Will this scheme turn out to be a bridge too far after all?

Published on Tuesday 12 August 2008 09:26

NEWS that the Forth Road Bridge might not be in as poor a state as first thought has led to calls from environmentalists to scrap the new crossing planned to replace it. But the pleas seem likely to fall on deaf ears.

While campaigners have been quick to claim slower than anticipated erosion means a replacement crossing is no longer required, plans for the new cable-stayed bridge are likely to stay right on track for the moment.

In December last year, it was announced the scheme – set to be the largest construction project in Scotland in a generation – would go ahead. Environmental surveys and ground investigations are already under way and feasibility studies will be completed soon.

The design for the bridge will be finalised early next year, while construction is expected to start in 2011 and take five-and-a-half years to complete.

The plans include dedicated "high occupancy vehicle" lanes for buses, as well as a central "multimodal" public transport corridor which could be used for light rail, a guided bus system or trams.

Wind shields – similar to those used on the Second Severn Crossing, which has never had to close due to adverse weather conditions in its 12-year history – will be a major feature, ensuring high-sided vehicles, those towing trailers, motorcycles and bikes no longer need to take the 40-mile diversion via Kincardine when the winds reach 50mph.

However, as the picture of how the new bridge would look emerges, engineers have not given up on saving the old one. Experts had previously warned that the Forth Road Bridge may have to be closed to lorries by 2014 and to all vehicles by 2019 because of the weakening of the cables which hold it up – but it now appears these were rather pessimistic estimations.

In February this year, the project to save the Forth Road Bridge's corroding cables took a significant step forward when the Forth Estuary Transport Authority (FETA) switched on the first section of the bridge's new dehumidification system.

Dry air is now being pumped through about a quarter of the total length of the bridge's two main suspension cables. The aim is to prevent further corrosion occurring by reducing the relative humidity within the cables to below 40 per cent.

New inspection findings indicate that the main suspension cables on the Forth Road Bridge have lost about ten per cent of their strength due to corrosion, but may be deteriorating more slowly than previously feared.

It is thought that the introduction of weight restrictions on the bridge as soon as 2014 will no longer be required.

But in order to properly safeguard the future of the bridge, the cables would need to be completely replaced, or a new cable added above or to the side of the existing main cable. Such an undertaking could mean contraflows and partial closures for up to seven years.

And if efforts to halt the corrosion fail, weight restrictions would probably need to be considered some time between 2017 and 2021. A third inspection will be carried out in 2011-12 to verify the effectiveness of dehumidification operation.

Barry Colford, chief engineer and bridgemaster, says: "The dehumidification works on both cables is scheduled to be completed by the end of 2009 and, although there is no absolute guarantee of the effectiveness of the system, there is a degree of confidence that the deterioration of the cables can be arrested prior to the strength loss reducing to a level where intervention is required.

"The timescale when intervention, in the form of loading restrictions, may be required now extends from 2014 to 2021, and it now appears more likely that the rate of deterioration is predicted to follow the more optimistic line."

Campaigners against the new crossing say this is a strong indication that the existing bridge can be saved.

Critics say the cost of a replacement crossing – estimated at 3.2-4.2 billion at 2016 prices, including VAT and the extra cost of strengthening and widening the bridge to accommodate public transport options – is too high.

A spokesman for Friends of the Earth Scotland said: "It is clear from the evidence that the impact of a second Forth road bridge in terms of traffic, congestion and greenhouse gas emissions would be unacceptable and would be contrary to government policy.

"The studies commissioned by FETA show that the strength in the existing bridge cable can be retained or added to without excessive traffic impacts.

"To commit up to 4.2bn on an unnecessary bridge is not, in our view, prudent use of public money and would lead to serious environmental consequences."

But mothballing the project at this stage is something the Government cannot risk.

Should further studies conclude that the bridge has a limited life span and cannot be saved, a plan must be in place to replace it. Having no crossing in place at some future date is not an option and work will continue towards this.

However, any firm decision on whether to proceed will be delayed for three years until a more accurate picture emerges.