The operators of the Forth Road Bridge have today announced plans for excavations and load testing to determine the long-term structural integrity of the bridge's main cable anchorages.
The main suspension cables are attached to the ground in concrete-filled tunnels, bored into the rock on either shore. The concrete is strengthened using post-tensioned, galvanised, high-tensile steel strands and the investigation will seek to establish the condition of the strands.
Consultant Fairhurst has been working for the Forth Estuary Transport Authority to determine the best method of investigation. The work will involve excavation to expose and inspect the strands and full-scale testing of some of the sockets where the main cables are attached to the anchorages. The inspection will also use various non-destructive testing methods such as acoustic monitoring, which it is hoped may provide some useful data when combined with the first two methods.
"The use of post tensioning in the buried concrete anchorage tunnels at Forth was considered innovative at the time," said chief engineer and bridgemaster Barry Colford. "Unfortunately, this form of construction can be vulnerable to corrosion and deterioration, especially in a saline environment such as is found at Forth." Frequent inspections are carried out to monitor for movement within the anchorage chambers and no signs of distress or movement have been recorded, he added. "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."
A sum of approximately US$10.9 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.
