Construction work has started on what will be the UK’s longest railway viaduct, and it’s to be built in north-west London for HS2. The HS2 railway emerges from tunnels alongside the mainline railway at West Ruislip, before curving northwards over the Colne Valley on a long sweeping viaduct.
A number of designs were looked at, including a dramatic cable-stayed bridge which would have meant fewer piers in the ground, but a far greater visual impact from a distance. The current design is aimed to be sleek and glide across the landscape without being intrusive on it from a distance, while also being visually appealing up close.
Apart from the concrete design, one of the notable aspects is that where was once proposed a concrete sound barrier running along the sides was replaced with a translucent one, which gives train passengers a view of the valley and reduced the bulk of the viaduct height.
Some of the viaducts spans will be as much as 80 metres long, carrying the railway around 10 metres 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.
Over the next year, engineers will construct 292 piles under the ground to support the viaduct piers. 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 6,000 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. An extensive programme of test piling has already been completed with engineers sinking 12piles at two locations with geological and structural data from these tests fed back into the design of the viaduct.
The main deck of the viaduct will be built in sections at a temporary factory nearby before being assembled from north to south.
When completed, at 3.4km long, it will be longer than the Forth Rail Bridge.