Bridges
Steel can be a solution for Viaduct Replacement in time to comes
The Wolvercote Viaduct in Oxfordshire is a vitally
important transportation structure as it not only
carries the A34 dual carriageway over the River
Thames flood plain, but also spans the Oxford to
Birmingham main line railway, the Oxford Canal as
well as the A40 Oxford to Cheltenham road.
Built in the early 1960s this strategic double
bridge viaduct has suffered extensive deterioration
due to the ravages of time and requires significant
and regular maintenance to ensure it remains safe.
Replacing a concrete viaduct with a new weathering
steel composite structure while ensuring minimal
disruption to the travelling public has presented
the construction and project design team with a
number of challenges.
One of the main objectives of the scheme is
to replace the existing 250m-long viaduct while
maintaining the current peak time traffic flows. To
achieve this a dual lane offline temporary viaduct,
capable of taking the A34 southbound traffic, is being
constructed adjacent to the existing southbound
bridge.
This offline viaduct will allow traffic to be diverted
off the existing northbound viaduct so it can be
demolished and a new structure built in the same
position. Once the northbound viaduct is constructed
the northbound traffic will be diverted back to its
original alignment.
This in turn will allow the project team to demolish
the existing southbound viaduct and construct
a new replacement structure in its place. This work
will essentially involve piling and then the installation
of new concrete piers.
“In order to reduce the project duration we will
then slide the 250m long x 11m wide offline viaduct
deck in to the original southbound viaduct alignment,”
explains Darren Dobson, Costain’s Project
Manager. “This jacking and sliding procedure will
take place in one night time shift in order to limit
the disruption to road users.”
The temporary southbound viaduct was begun in
the Summer of 2008, with piling work and piers cast
prior to steelwork erection beginning. The steel deck
was then completed in an eleven week programme
beginning in October.
“Although the southbound bridge is temporary
it still resembles a permanent structure and was
erected as such,” explains Simon Reavell, Project
Manager for steelwork contractor Fairfield Mabey.
“We have supplied interface plates which are placed
below the deck bearings, and they will aid the sliding
process.”
The 250m long temporary bridge has seven
spans, three fewer than the existing viaduct, and
was erected one span at a time. The length of each
of these spans vary, but they are all approximately
35m long. Fairfield Mabey brought the necessary
steel girders to site and assembled them into pairs
on the ground before lifting them into place. Each
span is made up of four main girders, which means
two pairs and two lifts per span.
The temporary southbound bridge is now nearing
completion. Once complete, traffic will be
switched on to the new structure allowing demolition
to start on the existing northbound bridge.
“The new northbound bridge is a similar
structure to the southbound bridge,” explains Mr
Reavell. “At 250m long and seven spans it will be
erected in the same way as the temporary off-line
bridge.”
Once demolition has been completed, Costain
will begin piling and then construct the bridge piers.
Steelwork for the second bridge is scheduled to begin
in July.
Meanwhile, the southbound temporary bridge
deck is scheduled to be moved onto its new piers
and abutments sometime in the Summer of 2010.
“Moving a 250m long deck, weighing in excess
of 5,000t, will be challenging manouevre,” says Mr
Dobson “But we anticipate moving the deck in one
six hour overnight phase.”
Once the deck has been slide to its new position,
the temporary bridge piers will be demolished leaving
no trace of it ever being there.
To keep vehicles flowing on the A34 near Oxford a temporary bridge will carry traffic while an
existing viaduct is demolished and then rebuilt. The deck of this temporary structure will later
be incorporated, after a sliding operation, into the new viaduct.
Steel solution for
viaduct replacement
FACT FILE
A34 Wolvercote
Viaduct
Main client:
Highways Agency
Main contractor:
Costain
Structural engineer:
Jacobs
Steelwork contractor:
Fairfield Mabey
Steel tonnage: 1,800t
Project value: £44.4M
Above: The Wolvercote
Viaduct spans road, rail
and canal.
Below: The initial
temporary bridge is being
built adjacent to the
existing viaduct.
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