almost 300 piles that will form the foundations for the Colne Valley Viaduct.
Designed by Grimshaw Architects, the 3.4km will become the UK’s longest rail viaduct; almost 1km longer than the Forth Rail Bridge.
The viaduct itself will carry the railway around 10m above the surface of the River Colne, Grand Union Canal and nearby lakes.
The structure will be supported by 56 piers, with the widest 80m spans reserved for where the viaduct crosses the lakes, and narrower spans for the approaches. This design was chosen to “enable views across the landscape, minimise the viaduct’s footprint on the lakes and help complement the natural surroundings”.
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 full 6,000t 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.
Align project director Daniel Altier said: “I have no doubt that the viaduct will become one, 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.
“I am delighted that today we have reached this important milestone in the viaduct’s construction.”
An extensive 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.