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Crossing the Firth of Forth

Contractors building the Kincardine Bridge on the Firth of Forth are using innovative techniques to cut costs

Project Kincardine road bridge, Fife
Client Transport Scotland
Principal contractors Morgan Est and Vinci JV
Designer Benaim
Ground engineer Cementation Foundations Skanska

Welcome to Kincardine, home of Scotland’s busiest single-lane road. But not for much longer.

The Kincardine road bridge is one of the main crossings that carry traffic across the Firth of Forth in Fife, central Scotland.

It does not quite get the attention attracted by its younger sibling downstream, the Forth Road Bridge, but it is a vital transport link for this part of Scotland nonetheless.

Every day the A876 carries 25,000 vehicles, 11 per cent of which are heavy goods, across the 71-year-old Kincardine Bridge and on through the town.

Although traffic problems were partially alleviated in 2005 with the opening of an eastern bypass around Kincardine, the dilapidated condition of the structure meant a need remained for another bridge across the estuary.

As well as diverting traffic away from the town, the new 1 km bridge will help to improve the health of local residents. At one point, air quality studies in the area found that Kincardine had a grossly high level of airborne pollutants, second only to London.

In 2006, after 13 years of debate, Scottish transport agency Transport Scotland chose a joint venture of Morgan Est and Vinci to build the latest Firth of Forth crossing, just to the east of the existing bridge at Kincardine.

Power station

The joint venture team chose a disused power station on the northern bank of the estuary as the main construction site, with a push-launch system.

The bridge deck is placed then pushed out over the water until it reaches the next pier, helping to minimise impact on the estuary’s wildlife.

The bridge at Kincardine is Morgan Est and Vinci’s fourth push-launch effort in the UK, after the two Channel Tunnel Rail Link Medway Bridges in Kent and the Thurrock Bridge in Essex.

This experience has proven invaluable. It has also meant the Kincardine Bridge has used some innovative techniques, according to project director John Osborne.

“There were two innovations incorporated in our design that gave us a competitive advantage,” he says. “The first is large drilled monopiles for the marine piers and the second is the use of partially reinforced externally pre-stressed concrete deck on a bridge, also a UK first,” he claims.

A pre-construction survey by Cementation Foundations Skanska found a number of mining cavities pre-dating 1850 that needed to be filled - one of the factors that increased project costs from £90 million to £120 million.

The next stage involved sinking 20 reinforced concrete steel-cased piles. Cornish specialist Seacore’s 1,450 tonne heavily modified jack-up barge Excalibur was used to drive the piles to a maximum depth of 34 m under the water, varying by 11 m across the span.

As well as being used as supports for the bridge, the piles were used to support temporary structures on site.

“There are 16 main piles with a 3 m diameter and four on either side of the navigation channel with a 3.85 m diameter,” says Mr Osborne. “The pile casings also act as cofferdams for the marine pier construction, while the piles will act as supports for the construction gangway.”

Marine piles

The walkway, just above high water level, allowed 8,000 cu m of concrete to be delivered to the middle of the river - a task which otherwise would have proven hugely challenging.

The concrete for the marine piles and the piers was pumped through a concrete supply pumping main installed on the gangway; piers 2 to 15 from the north shore, piers 16 to 21 from a temporary jetty built over the saltmarsh on the south.

To simplify the deck construction and save time, partially reinforced pre-stressed concrete was used on the deck pieces.

In total, 27,000 tonnes of concrete will be pushed across the Forth Estuary to complete the bridge, with each deck section cast in factory-like conditions on site and placed on a skidding beam.

From the shore at the bridge’s northern end, two hydraulic jacks, creating 400 bar of pressure, use a push and clamp method to move the deck forward at 150 mm a minute, for a total length of 45 m a push.

By Christmas, Mr Osborne and the team expect 27 of 29 launches to have been completed successfully.

“When we started, the cycle of creating the deck section to the end of the push took 20 days,” he says.

“We’re now working to an eight-day cycle and we’re hoping to get it down one further day before we’re done in February.”

Temporary bearings on top of each pile are topped with Teflon-coated steel plates.

During each push, at least two men must work at each pile to ensure contact between the plates on the bearing and on the deck sections is not broken. The aim is to keep the anti-slip reaction against the slide as low as possible.

“The biggest challenge here is ensuring an efficient sliding surface,” explains Mr Osborne. “The highest friction is created at the skidding beam at the start of the launch.

“Over the summer, when we had the best sliding conditions, we reached 11 per cent reaction. On previous projects this has been as much as 40 per cent.”

Once the entire deck is in place it will be jacked up to allow the sliding surfaces and temporary bearings to be removed. These will then be replaced with mushroom-shaped permanent bearings that have been cast on site.

Road network

As well as the main bridge work, the contract includes construction of 6.4 km of highways to link the crossing to the existing road network, incorporating five smaller bridges and a motorway interchange.

Due to the nature of the ground, several important geo-technical factors had to be taken into account for the roadworks.

Much of the area is within the flood plain of the Forth Estuary so the roads had to be built up above the existing alluvial ground level using special techniques to prevent excessive settlement.

The ground in the area is predominantly clay. Traditional methods to control the water involve drilling holes and filling them with sand, allowing the fluid to run off, but here canvas wicks have been placed at regular intervals to allow water to disperse.

Once the job is completed, road users will be travelling on the second longest push-launched bridge in the world, totalling 1,188 m. And residents of Kincardine can look forward to a quieter village.

Destressing prestressing

The Morgan Est and Vinci joint venture is using an innovative technique to help to speed up the bridge launch and keep costs under control.

The site team is partially prestressing the concrete deck sections, a technique suggested by London-based designer Benaim, which used it during the construction of two bridges in Ireland.

By doing so, it becomes possible to share the loading between the prestressing and longitudinal reinforcement in the deck.

This ensures the number of prestress tendons required is cut by as much as 50 per cent while the amount of cheaper rebar needed to compensate increases by just 20 per cent.

The deck is prestressed with a central force for the launching – this is needed as every section of bridge experiences both sagging and hogging effects during launch.

These cables are stressed off the critical path, making the casting cycle easier and faster.

On completion of the launch, a further series of cables is installed to carry the long-term loads.

The deck becomes fully compressed under all permanent loads, but under launching and traffic loads it acts as a reinforced concrete member.