When an articulated lorry smashed into one of the main supports beneath a bridge over the M6 in Lancashire last year, Highways England had no choice but to close the motorway in both directions immediately.
Miraculously, no-one was badly hurt in the collision but the impact was so severe that the truck demolished one of the concrete columns supporting the Turner’s Farm accommodation bridge between junctions 32 and 33 and the whole structure was in imminent danger of collapse.
For Highways England – not to mention users of this busy section of the motorway network – the main priority was to get the bridge stabilised as quickly as possible to enable traffic to flow again.
“Thankfully, the driver wasn’t seriously injured; however, the collision left the lorry wreckage as the only thing holding the bridge up,” says Mabey’s engineering manager, Matt Westhead.
“The lorry hit the southbound verge pier, creating impact damage at the top of the north column and crosshead,” Westhead explains. This resulted in major displacement of the north column where its bearing was removed, meaning it could no longer support the steel beam above.
“The reinforced concrete crosshead also suffered significant damage with severe cracking at its intersection with the south column,” continues Westhead. A visual inspection of the whole structure noted no other obvious defects to either the superstructure or the substructure.
Speed was of the essence, as the wreckage of the lorry could not be removed until the temporary support was in place. Very quickly Amey Consulting, Highways England’s designer responsible for structures, supplied parameters to Mabey so it could start designing the temporary support.
“Given the state of the bridge – and the urgent need to open the motorway again as soon as safely possible – a rapid temporary solution was required,” says Westhead.
“The most suitable solution was a structure consisting of two of our Mass 50 four-legged towers with base beams to spread the load on the carriageway.
“This design could be constructed quickly as a large percentage of the equipment could be pre-assembled in the local Mabey depot in Garswood, ensuring building timescales on site and manual handling were minimised,” continues Westhead.
Once the method had been agreed by all parties involved, the Mabey team worked through the night to complete the installation of the towers and base beams. Header beams and hydraulic jacks were located under the bridge soffit to permit a load transfer and provide equal load distribution through the supporting legs.
“Each temporary support tower had to be capable of hydraulically supporting a vertical load of approximately 48 tonnes whilst also offering lateral support to the bridge deck,” explains Westhead. “The system design had to ensure that support points were located at a position which would not overstress the existing structure, while also providing adequate room to extract the wreckage of the lorry safely and maintain two lanes of traffic flow following the operation.”
To get an accurate picture of the load paths and bridge articulation, Mabey’s instrumentation division installed a monitoring system to detect and relay a number of variables including load, temperature and displacement.
“To monitor vertical movements at the bridge support positions during the repair works, we installed digital levels on the abutment, while transverse and longitudinal movement at the abutment itself was monitored using displacement sensors,” says Westhead.
All this information fed into Mabey’s LIVEsite, a web-based portal that visually displays the data from the sensors to provide a real-time view of the bridge behaviour and monitor any movement. This ensured safe load transfer to the temporary works and realignment of the bridge deck back to its original position.
Directly following the successful completion of the emergency solution, Mabey designed and installed a ‘semi-permanent’ propping system to support the bridge over the long term. This is now in place and providing the necessary time Highways England needs to design a permanent replacement pier while allowing the carriageway to be fully reopened to traffic.
Mabey’s rapid response to this emergency impressed the main contractor, Kier, which has since added Mabey Hire to its designated supplier list for Area 13. “This means they will be able to directly appoint us to undertake any future works, effectively reducing the number of interfaces within the supply chain,” says Westhead.
Kier Highways general manager for Area 13 maintenance & repair, Graham Porter, described Mabey’s response as ‘first class’:
“From the initial call, Mabey engineers designed and delivered a solution that enabled us to reopen the road in little over 24 hours. The team worked tirelessly, and we are extremely grateful for their support,” he says.
This article was first published in the July/August 2019 issue of The Construction Index magazine