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Local authority engineers at Barnet Council are having a tense time replacing two old rail bridges over the last few months. Report by Adrian Greeman.

The very public attention given to Network Rail project delays over Christmas cannot have helped calm the nerves of engineers at Barnet Borough Council in north London. They too have been carrying out major work on the railway, replacing two bridges on the Midland Mainline out of St Pancras.

"Rail bridge replacement is a big task for a local authority," says Barnet chief highway officer Ian Caunce. He says this work is beset with potential scheduling and logistics problems. Difficulties were compounded by a very tight timescale which meant the council had to get everything done in two years, from the approval to completion. "Otherwise we lost the funding" explains Caunce.

The parallel projects have each involved multiple possessions of the tracks in a sequence over the last few months. This culminatesd in the successful demolition of a Victorian arch bridge before Christmas and its replacement with a slimmer and much longer steel bridge during a 54 hour blockade.

The new deck, preassembled off site, was moved into place on multi-axle low loaders and lowered onto bearings on four cylindrical concrete piers, one for each corner. The piers are continuations of giant single 2.4m diameter concrete pile foundations, bored over the last few months alongside the live railway and during several earlier possessions (NCE 13/20 December 2007).

The brick arch was demolished and the abutments excavated back around these piers to drop in the new deck. Comprising steel I beams and cross beams, this was the slimmest design that could be achieved, to gain maximum headroom. The road below is also being lowered to help achieve a total 2.5m gain which will make it passable for high vehicles. Collisions are notoriously frequent at present.

A second operation is due at the end of January for the other bridge, when final sighs of relief can be breathed by Barnet and its contractor Norwest Holst.

The scheme is needed to open up a traffic bottleneck on the old Aerodrome Road which links Barnet to Colindale, explains Caunce. Three lanes instead of currently two very narrow lanes, and passage for buses, will give crucial access to series of major new housing projects. An initial 4,000 houses and eventually 10,000 are being built.

The rail projects were critical therefore and could be funded by additional money. But Barnet was caught by a spending deadline on the grants.

"A large part of the £12M for the scheme comes from Growth Area Funding" explains Caunce "but that has a time limit on when it must be spent, essentially by next April." The £7M grant was awarded in 2005.

The first issue was to book possessions. Both arches carry important tracks, the fast mainline service and the Luton and Bedford "slow line" running alongside the M1 motorway where it heads out through the north London suburbs. Almost two years notice is needed on such busy routes, and the council was advised by an initial feasibility consultation with Parsons Brinckerhoff that it needed to get in early.

So before the council knew exactly what the designs would be and whether it could get approvals, it had to dive in and take some risks, booking the possession time needed and committing funds. A dozen assorted possessions were booked, four of 54 hours and other shorter.

Negotiations then followed with Network Rail to establish a so-called underbridge agreement, essentially spelling out the work and its implications, making asset protection agreements and agreeing long term maintenance cover for the changes. These can take time because of the multiple concerns about safety and schedule disruption on railways.

"Network Rail is very commercially minded" says Caunce "and insisted on various additional works like track renewal beyond the bridge." This put more pressure on the programme, "and meant we were in a bad position for sticking firm in negotiations, so had to accept things [as they were]" he says.

Design work was carried out before it was known whether the work would be allowed so the project could keep on track with the tight programme.

Luckily he says, the council's committees were supportive of the project.

A design and build contract for the scheme was ruled out because of time pressures. Instead tenders were invited 12 months ahead of the start of construction, on the basis of detailed designs worked up by consultant Atkins. From an initial six, Norwest Holst was selected with foundations and steel superstructure all in the one contract.

Fortunately says Caunce the eventual design and construction was relatively straightforward, with the four main piles for each bridge to go in first and then the demolition and replacement to be accomplished around those. Piling work, carried out by Simplex and the Westpile division of Bachy Soletanche, was under a £2M joint venture subcontract and involved a large rig installing the piles from temporary platforms.

Norwest Holst helped develop special steel cylinder casings to surround the pile tops which are just under 2m from the track side. Using these, enabled work to continue near the live railways to create the full pier heights.

The steel casings have to be cut and removed during the main demolition and replacement possessions this winter, part of the complex sequence to remove the old brickwork, excavate back the embankments and install the new deck with tracks.

Both bridges will now need the final touches added.

The road too has to be excavated to a new depth and during this process, two horizontal precast concrete ground beams will be installed to link the two piles on each side of the track. These have to withstand high horizontal loads from braking trains.

"In the meantime we have steel lock elements at the top rather like seismic restraints" says Chrysostomou "and these will be removed."

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