UK: The EKFB joint venture of Eiffage, Kier, Ferrovial Construction and BAM Nuttall reports significant progress in the construction of two major viaducts on the section of High Speed 2 it is building in Buckinghamshire.
EKFB is responsible for constructing 79 km of the route from the northern Chiltern Tunnel portal to Long Itchington Wood in Warwickshire. This section includes the viaducts Wendover Dean and Small Dean.
Double-composite structure
The deck for the 450 m long Wendover Dean Viaduct was slid into its final position on August 11, marking the end of a complex eight-month-long process. An initial section of deck was pushed into place in January; these first elements of the deck weighed 590 tonnes, but by the time the fully assembled deck had been secured across the valley, this had risen to 3 700 tonnes.
Project promoter HS2 Ltd says the viaduct is the first major railway bridge in the UK to be built with a ‘double-composite’ structure. The deck is formed from weathering steel sections between 23 m and 25 m long, which were welded together to form two parallel steel beams. Precast concrete panels are then placed at the top and bottom of the beams to form the hollow double-composite arrangement.
The project promoter says this design creates a strong and lightweight structure which requires significantly less carbon-intensive concrete and steel than more traditional approaches.
The ‘push launch’ of the viaduct involves sliding pre-assembled sections of bridge deck on temporary bearings on top of the piers, between which Polytetrafluoroethylene pads are placed to reduce friction. The entire structure is then ‘pushed’ — or more accurately pulled — over the next set of piers, with a series of winch and pulleys system used to effectively drag the structure forward.
Now that the deck has been moved into its final position, the process of removing the temporary bearings can begin. This will involve lowering sections of the deck by 600 mm in 200 mm increments until the section is resting on its permanent bearings.
Small Dean span
A similar approach is being adopted for the nearby Small Dean Viaduct, located just to the north of Wendover Dean. However, unlike its near-neighbour which was built in sections, the deck for this viaduct is being constructed in a single phase. This is made possible because there is more room to assemble the 345 m long deck at the Small Dean site, and a single slide is required to minimise disruption to the London – Aylesbury railway and the A413 trunk road, which the viaduct will cross.
Assembly of the deck is now approaching completion, and EKFB is aiming to slide it into place early next year. This is expected to require a one-day closure of the A413 as a temporary guide at the tip of the deck is moved over the road. However, the road will remain open during the remainder of the work, although the railway will be closed for the duration — expected to take four days.
Once in position, precast concrete parapets will be installed on the deck, and the upper and lower precast panels will be covered with a further layer of concrete to bind them into the main structure.
