Published: Thursday, 25th June, 2009 7:42am

Bridge bosses face £6.4 million question over anchorages

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THE £6.4 million question for concerned Forth Road Bridge bosses is "Has corrosion spread into the anchorages that hold the whole structure up?"

They"ll spend at least that much to find out after deciding they have to know if the end of the main cables, encased in concrete, are rusting too.

The Forth Estuary Transport Authority (Feta) discovered corrosion in the bridge"s main cables – which they"re trying to arrest with a £10 million dehumidification system.

But where the cables are anchored into the ground on both sides of the Forth, no-one has been able to check their condition since the bridge was built 45 years ago.

Feta convenor Councillor Tony Martin said, 'It could well be that they"re absolutely perfect but it"s very expensive to find that out.

'Barry Colford (the bridgemaster) thinks that it"s unlikely they"ll be corroded but we thought it was unlikely that the main cables would be corroded.'

Feta"s accounts show they have put aside £6.438 million 'to carry out an investigation into the long- term structural integrity of the bridge anchorages'.

The report adds, 'This is a unique project without precedent and there is a high risk attached to this work.'

It"s proposed that the existing Forth Road Bridge will be used for public transport only when the new crossing opens in 2016 but the rust in the main cables is an unknown factor.

The results of the dehumidification system won"t be known until 2012 at the earliest and the main cables may then have to be replaced if the corrosion has led to a drop in the safety factor.

Mr Martin said, 'I don"t want people in any way to think that the bridge is unsafe. It isn"t.

'Everything is perfectly fine but we need to see these individual strands to see what condition they"re in.

'What we"re trying to do is prove everything is OK and that we can use these chambers again if we need to.

'But hopefully we won"t have to replace the main cables.'

He added, 'Because it"s a problem we have only on the bridge, we might estimate it"ll cost £5 million and the tenders may come back and say it"ll be £9 million.

'It"s work that no-one has done before so the risks are enormous and contractors really want us to take a lot of that risk.'

A Feta spokesman explained, 'There"s no external sign of any problem but we want to establish the long-term structural integrity of the anchorages.

'Depending on how effective dehumidification has been, we may want to replace the main cables so you"d want to make sure the anchorages you"re attaching the new cables to are sound in the long term.

'The Forth Road Bridge"s construction is unique in many ways and because the anchorages are filled with concrete there"s no way of knowing the condition of the steel rods inside the anchorages.

'Consultants have looked at non-invasive ways such as ground radar, ultrasound and acoustic monitoring but they"ve concluded none of these techniques will provide the reassurance we need.

'We think we"re going to have to excavate, open up the top of the anchorages very carefully and look at the steel inside.'

He continued, 'The anchorages are basically tunnels built into the rock on either shore and then they"re back-filled with concrete.

'They act as giant tent pegs to anchor the main cables to the shore and to hold 14,000 tonnes of load.

"There are four anchorages, one at each end of the cable, and they"re in covered chambers.

'The excavation will be through very hard rock. These locations were chosen for good reason, because the rock was so hard, but it makes it extra difficult now.'

There will also be load testing – 'which is like pulling on the cable to see what load it can take' – but it"s thought the project is already 18 months behind schedule.

The spokesman said, 'It"s a one-off project and it"s hard to be firm on costs and the timescale.

'Because the construction is unique in certain ways, you can"t point and say this is how it was done elsewhere.'

And Mr Martin concluded, 'It"s a challenge just being able to find a consultant engineer that will do the work and we"ve still got the tenders out.'