Anchorage Investigation Contract Awarded

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.

The investigation will involve excavating down through the rock to the top of both of the south anchorage tunnels over a length of about ten metres. The work will be carried out in a carefully controlled manner using a combination of mechanical excavation and hydrodemolition (very high pressure water jets).

Further careful hydrodemolition will be used to remove the concrete inside the tunnels to expose some of the steel ducts housing the pre-tensioning 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 the capacity of the anchorages to be evaluated.

On completion of the investigation, permanent access chambers to allow future monitoring and inspection will be constructed and the site reinstated to its current levels.

The works are programmed to start on site in August this year. 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 project will not cause any disruption to bridge traffic. However, one way working using temporary traffic lights will be required on Shore Road in South Queensferry for the duration of the excavation work.

Barry Colford, Chief Engineer & Bridgemaster, said: “This is a very important investigation into what are critical components of the suspension bridge. I would stress that there is no external evidence of any problem but it is our responsibility as a bridge authority to inspect every part of the structure in order to ensure there are no hidden issues.”

Michael Mitchell, Head of Business Development (Civils) at GRAHAM Construction said: “It is a tremendous endorsement of GRAHAM’s technical expertise to be involved in such a high profile project on one of Scotland’s most iconic structures. We are looking forward to working closely with FETA and their professional team to deliver this challenging project. “