Plugging the gap in checks on cables

Concrete “plugs” which anchor the bridge’s main cables are being unearthed for the first time in nearly 50 years as part of the corrosion checks.

The anchorages, which stretch up to 175ft underground, are the last parts of the cables to be inspected.

Bridge engineers do not expect to find serious corrosion – but remain in the dark because the concrete-filled tunnels have lain untouched since the bridge was completed in 1964.

Bridgemaster Barry Colfold said if there had been any problems, movements in the exposed part of the wire strands would have been detected.

However, there is uncertainty about the state of the anchorages on the south side because of flooding following an explosion caused by methane gas during construction, which severely burned five workers.

Hydrogen gas was given off, which could be a sign of the zinc coating of the wire strands reinforcing the concrete corroding.

The £3.5m contract involves excavating 25ft (8m) of soil and concrete to uncover the anchorages, with water cannons 750 times as powerful as a garden hose being used to reach the steel ducts housing the wire.