Costain keeps traffic flowing

Replacing a busy viaduct while keeping the traffic flowing is no simple task. To minimise disruption, Costain has erected a temporary bridge, which will then be slid into position as part of the new structure. Martin Cooper explains.

The Wolvercote Viaduct in Oxfordshire is an important transportation structure, carrying the A34 dual carriageway over the River Thames flood plain, while spanning the Oxford-to-Birmingham mainline railway, the Oxford Canal and 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.

"There is water leaking through the deck joints," explains Highways Agency project manager, Ian Johnson. "Maintaining the viaduct would be an expensive undertaking, so it's better value for money to completely replace it."

Replacing a concrete viaduct with a new 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.

Diverted traffic

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 11-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."

Highly engineered

Sandy Forshaw, project engineer for Jacobs, says the deck has been highly engineered to include more flexibility than normal as it will be under strain when it is moved. In particular, the bracing arrangements have been designed to accommodate this movement. "As the deck will be moved in one shift, its expansion joints, located at each abutment, have been designed for rapid de-mounting and mounting," Forshaw adds.

The 250m-long temporary bridge has seven spans, three fewer than the existing viaduct, and was erected one span at a time. The lengths 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.

Once each pair of main girders had been assembled with its cross member beams, all formwork and edge protection was then also attached prior to the lift. "This is for health and safety reasons," explains Reavell. "By lifting as much as possible, we reduce the need for working at height later in the programme."

Logistically, lIfting bridge decks over road, rail and a canal called for forward planning. The initial bridge deck lift involved the span over the A40, which was closed for one night, while a night-time rail possession allowed the span over the Oxford to Birmingham railway to be installed.

Depending on which span, each lift has ranged in weight between 75t and 125t. However, because of the site's location and access restrictions, Fairfield Mabey has predominantly used a 1,000t capacity mobile crane.

"We've also used a 700t mobile for some lifts, but the big crane has been used for its reach and its lifting capacity," explains Reavell.

From the compound adjacent to the new bridge, the lifting process has been fairly straightforward. But there have been lifts, primarily the spans over the rail and the canal, where the larger crane's reach has been fully utilised.

Temporary bridge

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 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 Dobson. "But we anticipate moving the deck in one six-hour overnight phase."

Once the deck has been slid into its new position, the temporary bridge piers will be demolished, leaving no trace of them ever having been there.

Summing up the project to date, Dobson says: "The constant challenge is limiting the disruption to road users and the local community while we replace the viaduct. We not only have to take into account road users, rail lines and a canal, but also various bridleways, footpaths and cycleways. To date, we have been successful in achieving our goal."

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Fact file

• A34 Wolvercote viaduct replacement
• Main client: Highways Agency
• Main contractor: Costain
• Structural engineer: Jacobs
• Steelwork contractor: Fairfield Mabey
• Project value: £44.4M