A complete, 160-year-old masonry arch bridge has been lifted 900mm for the first time, offering an alternative method of providing additional clearance for rail electrification works.
The lift was carried out by a team from Elevarch, a subsidiary of contractor Freyssinet, and is part of a full-scale test funded by the RSSB Innovation Railway programme.
The company estimated that there are around 500 masonry arch bridges across the country that are over rail lines which are due to undergo electrification upgrades.
When the clearances between the train and the underside of the arch are too small to install the new cables, currently the track is either lowered – which requires possession of the line for six weeks while works are being carried out – or it is necessary to demolish and replace the structure with another bridge with a higher clearance.
Freyssinet said that by “simply” lifting masonry arch bridges enough to gain the required clearance, it hoped it would provide an additional option requiring only one 24-hour possession of the railway.
It said that 25% of the cost of electrification was attributed to reconstruction of bridges. For example, through 10% to 20% innovation savings, the company said it could yield a financial saving of £25M to £75M on the Great Western mainline.
“The Elevarch technique is not expected to replace bridge reconstruction or track lowering completely, but rather join that pair as a third option to be selected by the engineer as the solution when the situation is most appropriate,” said Freyssinet sales and technical director Kevin Bennett.
“It is expected to be of significant interest in heritage situations when track lowering isn’t viable – it’s better to sympathetically move a heritage structure than remove it altogether.”
The bridge, which spans 10.1m and is 4.3m wide, was above a section of the mothballed east-west rail phase 2 route between Bicester and Bletchley railway track. The structure was subject to live monitoring of the movements during the entirety of the lift.
The team behind the scheme said that it had carried out specific strengthening works on the bridge prior to making a vertical and horizontal cut in the brickwork through the abutments. Eight 50t capacity jacks then lifted the bridge to within 0.1mm of each other in a synchronous, staged lift.
After each stage, hardwood timber was inserted into the space created to support the bridge. The jack foot was then retracted and the process repeated another cycle.
The horizontal thrust forces provided by the abutments were transferred to the arch across four vertical slip bearings, which were installed within the two gaps created by the cuts.
After the bridge has been tested while raised to 900mm, it will then be lowered by 465mm. The approach ramps do not need to be reprofiled for the purposes of the trial. The gap in the abutment will be flooded with concrete and covered by a new facing brickwork to restore a permanent support.
“With something in the region of 500 overbridges getting in the way of Britain’s electrification programme, the future for this bold but simple technique is expected to be bright,” added Bennett.