Why the M4 bridge has to go

NATIONAL Highways has explained why the Badminton Road bridge over the M4 needs to be replaced, how it will be done and how long it will take.

In December the government agency announced that the bridge, which carried 16,000 vehicles a day on the A432 between the Avon Ring Road and Yate, could not be repaired and would have to be replaced, despite being designed to last 120 years from its opening in 1966.

It has been closed since engineers found “accelerated deterioration and cracking” in the concrete last July.

After months of detailed inspections, the agency’s experts concluded that the bridge is not an immediate risk to the 87,000 vehicles a day passing underneath on the M4 – but would soon become unsafe if heavy traffic was allowed to use it again.

Before work can start on demolition, services including electricity cables, water mains, BT fibre optic cables and gas mains all have to be moved and buried in trenches under the M4.

National Highways’ timetable is:

• January 2024: ecology work and habitats

• February 2024: ground investigations and boring.

• March to May 2024 : moving and re-routing pipes and cables

• Summer/Autumn 2024: demolition

• Late 2024: New build starts

• Early 2026: New bridge opens.

Route manager Sean Walsh said the demolition will happen in a 60-hour window when the M4 will be closed, so they can remove the central span of the structure. 

There will be lane closures and overnight closures on the motorway at other times while the work is underway.

Mr Walsh said: “The build is a 12 to 18-month project – much shorter than normal, as we realise the importance of accelerating this build.”

He said that none of the 164 other similar ‘post-tension’ structures the agency was responsible for in the South West had major issues, adding: “It’s very rare we have to demolish and rebuild a whole bridge.”

Mr Walsh said the 120-year design life was predicted when traffic numbers, lorry sizes and other factors were all very different to today.

Engineering team manager Terry Robinson said all road bridges undergo ongoing maintenance but the issues found by inspectors in the summer were exceptional.

He said: “The examination showed structural cracks going right through the bridge.

“We’ve been working solidly since July to examine the bridge and find out how bad the problems are, and what the possible solutions are. 

“The bridge is not unsafe and is not at risk of collapse, but it’s unable to carry heavy traffic.

“Traffic will cause the carriageway to move and because of the cracks, that could result in a piece of concrete landing on the M4 below, so that is why it has to be closed now.

“We have assessed it as perfectly safe for pedestrians and cyclists.

“We’re speeding things as quickly as we can to get the bridge back open as soon as possible.

“The whole program has been condensed, as we know it’s important to local people.”

National Highways says the replacement bridge will be 20 metres (66ft) wide, and will “allow for more sustainable transport options to be considered”.

Mr Walsh said the bridge demolition and rebuild would be a “multi-million-pound” project of great complexity – but due to timescales and costs, it would not be possible to install a temporary bridge during the works.

Mr Walsh said: “We are working with partners to expedite the process as quickly as possible. 

“We fully understand the disruption the closure has caused and will cause, and we thank people for their patience as we move this project forward as soon as possible.”