THE HULKING concrete sections spanning the South Esk river at Montrose, an hour south of Aberdeen, appear solid enough. Even suspended 14 m above the sea bed from a crane the size of a roll-on roll-off ferry, a 2,360-tonne span looks fairly indestructible.
But even when cars were still streaming across the bridge, Angus Council's roads department knew that appearances were deceptive.The structure was riddled with cracks caused by alkali silica reaction.
Unbeknown to the engineers who constructed the bridge in 1931, the type of aggregate they were using would in time react with the cement to form an expanding gel, breaking apart the concrete.
The problems were first noticed in the 1970s.The council used extensive cladding and strapping to bandage the bridge but it could only soldier on for so long.The worst-hit areas were the counterweights on the abutments. If these decayed too far, the bridge would topple.
'We have been monitoring crack width for the past 10 years, ' says Angus Council director of roads Ronnie McNeil.'Without wishing to worry the public, we were a few months away from imposing weight restrictions.
There was a danger of something punching a hole in the bridge deck.'
Balfour Beatty won the design and build contract to demolish the old bridge and replace it in October 2003 but obtaining statutory consents from the Scottish Executive caused a nine-month delay between award of contract and commencement of work.
'There was nothing we could do to speed up that process, ' says project manager Keith Bowman.'We just had to wait.The good thing about it was that I had time to think about how to demolish the old bridge.'
Mr Bowman was extremely reluctant to demolish the bridge in situ on account of the attendant health, safety and environmental risks.
'Our first consideration was safety, ' he says.'I didn't want the team to spend much time working over the water. It's a cold and windy spot and the river can flow at speeds of nine knots.This area is also environmentally very sensitive so it made sense to minimise the risk of pollution by getting the works on to land as quickly as possible.'
Salmon and sea trout swim the South Esk river and the fisheries board was keen to prevent any disruption to the stocks.And a little further inshore from the bridge is Montrose basin, a wetland registered under international treaty as ecologically significant. It is a unique habitat for birds, fish and plants.The flat mud basin, dry at low tide and wet at high tide, contains salt water from the sea and fresh water from the river.
Pollution from the demolition might upset this delicate balance.
Mr Bowman decided to investigate the possibilities of removing the bridge to land to demolish it. Consulting with Scaldis, a Belgian marine and salvage contractor specialising in big lifts, he found that the structure could be lifted out in three sections.
He says: 'I went on the internet and typed 'big crane' into Google. I emailed a lot of companies I found and Scaldis responded immediately.
Then it took a year of negotiation and planning to put this lift together.'
The most difficult decision was where to put the slings. Initially the concept was to put beams under the bridge spans and keep the slings from touching the bridge altogether but this created problems when the lift was completed. It would be too difficult to get the beams out from under the bridge once it had been put down. Instead, the Balfour Beatty/Scaldis NV lift team decided to reinforce the towers against compression forces with an extra strut and put the slings directly around the bridge.
Mr Bowman then had to put his neck on the line and pick a date for the lift six months before the day.
'The utility diversions weren't guaranteeable, ' he recalls.'BT has to give a certain period of notice when they move a fibre optic cable but they came through for us and got them done in time.'
On the day of the lift the client is not the only one watching closely.The river bank is lined with what appears to be the entire population of Montrose. Schools have been deserted and workers have taken an unofficial day off.More than 6,000 spectators wait with bated breath for Rambiz, the huge floating crane, which costs £45,000 per day to hire, to grind into action.
First the central section of the bridge, a puny 300 tonnes, is lifted and placed on the dockside.Then the contractor must wait for high tide to lift each of the massive 2,340-tonne spans.The river bed is so high in the vicinity of the south span that the lift initially causes the crane to catch on the sand bank, as the team predicted. By lowering the span into the water and reducing the load to 1,600 tonnes they free the crane and move into a deeper channel.
Once all three spans are safely on the dockside, they can be broken up at the contractor's leisure.
Despite the team's best efforts to sell the structure on online auction site Ebay, there were no takers.Mr Bowman jokes: 'When they realised they would have to pay postage and packing - about £1 million - everyone lost interest!'
Mr McNeil was impressed by Balfour Beatty's method of solving the demolition problem.He says: 'The demolition is on the critical path for the entire bridge replacement project and this job is a race against time.The insitu programme would have taken three months whereas this is over in three days.'
The council may be keen to forget the lift and move on with the construction of the new bridge but Mr Bowman still has it on his mind.He is in touch with the publishers of The Guinness Book of Records, who are checking their facts. It may well be the biggest on-shore lift in the world.