Vale of Aylesbury viaduct unveiled
HS2 contractor EKFB has been busy on the northern outskirts of Aylesbury preparing the groundworks for the Thame Valley viaduct, which will carry the high-speed line at low level across the river’s floodplain.
MAIN construction has begun on the 880 metre-long Thame Valley Viaduct to the north of Aylesbury.
The viaduct will have 36 spans, each formed of two 90 tonne precast box girders (beams) placed longitudinally on a series of 70 precast piers, which in turn each rest on three 45m-deep piles. The piers are only 3.3m above ground level, so this will be no Welwyn or Harringworth viaduct in appearance, despite the estimated total of nearly 6km (3¾ mile) of piling required.
32 spans are 25ms long and four are 20m, which when complete will have created the longest viaduct in what is known as the C23 Programme of HS2. This runs for 80km (50 miles) from just south of Beaconsfield to Greatworth (south of Warwick), and will eventually boast 15 viaducts.
Off-site manufacture
The post-tensioned, precast box girders are manufactured by Pacadar on the Isle of Grain in Kent using the same mould to ensure consistency.
They are transported individually from there to site overnight by road.
Each end of the beam is labelled north or south. As there is no area on site large enough to turn one of the delivery lorries, the beams must be loaded the right way round in Kent for lifting off the trailer and onto the viaduct piers.
Once a beam has been lifted clear of the trailer by two crawler cranes, the lorry drives away and engineers check the beam’s dimensions the oldfashioned way by using a spirit level and tape measure. The beam is then lifted, swung and lowered onto the pre-prepared pier baseplates, each with four vertical bolts to secure them onto their designated pier, while 10 rods create a strong connection between the beams. A complete lift is carried out in about 15 minutes.
The viaduct’s deck, formed of reinforced concrete slab partly precast and partly created on-site, will be placed on the beams later for the rails to be installed upon.
The original plan was to have four precast beams, but a redesign halved this. As a result of this change benefits include environmental, cost and flexibility in the construction sequence, plus a reduction in temporary works during construction.
Just like Lego
Construction of the piers began several months ago.
All the major elements of the bridge – the concrete piers, beams, reinforced concrete deck planks and parapet – are manufactured off-site on the Isle of Grain and assembled on site, as HS2 puts it: “Like a giant Lego set!”
HS2 claims this is the first prefabricated viaduct to be built in the UK.
Contractor EKFB is a joint venture between Effiage, Kier, Bam Nuttall and Ferrovial construction, a Spanish company.
A spokesperson said: “This structural design is based on similar viaducts built as part of the Spanish high-speed rail network, and reduces construction time, workforce numbers, and embedded carbon.”