HS2’s engineers have started sliding nearly half a kilometre of bridge deck over a series of pylons to span a valley just to the south of Wendover.
The temporary ‘nose’ of the beam resting on the first pier during the Wendover Dean deck slide (c) HS2
The 450m-long viaduct will be the first major railway bridge in the UK to be built with a ‘double composite’ approach, which uses significantly less carbon-intensive concrete and steel than a more traditional design – and has allowed HS2 to halve the amount of embedded carbon in the structure.
Instead of using solid pre-stressed concrete beams to form the spans between the viaduct piers, the ‘double composite’ structure uses two steel beams sandwiched between two layers of reinforced concrete to create a lightweight and super strong hollow span.
Due to the length of the viaduct, the deck is being assembled in three stages, ranging from 90m to 180m sections, with each one pushed out from the north abutment before the next section is attached behind it. This painstaking process means that the weight of the deck will increase with each push, up from an initial 590 tonnes this week to 3700 tonnes by the end of the year.
The viaduct is being built by HS2’s main works contractor, EKFB – a team made up of Eiffage, Kier, Ferrovial Construction and BAM Nuttall – with the manufacture and installation of the beams being led by specialists at Eiffage Metal.
A winch is used to push the deck forward at a speed of around 9 metres per hour, sliding across Teflon pads to reduce friction – a material usually found to the surface of a non-stick frying pan.
It is the longest deck slide on the HS2 project to date.
CGI of the Wendover Dean Viaduct (c) HS2
At this point in the Chilterns the railway will be on a slight gradient, so to help maintain control, the deck – which will eventually weigh the same as 264 double-decker buses – is being pushed slightly uphill, with the finish point approximately 1.8m higher than start point.
James Collings, EKFB’s senior engineer, said: “The project team has reached a fundamental milestone in the build of this industry-leading viaduct. We’re progressing well with the first-of-three steel launches being conducted this year and already looking forward to seeing this viaduct come to life. Along with our supply chain partners, Eiffage Metal, we’re proud to be delivering this viaduct safely and to programme.”
The beams are made of ‘weathering steel’ which naturally fades to a dark brown colour over time and will help match the natural tone of the surrounding countryside. A similar approach – using weathering steel and a double composite structure – is also being taken at the nearby Small Dean Viaduct which is currently at a much earlier stage of construction.
Weathering steel gets its characteristic colour from a surface layer of corrosion which protects the steel and removes the need for regular painting.
The Wendover Dean Viaduct will be supported by nine evenly spaced piers, some of which will be up to 14 metres high. These consist of a series of hollow pre-cast concrete shells – manufactured in Northern Ireland – which are placed on top of each other and filled with concrete and steel reinforcing.
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