Go With The Flow
15 February 2010
March 27 is the date ringed in the diaries of Costain
Project Manager, Darren Dobson, and his team working on the new
Wolvercote Viaduct near Oxford.
That night will see the climax of the two-year Highways Agency
project to replace the existing viaduct when a 5,000-tonne, 250
metre-long bridge deck is jacked 16 metres horizontally across from
its temporary bearings and plinth to become the viaduct's new
southbound carriageway.
Once settled into its new position, it will be stitched together
with the new northbound carriageway and Darren reckons the project
will be finished by June - three months ahead of schedule and on
budget.
The existing viaduct, which carries the busy A34 dual
carriageway across a flood plain, the A40 Oxford - Cheltenham road,
the Oxford - Birmingham railway line and the Oxford Canal, is a
relatively young structure, having been built in the early
1960s.
However, water has penetrated the viaduct's deck joints to such
an extent that the structure has deteriorated badly and requires
substantial, disruptive ongoing repairs. A new bridge is the more
economic solution.
Darren's team initially built a temporary viaduct (but with a
permanent deck) beside the southbound carriageway.
The process then went like this:
• Switch southbound traffic on to the temporary viaduct;
• Switch northbound traffic on to the old southbound
carriageway;
• Old northbound viaduct demolished and replacement
built;
• Northbound traffic switched back to new northbound
viaduct;
• Old southbound viaduct demolished;
• Rebuild southbound viaduct foundations and piers.
At this point, that bridge slide takes place during an overnight
closure. Afterwards, the temporary viaduct's foundations will be
demolished and the area landscaped.
The advantage of this method is that the A34 has remained open,
without disruption to traffic, throughout the contract. And making
the new deck atop the temporary supports a permanent structure from
the outset, rather than a temporary one that would have to be
replaced, "has taken millions off the cost and saved several
months", says Darren.