Engineers who specialise in bridge lifts have backed calls to raise a grade II listed masonry arch bridge that is causing problems for the electrification of the Great Western Main Line.
ElevArch rail business development manager John Kennils said it would be possible to avoid demolishing the bridge by jacking it up to create the required clearance and then reprofiling the road on either side.
It comes after several New Civil Engineer readers suggested lifting the bridge, after Network Rail lost its application to demolish the Steventon bridge and replace it with one with a higher clearance.
ElevArch, a subsidiary of French engineer Freyssinet, and Bill Harvey Associates carried out a successful trial in 2016 to lift a 160 year-old single span, 220t masonry arch bridge 900mm using 10, 50t jacks. Kennils believes similiar works could be carried out at the Steventon bridge site.
“We believe it can be done,” Kennils said. “We’re now looking into some of the differences to what we did to the trial site. The big difference is that that was a single span and this is a multi-span, but we still think it’s achievable.
“We’re looking at slightly amending the methodology using different types of jacks so we don’t have to have timber chocks underneath. It’s in its development at the moment.”
The Steventon overbridge differs from ElevArch’s successful trial in Buckinghamshire as it has three spans and is skewed across the railway.
Overhead lines have been installed under the bridge, but speed restrictions will be imposed when the lines are electrified in December this year, due to the steep gradient of the cables and the risk of the train’s pantograph hitting the bridge.
Kenills expects the overall cost of raising the bridge to be significantly less than bridge reconstruction or track lowering, with demand for track possession time also reduced. However he said the technique was not supposed to be a solution for every situation.
“The ElevArch technique is not expected to replace bridge reconstruction or track lowering completely, but rather create a third option when the situation is most appropriate,” he said. “Ultimately, it will provide an effective alternative to overbridge demolition, particularly attractive for listed structures.”
In the 2016 trial, the arches of the bridge were cut free from their abutments and wing walls and jacks inserted into holes created in the brickwork to take the vertical weight of the bridge and lift it off its supporting abutments.
Elevarch at 300mm into the lift
The horizontal component of the thrust force was taken by four vertical slip bearings which were inserted into slots cored through the four wing walls.
As the jacks lifted, hardwood timber crib stacks were inserted beneath to support the bridge each time the jack foot retracted.
The bridge was raised in incrementally over a period of six hours with continuous monitoring to ensure the movement was within the required tolerances.
Kennils said each of the 10, 50t jacks was computer-synchronised from a central unit to within 0.1mm of each other, guaranteeing a fully balanced lift.
The gap created by raising the bridge was then faced with brickwork and flooded with concrete to restore permanent support.
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