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.

The bridge, which carries thousands of cars a day on the A432 between Downend and Yate, was designed to last 120 years from its opening in 1966.

But last July engineers found “accelerated deterioration and cracking” in the concrete and it was closed to motor traffic.

After months of detailed inspections, the government 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 they said it would soon become unsafe if heavy traffic was allowed to continue crossing over it, as pieces could come loose and fall onto the motorway were it to be used by vehicles again.

The structural failures mean that after 57 years of use – less than half its intended lifetime – the bridge by the Wick Wick roundabout, near the Willy Wicket pub, will have to be replaced.

The timetable

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, which is responsible for the bridge as part of the motorway network, says the timetable will be:

• 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

National Highways 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.”

‘Very rare’ problem

Mr Walsh said National Highways is responsible for 2,200 structures in the South West.

There are 164 similar ‘post-tension’ structures to the Badminton Road bridge, but it is the only one with major issues.

He said: “In 16 years in this job, this is the only bridge I’ve seen that has this magnitude of work needed.

“It’s very rare we have to demolish and rebuild a whole bridge – this is the only one in the region.”

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 during their lifetime and, behind the scenes, teams were always working to do this. 

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.”