Dr Bill Harvey, an engineer who has worked on bridges for over 40 years, explains the Elevarch project, a remarkable cost-effective solution to a common rail engineering challenge that won the Award for Outstanding Value at The Structural Awards 2017.
It’s estimated that 500 masonry arch bridges in the UK are too low for Network Rail’s electrification programme. Elevarch is designed as a sustainable solution, by cutting free the bridge arch and raising it on jacks. The method promises to save the demolition and rebuild of hundreds of overbridges - you get the same bridge simply lifted into a new position. It’s hugely cost effective (the saving could be as much as several hundred thousand per bridge) and has heritage applications preserving masonry rail infrastructure across the UK.
The concept was selected as one of four in phase two of the “Avoidance of Bridge Reconstruction competition” and a suitable ‘demonstrator’ structure was located - Moco Farm bridge, which carries farm traffic over a railway that is being recommissioned.
The patented technique sees a sequence of operations - a horizontal saw cut is made through each abutment, just below the arch springing, while five holes are cored horizontally into each abutment. Vertical lifting jacks are inserted into these holes, supporting the weight of the bridge.
The horizontal component of the thrust force is taken by four vertical slip bearings which are inserted into slots cored through the four wing walls. These bearings prevent the arch from spreading horizontally, while allowing vertical movement. Once they have been grouted in place, it is safe to wire saw cut the rest of the wing walls to free the arch from the foundations - then the lifting can begin.
The lift took place in October 2016. Computer-controlled jacks provided a fully balanced, synchronous lift, as the 160-year-old, 220 tonne bridge was jacked 900mm. As the jacks lifted, hardwood timber crib stacks were inserted beneath to support the bridge each time the jack foot retracted.
The lift took about six hours, during which constant monitoring verified that the arch was behaving as predicted. The arch was then lowered by 465mm so that re-profiling of the approach ramps was unnecessary. The bridge was left 435mm higher than its starting position and the gap in the abutment was faced with brickwork and flooded with concrete to restore permanent support.
The main engineering challenge in this project has been conveying the project in ways that other engineers and clients can understand. Perhaps the hardest thing is to explain that, in modern terms, these bridges really aren’t that heavy. You could hire a crane and lift the whole bridge out in a piece with a little planning.
In some ways Elevarch would have been better understood and accepted by mediaeval masons than by modern engineers. Modern design rules, even for masonry, are fixated on modern engineering concepts, particularly analysis – it has rather fallen from favour to think that you need to explore the boundaries of possible behaviour to be confident the work is OK.
Elevarch has potential for much wider application: both applied to canal bridges, and as an alternative to demolishing bridges that are no longer wanted: when arches are demolished, they are usually broken out and dropped on the track – by lifting them out a huge amount of railway stoppage time would be saved.
We hope to see the solution rolled out across the UK and in other countries, and are delighted to have won the Structural Award for Outstanding Value.
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