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Concentration through fabrication

Bridges - Pioneering use of robotic fabrication is cutting renovation of the Leven Viaduct from two seasons' work to a single blockade.

For around 150 years the wind that comes whipping in from Irish Sea and across the flat sands of Morecambe Bay has drenched the Leven Viaduct with sand and salt spray. This has taken a heavy toll on the ironwork and brick of this 500m long multi span structure, one of several estuary crossings on the Carnforth to Barrow rail line.

Major repairs in 1915, and again in the 1950s, strengthened the bridge but age has brought it near to closure.

So owner Network Rail is investing ú14M in a complete rebuild of the viaduct, ripping out the old deck and tracks over 48 of its 49 spans and replacing them with new. The 20m central navigation span is being refurbished. Work started in March and there are two months remaining for main contractor Carillion to complete the project.

The same winds that have buffeted the bridge also make work cold, uncomfortable and potentially dangerous. The bridge is not exceptionally tall but far enough above the shifting sands and sudden tides to cause serious injury. And there is limited space on the two track deck area.

'You could say that we tick most of the boxes when it comes to safety hazards, ' says Carillion project manager Matthew Wylde.

Logistics are also a challenge, with site access highly restricted.

Network Rail's original belief was that the work would need two seasons to complete, working on one track at a time and using the other to move in and out of site.

Blockades of 16 weeks were prepared for spring this year and next, when the weather is most likely to be favourable.

Carillion's design and build tender proposed a faster system, concentrating all the work into the same season. The speed up comes from using two specially commissioned overhead lifting gantries rather than on-track cranes for lifting out old sections and later placing the new.

It also, for the first time, uses off-site robot welding to fabricate the new steel viaduct deck.

This has been critical to delivery of new steelwork in time, against a tight schedule, says Wylde.

Existing spans are composed of six longitudinal steel girders with cross connecting angle pieces, explains Network Rail project manager Neil Jones.

'This supports a variety of deck plates, some the original wrought iron but as you can imagine this is often like lace and in many places has been replaced with cross timbers or newer iron plates' he adds.

Above, the bullhead rails are mounted on longitudinal timbers that run along the main support girders. Spans are in the main 9m to 12m long, although there are a few shorter 7m spans.

To use gantries, Carillion has had to create special rails running down either side of the track on walkways replacing the viaduct's original maintenance walkways. The portal frame gantries have a 30t capacity.

First, cross members connecting the innermost longitudinal girders are flame cut and removed, splitting a span in half lengthways. Old walkways are removed and new walkway spans installed using small roadrail lifting machines.

Next, the gantry is positioned and hoists put in place to support the deck section once it has been cut free. Cutting is carried out with conventional flame cutters and extreme high temperature thermic lances.

Spans are removed in two layers ? the deck and the main support girder assembly.

'That leaves the span empty, which gives us room for work on the pier tops and installation of bearings, ' says Wylde. Part of the contract is to repair any damage to the brickwork-clad iron piers. A number of precast concrete replacement tops sit in the main depot ready for use.

'A key reason for using two gantries was to allow this space for pier work, ' says Wylde.

New full-width trough section girders are being installed.

'Steelwork for the new walkways was first to be erected and by the end of April we were over halfway through installing them all, ' Wylde says. During early May the deck replacement began and this is expected to take 10 weeks in total.

Although most of the viaduct's 49 spans are around 9m long, a detailed survey of the foundations revealed 16 different deck geometries which required unique 3D models.

'The use of automation for material preparation and welding based directly on the 3D model made this variable geometry well suited to the robots, ' says Lloyd.

'Timing was critical on this project, ' he adds. 'By utilising robotic welding we are able fabricate the girders faster and have confidence that the finished product will be right first time.' Rails will be mounted directly using the relatively new Vipa Baseplate system of rail chairs bolted directly to steel stools on the deck.

All has been going relatively well, although this year's late spring has meant more high winds than expected, costing Carillion the 10 days of 'float' it had allowed for weather stoppages.

On an already tight six day working schedule, this has meant re-programming to do three rather than two spans daily. Seven day working may be needed at some point, adds Jones.

But work should be complete by the scheduled blockage end of the blockade on 17 July.

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