Anchorage Investigation Plans Backed by Industry Peers

A group of leading bridge engineers have confirmed the need to carry out excavations in order to prove the long term structural integrity of the Forth Road Bridge’s main cable anchorages.

The anchorages are concrete filled tunnels bored into the rock on either shore, which hold in place the bridge’s main suspension cables. Concerns over the possibility of corrosion were first raised in 2005 following the discovery of papers highlighting problems during construction, particularly in relation to early depletion of galvanising on the steel pre-tensioning strands that hold the concrete in the anchorages together. Bridge authority FETA instructed consulting engineers WA Fairhurst & Partners to investigate the best way of proving the anchorages’ structural integrity, and a Peer Review Panel commissioned to check these plans has now published its final report.

The Peer Review Panel concluded that:

• The Panel concurs with FETA’s decision to carry out an intrusive inspection of the anchorages, by excavating down to inspect the pre-tensioning strands. These inspections will give FETA sufficient confidence at this stage to either rule out the issue of corrosion altogether, or to consider suitable remedial actions.
• The Panel is satisfied that the process of assessing risks to users of the bridge and determining the best method of investigation has been advanced in a timely and thorough manner.
• There is no current and direct evidence that the bridge is in any way unsafe.  Rather this investigation is the only effective way to deal with concerns over the longer term integrity of the anchorage strands.
• Carrying out the investigation now gives FETA the ability to carry out a proactive assessment of the safety of the bridge rather than having to react in an ad-hoc manner.  
• Doing nothing is not considered a viable option.

The report recommends that a monitoring system be installed to detect future corrosion within the anchorages, but advises against full scale load testing of the anchorage sockets, with cutting of pre-tensioning strands for sampling and testing only to be considered as a last resort.

The Peer Review Panel comprised David Mackenzie of consulting engineers Flint & Neill, Pete Sluszka of US consultants Ammann & Whitney and Bill Valentine, Transport Scotland’s Chief Bridges Engineer.

The Panel’s endorsement means the process to recruit a contractor to carry out the works can now proceed. Work on site is expected to start in autumn 2011, with excavations taking at least a year to complete and the first indications of the anchorages’ condition emerging in 2013.

Excavations will take place at the south anchorages only, where any deterioration is likely to be worst as ground conditions are poorer and there are more contemporary accounts of problems.

The initial cost of the work has been estimated at £7.5 million, although the full extent of the work required will not be known until the excavations have been completed.

Barry Colford, FETA’s Chief Engineer & Bridgemaster, said: “I welcome the comments and recommendations of the Peer Review Panel. Our intention now is to press ahead with the investigation. This is the only way to deal with long term concerns over the structural integrity of the anchorages.

“Excavating down to expose the steel will allow us to assess the current strength of the anchorages and will help in the difficult task of estimating future strength. Depending on the findings, challenging engineering decisions may have to be taken.

“However, it must be stressed that there is no direct or current evidence that the bridge is in any way unsafe. Frequent inspections are carried out to monitor for movement within the anchorage chambers and no signs of distress or movement have been recorded to date.”