M20 Footbridge

Dealing with the recent M20 bridge collapse was the first big test for us as Highways England’s’ new Area 4 maintenance provider.

We were not even three months into the new contract to manage Area 4 of the strategic road network in Kent and Sussex known as the ‘gateway to Europe’ when we were confronted with the high profile collapse of a footbridge on the M20 on the August bank holiday Saturday at around midday. This is one of the busiest times for the motorway that links London and the Port of Dover. Thanks to a huge effort and close collaboration between all parties the road was opened to traffic in just over 24 hours.

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And a week later another huge operation to remove a surviving section of footbridge was completed 18 hours. A number of hours ahead of schedule, again allowing the motorway, which had had to be shut for the operation, to be opened quickly to traffic.

Current understanding is that the concrete footbridge was hit by a third party digger machine being transported on a low loader on the London bound carriageway which dislodged a section of the bridge. This crashed onto a lorry and very fortunately no one was seriously hurt, though a quick thinking motorcyclist broke his ribs when he threw himself off his vehicle before hitting the fallen bridge section.

Traffic was halted between junctions 1 and 4 of the M20 leaving drivers to survey the fallen drop in section of bridge, its ramp and surviving cantilever section that straddled the coast bound carriageways.

For Highways England and the A-one+ Area 4 team this is when all the training, planning and experience kicked in. A-one+, Scheme Delivery Manager, Karen Davies said; “Safety is the top priority for all of us, so the first job was to secure the site and release the drivers caught in the queues.

“We then considered how best to clear the debris and re-open the road as well as what to do with the surviving section of bridge. We were new area maintenance providers being faced with a major challenge but the collaboration and support we received from Highways England and our supply chain was crucial and the experience has provided us all with some excellent learning opportunities.”

Working with Kent Police the thousands of drivers caught in tailbacks were released from the scene by 3.30pm on the Saturday and diversions were put in place.

That was when A-one+ and our supply chain partners could move onto site to break up and remove the demolished section of bridge while structural engineers assessed the stability of the surviving cantilever section. In less than 24 hours the team had removed all 100+ tonnes of debris from the carriageway and the stranded vehicles. Our supply chain provided two thirty tonne cranes, HGV recovery vehicles to remove extensively damaged heavy good vehicles, croppers to cut twisted steel on the structure and four sweepers and black top repairs lining. Over 100 people worked on the incident clear up operation.

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The cantilever section was deemed safe following inspections by chartered engineers and extensive engineering monitoring and calculations were carried out to allow traffic to run underneath it, with restrictions. Two lanes of the coast bound M20 were reopened, operating at 50MPH, by 4.30pm the next day, the Sunday and all lanes on the London bound carriageway were reopened less than an hour later.

“The remaining section of bridge was constantly assessed and was safe for traffic to pass underneath with the temporary speed limit,” says Highways England’s chief highway engineer Catherine Brookes. Sensitive movement detectors were installed and the structure was closely monitored. And 24 hour security as put in place at the site to prevent anyone getting on to the remaining section of footbridge.

“We naturally needed to remove the remaining section,” Brookes said. Any change in the condition of the bridge would have required an unplanned closure of all or part of the motorway which would have created serious disruption; planned closure and orderly removal of the section with people aware of a closure created less problems, she explained.

Highways England was also aware that there was a risk to driver experience and site access by leaving the remaining section standing for any length of time.

The decision was taken to do the work quickly and in safe, controlled conditions the bridge came down the following weekend.

“We could also use the closures to carry out as much work as possible, including barrier repairs, resurfacing and litter picking,” Brookes said.

East Street footbridge, near the village of Addington was constructed in 1971 of reinforced and prestressed concrete and consisted of three sections. The first, cantilever, section projected over the coast bound carriageway; the second was a ‘drop in span’ which collapsed was supported from the end of the cantilever and spanned over the London bound M20 carriageway. And the third section consisted of a spiral ramp. Measured headroom above the road was 5.17m, greater than the minimum maintained headroom to which all new bridges are designed of 5.03m under national standards. Its last general inspection was in March this year.

The only safe way to remove the cantilevers was to use pulverizes to cut the bridge away in small chunks, with the risk that once the works started the pier and back strap would move or collapse. The decision to opt for a 58 hour weekend closure allowed us plenty of time to remove the cantilever section in safety.

This involved not just closing the M20 between junction 1 and 4 near the bridge but also the coast bound M26 from M25 junction 5. In a major communications exercise, a newsletter explaining why the work was necessary and setting out detail of the closures was distributed to every resident in the area; all emergency services were notified including blood banks and Maidstone Hospital; and strategic traffic generators such as Gatwick Airport, the Port of Dover and Eurotunnel were informed of the closure and planned diversion routes. Local MP Tom Tugendhat was also kept fully informed.

From the Friday night more than 100 people worked 4,200 man hours to safely demolish and remove the structure.

“Four pulverisers cut up the bridge with 100 plus tonnes of concrete and steel rubble from the carriageway and verge removed using tipper trucks. In the same possession routine maintenance was carried out to reduce the need for future closures. This included 10,000 sq. meters – around five miles – of new surfacing, over double the amount laid during overnight closures, five miles of verge cutting, 2.5 miles of central reservation vegetation clearance, five miles of white lining and nearly eight miles of litter picking,” said Davies

The road was open on Sunday evening rather than the anticipated Monday morning thanks to a sterling effort from the whole team. “We’re very proud of what we have achieved and that we were safely able to remove the remaining structure and re-open early,” Brookes said. “This was good news for drivers on the M20 and M26. And to further reduce disruption for road users, we took that opportunity while the motorway was closed to complete the extra work.”

Highways England is responsible for 560 footbridges in England and is considering whether as a result of the M20 incident, there are any wider implications it needs to address. Between October 2015 and March 2016 there were 12 recorded bridge strikes across the Highways England network, with the majority caused by over height vehicles or improperly secured loads.

“And we will start planning the replacement for East Street footbridge in due course,” Brookes said.