HS2 Thame Valley Viaduct, first beams lifted into place
Text edited from a script I wrote for a video which can be found here
At 880m long and only 3m high the Thame Valley Viaduct in Buckinghamshire won’t be as impressive as the Colne Valley Viaduct in Denham. However, the way in which it is being constructed will make the viaduct pioneering, at least here in the UK.
The viaduct will pass over a large floodplain to the west of Aylesbury that is situated between the A41 and A418. Trying to build a viaduct that is almost a kilometre long within a floodplain provides a whole host of challenges, which is part of the reason why the viaduct is being constructed in the way that it is, using prefabricated elements for much of the viaduct's structure.
In order to build the viaduct an access road first had to be constructed, which had to be strong enough to allow cranes, ADTs (articulated dump trucks) and excavators to pass over it, but could not contaminate or block the flow of water.
Therefore several culverts were installed underneath the road to stop it from becoming a dam, in addition, the base for the road was made with coarse stone, which was free of fine material that would otherwise reduce permeability and could contaminate the water.
The road which has been constructed to a height of approximately 1.8m is a feat of engineering in itself, but is only temporary and will have to be removed once the viaduct is complete.
With the completion of the haul road work could begin on the difficult task of trying to sink piles and excavated material for a pile cap within a floodplain. The pile cap is a steel-reinforced concrete box which sits on top of the piles and is used as a base to support the piers. There are typically three piles underneath each cap, which have been sunk to a depth of 45m to provide a solid foundation for the piers and ultimately the viaduct to rest on.
Before earth could be excavated for the pile cap a cofferdam had to be constructed, which used sheet piling that was driven into the ground. The earth was then excavated to the top of the piles before a mould was placed inside the hole into which steel reinforcement was placed and concrete poured.
The construction of the haul road and pile caps is interesting, but not particularly groundbreaking. The pioneering part began when the construction of the viaduct's structure commenced, as it is thought that this will be the first time that a viaduct in the UK has been constructed almost entirely out of pre-fabricated sections. This includes the steel reinforcement cages for the pile cap, the concrete piers and concrete beams used to support the deck, as well as elements of the deck.
The 68, three-metre tall concrete piers that each weigh 42 tonnes and 72 concrete beams that weigh 97 tonnes have been constructed 90 miles away on the Isle of Grain by PACADAR.
Using prefabrication in a controlled environment has allowed the bridge design to be improved which it is thought will save as much as 19,000 tonnes of concrete. In addition, it will cut down the number of lorry movements required, with each beam transported on a single vehicle, which would otherwise require as many as five, four-axle lorry loads of materials to be delivered to the site.
The 42-tonne piers alone would have required the equivalent of around two, 12m3 cement mixers worth of concrete to be delivered to the site. In addition, using prefabrication means fewer workers are needed on site which further reduces vehicle traffic in the area.
The first piers were installed in June this year (2023) and so far 36 have been installed. With the piers now installed the first beams are beginning to be placed on top.
The lift was carried out using two crawler cranes, one with a lifting capacity of 300 tonnes and another with a 350 tonne capacity. The cranes worked in tandem to first lift the beam from the transporter and then slew the beam into place on top of the piers.
Once installed the beams will be post-tensioned to the adjoining beam, using short threaded bars that will be slotted into holes within the ends of the beams.
Once several beams have been installed the deck sections will be lifted into place on top of the beams. The bridge deck will be constructed using pre-assembled panels, which will be covered in a final layer of concrete that will be poured in situ to form a continuous concrete deck. The deck will support pre-cast slab track sections which will in turn support the rails.
Although the piles, pile cap and top layer of the deck are being poured in situ, it is the use of prefabrication for the beams, piers and sections of the deck and slab track that make the construction of this viaduct unique within the UK.
So whilst it may not get as much attention as the Colne Valley viaduct, The Thame Valley viaduct is no less important and is helping to push the boundaries of UK construction.
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