Capital Projects:

Contents:

1. Background
2. Peer review conclusions
3. Next steps
4. Related documents

Background

The anchorages of the main cables of a suspension bridge are critical elements of the structure.  At Forth, tunnels were formed within the rock at each of the four anchor points and filled with concrete.  The main cable wires splay out in the anchorage chambers and loop round strand shoes which are in turn bolted to the face of the concrete tunnels.  Friction between the concrete tunnel and the rock, and the weight of the overburden above, prevents the cables pulling the concrete tunnel out of the ground.

The concrete in the tunnel itself is not strong enough to withstand the forces from the cables and was strengthened using pre-tensioned galvanised, high tensile steel wire strands.  This use of pre-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.

In the course of a study into the feasibility of replacing or augmenting the main cables, completed in 2008, it became apparent that further work would be required to prove the long-term structural integrity of the anchorages. Records and papers acquired relatively recently relating to the construction of the existing anchorages highlighted various problems during construction particularly in relation to early depletion of the galvanising protecting the post tensioning strands which are housed in grouted ducts set in the concrete tunnel.

The current safety of the bridge is not in question.  This investigation is about ensuring the long term structural integrity of the anchorages and is a pro-active measure to ensure that all accessible parts of the structure are inspected. Frequent inspections are carried out to monitor for movement within the anchorage chambers at the tunnel/strand shoe interface and no signs of distress or movement have been recorded.

However, these reports determined the need to carry out a special inspection or investigation to try to establish the existing condition of the pre-tensioning strands. Work has been ongoing since 2008 to determine the best way of doing this. The anchorages' unique design makes this an extremely difficult task.

There is guidance from the UK Department of Transport for inspecting post tensioning in bridges as it is acknowledged that there can be problems with this type of construction.  The guidelines refer mainly to the difficulties in establishing the condition of post tensioning strand in bridge decks.  These difficulties are exacerbated in a tunnel.  Following much discussion, research and consultation with various specialists, it was concluded that three separate methods of investigation should 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 load testing of a number of the sockets within the anchorage chamber
• Other methods of non-destructive testing, such as acoustic monitoring - these would not solve the problem alone but may provide useful data when combined with the first two methods.

Peer review conclusions

Consulting engineers Flint & Neill Ltd were appointed to chair a Peer Review Panel in order to audit and review the work being carried out by Fairhurst.  The panel comprised David Mackenzie of Flint and Neill; Peter Sluszka of the NY consultants Ammann & Whitney and Bill Valentine, Chief Bridge Engineer, Trunk Roads Network Management, Transport Scotland.

In August 2010 the Peer Review Panel produced their final report, concluding the following:

• 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 recommended that a monitoring system be installed to detect future corrosion within the anchorages, but advised 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.

Next steps

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.

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.

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.  The excavation work and visual examination and testing of the strands would be carried out in the first instance as a separate contract. Further work would depend on the results of this examination.

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 insitu testing of the anchorage sockets will also be challenging as access is very difficult within the anchorage chambers.  Safety of the workforce, the bridge and users will govern all aspects of this complex project.

Work on the anchorages is not anticipated to involve major disruption to bridge traffic.  However, given the nature of the works there will be significant environmental issues to be dealt with especially with regard to noise, dust and discharge from the excavation and hydrodemolition.  To assist the community it will be a requirement of the contract that that the contractor appoints a named member of staff as liaison officer. Prior to the commencement of the works, it is intended that a number of meetings will be held with local residents and the community councils to provide as much information on the works to be made available and to establish points of contact for the duration of the works.

Related documents:

• Anchorages update (27 August 2010)
• News release: Anchorage investigation plans backed by industry peers (23 August 2010)
• Main cable and anchorages update (17 February 2010)