Last updated on 3 Oct 2012
Current works on site:
The investigation to determine the current condition of the anchorages started on site at the southern anchorages in August 2011 and is progressing well. A number of ducts are now visible at the crown of the south west anchorage tunnel and their external condition appears to be good.
The contractor carrying out the works is GRAHAM Construction and the contract involves excavating down through overburden and rock to the top of both tunnels to expose the concrete forming the tunnels over a length of around ten metres. This work is being carried out in a carefully controlled manner utilising a combination of mechanical excavation and hydrodemolition techniques. Side slopes are being strengthened by subsurface soil nailing and rock anchors.
Further careful hydrodemolition is being used to remove the tunnel concrete to expose a number of the steel ducts housing the pre-tensioning steel strands. These ducts will be carefully cut open and the grout washed out to expose the strands. Once exposed, the strands will be inspected and tested in order to allow an evaluation of the capacity of the anchorages to be determined. It is proposed that permanent access chambers will be built to aid future monitoring and the site reinstated back to the original ground level profile.
The work is not causing disruption to bridge traffic. However, temporary traffic management is in place on Shore Road and this involves one way working using temporary traffic lights for the duration of the excavation work.
Given the nature of the works there are significant environmental issues to be dealt with especially with regard to noise, dust and discharge from the hydrodemolition work. The Contractor is required to comply with the requirements set out in the contract which include special requirements of SEPA. With regard to noise, guidance on levels is given in the contract documents but the Contractor is responsible for seeking formal consent from the local authorities for the proposed method of working.
If the investigation does reveal significant deterioration in the steel strands within the anchorage tunnels then, depending on the level of that deterioration, measures may have to be considered to limit loading on the bridge. Full scale testing of the sockets within the southern anchorage chambers is not being carried out but may be re-considered depending on the outcome of the inspection. Any work required on the northern anchorages will also be dependent on the outcome of the results from the southern anchorages investigation.
It is hoped that information on the condition of the strands will start to become available in late 2012 and further information on the condition and strength of the strands during 2013, as more of the ducts are uncovered and opened to allow inspection and testing.
The anchorages are concrete filled tunnels bored into the rock on either shore, where the bridge’s main suspension cables are attached to the ground.
Concerns were first raised several years ago over the possibility of corrosion in the steel pre-tensioning strands that hold the concrete in the tunnels together. A feasibility study was carried out to determine the best way of proving their structural integrity and this study’s findings were subsequently confirmed by peer review.
In June 2011 the Forth Estuary Transport Authority awarded a £3.5 million contract to GRAHAM Construction to carry out works to facilitate an investigation of the structural integrity of the main cable anchorages. Excavation work at the South Anchorage began in August 2011 and will continue through 2012. It is estimated that excavation will take 12 months and a further 12 months will be required to carry out the inspection and evaluate the findings.
The contract with GRAHAM Construction is a standard form New Engineering Contract (NEC) 3 Option C Target Cost Contract with a tendered Target Cost of £3,497,849. There is a pain/gain element to the works. If the final cost of the works is above the Target Cost then the Contractor pays a share of that increased cost. Conversely, if the final cost is below the Target Cost, then the Contractor receives a share of the saving. It should be noted that the Target Cost can be adjusted during the contract to account for unforeseen work. At present the investigation is scheduled to be completed in December 2013.
Some £220,423 of Compensation Events (items of work unforeseen at tender stage) have already been agreed with the Contractor and these items of unforeseen works have caused some delays.
It should be noted that by the very investigatory nature of this particular project the estimate of cost was always going to be difficult to determine. As previously reported to Members, prior to the 2011 Spending Review a budget of £7.6 million had been allocated to cover all of the anchorage investigation work.
From examination of the ducts recently exposed, it is apparent that they are some 400mm deeper and at a steeper angle than recorded on the as built information. Added to this, the rockhead is higher than expected and the extent of ancillary foundations from the original construction, which have to be removed, is also greater than anticipated. All of this means that the Contractor has to spend far longer than originally programmed and the additional resources required has led to the submission of further and more significant claims for compensation. The Authority’s Engineer, Fairhurst, is still evaluating the likely cost of these additional works.
Notwithstanding the final agreed cost, it is likely that the final Target Cost of this investigation contract will be significantly higher than the original estimate for the reasons outlined above.
It should also be noted that there is a still a significant risk of additional costs being realised during this contract. Not only is the work an investigation, but this type of the work has not, as far as can be determined, been carried out on any other bridge. Therefore, the techniques for inspection and wire sampling, and the evaluation of strength, are being developed specifically for this project.
The current funding of the anchorage investigation will not cover any work on the north anchorages or for a replacement anchorage system.
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 proactive 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.
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.
The Panel’s endorsement meant the works themselves could now proceed. Work on site began in autumn 2011, with excavations expected to take at least a year to complete and the first indications of the anchorages’ condition emerging in early 2013.
The initial cost of the work was estimated at £7.5 million, although the full extent of the work required will not be known until the excavations have been completed.
Initially, iwork will commence 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 is being carried out in the first instance as a separate contract. Further work will depend on the results of this examination.
The excavation is 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 managed, especially with regard to noise, dust and discharge from the excavation and hydrodemolition.
Facts & Figures
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