Plans for Anchorage Investigation

The anchorages are concrete filled tunnels bored into the rock on either shore, where the main suspension cables are attached to the ground. They are required to resist almost 14,000 tonnes of load trying to pull them out of the rock.

The concrete in the tunnel itself is not strong enough to withstand the forces from the cables, so is strengthened using post tensioned, galvanised, high tensile steel strands. It is the condition of these strands that the anchorage investigation will seek to establish.

Three separate methods of investigation are to be taken forward:

• Excavation behind the anchorage chambers down to the top of the tunnel to expose and inspect the post tensioning strands
• Full scale testing of a number of the sockets where the main cables are attached to the anchorages
• Various other methods of non-destructive testing such as acoustic monitoring, which would not solve the problem alone but may provide some useful data when combined with the first two methods.

Initially, it is proposed to commence work at the south anchorage as it is expected that if corrosion of the strands has occurred it will be worse at this location.  Further work would depend on the results of this examination.

The need to carry out an investigation into the condition of the anchorages was determined in the course of a study into the feasibility of replacing or augmenting the main cables, completed by consultant engineers Fairhurst in 2008. Records and papers acquired in the course of this study highlighted various problems during construction of the anchorages, particularly in relation to early depletion of the galvanising protecting the post tensioning strands. Fairhurst have been continuing work to determine the best method of investigation since 2008.

Barry Colford, Chief Engineer & Bridgemaster at Forth Road bridge said: “The use of post tensioning in the buried concrete anchorage tunnels at Forth was considered innovative at the time.  Unfortunately, this form of construction can be vulnerable to corrosion and deterioration, especially in a saline environment such as is found at Forth.

“The current safety of the bridge is not in question. Frequent inspections are carried out to monitor for movement within the anchorage chambers and no signs of distress or movement have been recorded.

“However, this investigation is about ensuring the long term structural integrity of the anchorages and is a proactive measure to ensure that all accessible parts of the structure are inspected.

“The excavation will be difficult as the ground conditions vary and there is methane present within the shale.  The work is further complicated by the close proximity of the foundations to the viaduct piers.  Any in situ testing of the anchorage sockets will also be challenging as access is very difficult. 

“Safety of the workforce, the bridge and users will govern all aspects of this complex project.”

Consultant engineers Flint & Neill Ltd have been appointed to chair a peer review panel including New York consultants Ammann & Whitney and Transport Scotland’s Chief Bridge Engineer in order to audit and review the work being carried out by Fairhurst.

A sum of approximately £7.3 million has been set aside for the initial investigation, although costs could rise if additional work is deemed necessary.  Work on site is expected to start at the south anchorages in summer or autumn 2011, possibly continuing until 2014

The anchorage investigation is not expected to involve major disruption to bridge traffic. However, given the nature of the works it is expected that there will be significant environmental issues, especially with regard to noise, dust and discharge from the excavation. 

The bridge authority has arranged two public meetings for local residents to discuss the impact of planned bridge works. These will take place at 7 p.m. on Monday 15 March at the Forth Road Bridge Administration Office in South Queensferry, and at 7 p.m. on Tuesday 16 March at the Queensferry Hotel in North Queensferry.