The award of the contract to facilitate the investigation comes after Graham scored highest in a competitive tendering exercise held by the Forth Estuary Transport Authority.
Concerns were first raised several years ago over the possibility of corrosion in the steel strands that hold the concrete together in the anchorage tunnels. 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 10m. The work will be carried out in a carefully controlled manner using a combination of mechanical excavation and hydrodemolition.
Further hydrodemolition will be used to remove the concrete inside the tunnels to expose some of the steel ducts housing the strands. These ducts will be 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.
“This is a very important investigation into what are critical components of the suspension bridge,” said chief engineer and bridgemaster Barry Colford. “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.”
