Written by Mungo Stacy.
Arnside Viaduct is the subject of major activity. Close to hand in the give-away-the compound, four sets of double-decker cabins indicating the number of workers involved, a large car park, full, formed of temporary metal plates, stockpiled materials recovered for reuse, and, on the far side, yellow road-rail machines.
Reconstruction
The approach is a temporary track a kilometre long through the fields of Meathop farm, where the speed of site traffic is likely to be governed by a flock of sheep. However, there is nothing sedate about the construction work.
The ‘board room’ walls are lined with a repeating pattern. Eight panels plan in daily detail how all 51 spans will be reconstructed over the course of a 16-week blockade from 28th March to 18th July 2011. Coloured bars run up and down diagrams of the double-track viaduct scheduling the different activities.
Production rates are intense. The deck is split in half for removal so there are a total of 102 deck units to come out and 102 to go in. The plan assumes five spans to come out daily and, later, three per day to be installed.
In between are other activities: drilling sockets for the new bearings, at the rate of 48 holes per day, and placing bearings at 12 a day.
The £11.1 million project was tendered as a design and build contract and awarded to May Gurney.
Project manager Steve Richardson explains, “At the start of the project we got all the main subcontractors together in this room and mapped out how the works would fit together”. This helped all concerned understand the critical activities and how the access would work for each stage, he says.
Key to the programme is the clever method of replacing the decks. Although ingenious, it has been done before, for this viaduct is almost identical to the 49-span Leven crossing ten miles further west which was rebuilt in 2006.
Client Network Rail gave a strong hint of its expectation by including the as-built drawings for the Leven Viaduct in the tender package.
Red gantries
Leven was memorable for the red gantries which ran up and down putting in the new decks. The same technique is being used at Arnside. The red gantry could be mistaken for the same one – in fact, it has been supplied by Mammoet specifically for this project to a design by Chris Booth.
The quantum leap at Leven was to install new maintenance walkways as independent structural units and run the gantries along them. Placing the walkways first, outside the existing bridge, allowed the deck to be reconstructed between them.
This innovation allowed the contractor to complete the works in half the time originally envisaged.
The problem was that the original viaduct could not support the weight of its own removal, in particular the high point loads which would be imposed by cranes reaching over to pick up adjacent deck units. The gantries overcame this by loading only the new structural elements.
Arnside Viaduct has similar issues. It was constructed in 1856, initially accommodating just a single line, then doubled in 1863. Four longitudinal wrought iron girders per span provide direct support below each rail, with lattice girders picking up the deck edges.
Exposure to the sea-salt environment over 150 years has caused the girders to deteriorate to the extent that replacement was deemed the most economic long-term option.
Trains appeared to make deliberately stately progress across the viaduct. Some passengers surely savoured the slowly-changing vista of the southern Lakeland fells and the intimate observations of wildlife disturbed by the rumble of the bridge.
However, train speeds were governed not by delight in nature but by a permanent 30mph restriction. Within the last four months this was reduced further to 20mph to ensure dynamic loading effects remained within the safe capacity of the girders.
Lessons learned
May Gurney sought to improve on the method used five years ago. It commissioned full-scale load testing of the road-rail plant to determine the imposed wheel loadings. Assessments were carried out to prove that the remaining girder cross sections could safely carry the point loads as the new walkways were craned in.
Additionally, there was sufficient capacity for a pair of road-rail machines working in tandem on one line to lift out the old girders from the adjacent line.
This gives important flexibility in the programme, says Richardson, since the Down line deck units can be removed in parallel with walkway installation rather than having to wait for the walkways and gantry. It also means only one gantry is needed, compared to the two used on Leven Viaduct.
There is also a subtle, but important, change to the walkways. A box girder section is used, supported by a single central rocker bearing at each end, rather than twin I-girders.
This allows the walkway unit, complete with new parapet, to be installed without removing the handrail from the existing deck. Full edge protection is therefore provided throughout the reconstruction.
Deck detail
One line, initially the Up side, remains open throughout to provide access for the full length of the viaduct. The new welded-steel deck units are brought in from the Arnside end on road-rail trailers.
The gantry has compressed air winches to lift the deck unit, transfer it across to the Down side and lower it into position. Hydraulic tirfors allow the gantry to move itself to the next span to repeat the operation.
Once the Down side is complete, rails are laid and the process repeated for the Up side. Story Rail is installing the permanent way and providing the road-rail plant. Redundant materials are taken to a road access point on the Grange-over-Sands side for removal.
The decks are designed by Gifford. They will provide full RA10 loading capacity and restore linespeed to 60mph. Derailment containment is provided by upstands whilst steel stools at nominal 600mm centres support the rails.
Mabey fabricated the decks from October 2010 to May 2011. Spans are called off as required from the stockpile at Mabey’s Chepstow factory.
Original construction details record a nominal 30-foot length for the spans. In practice there are variations between the piers so the individual deck units are made specifically to suit each span.
Length typically varies by up to 100mm around the average 8550mm between bearings; typical unit weight is 24 tonnes.
Span 5 is over the river channel and until 1865 could be drawn back to allow boats to pass. It is longer at 16250mm. It causes some disturbance to the regular production-line construction sequence as special measures are needed for its removal.
In particular, one side of the centre beam top flange needs to be removed to allow the new Down-side deck to fitted alongside, while it remains supporting the Up line.
The piers were rebuilt around 1914-16 due to oxidation and failure of the original cast iron columns at low-water level. A full survey of all the brick-clad concrete piers is being carried out as part of the project and remedial measures implemented, including stitching across cracks.
The estuary sands are a protected environment with just about every acronym in the book -SSSI, SPA, SAC, AONB – and they are on the edge of the Lake District National Park. Extensive negotiation was needed to permit access to the piers from the estuary.
This was essential to install the pier head scaffolding prior to the blockade. The incoming tide is treacherous and can achieve nine knots, faster than can be outrun. Specialist protection is provided on the sands including control of working hours and a jet-ski in attendance.
Local news
The wind funnels into the valley creating a near-constant breeze on the viaduct. In Week Two of the blockade it gusted to 60mph, stopping work for four days.
“I would not let anyone walk across the viaduct, let alone work on it”, says Richardson. Progress recovered quickly, with the team showing they could remove up to 15 spans in a day.
We walk on towards Arnside and the wind drops as soon as we reach the causeway at the end of the viaduct. A little further, Arnside Station is the temporary terminus for trains. A replacement bus service operates.
The next station at Grange-over-Sands is three miles by rail but a half-hour journey by road. Additional bus services therefore serve Grange, providing quicker links to and from the West Coast Main Line at Oxenholme.
Keeping the station open has been a big plus.
“Initially we thought we could not run beyond Carnforth”, says Network Rail’s project manager Stephen Townley. With May Gurney appointed, the team’s review of the methodology showed that the construction compound could be sited beyond the station.
Further discussions with the train operators and Network Rail operations staff developed a viable operating plan involving a section of single-line working. A temporary crossing provides level access to both platforms.
Local interest in the project has been strong. A public open day at Grange attracted around 300 people and one at Arnside double that. A key issue was potential provision of a public walkway across the bridge.
“We’ve listened to what local residents are saying and appreciate there is a big desire for it,” says Townley. Crossing the estuary sands is a popular charity event.
Many parties would be involved, including the Crown Estate, not least in resolving the land issues associated with creating new paths to each end of the bridge. “We have considered it in our design and made allowance for a bolt-on walkway, but it is a scheme for the future”, Townley insists.
This interest is in evidence as we take lunch at Arnside, our table on the promenade incidentally allowing the project managers to keep an eye on the gantry.
Even as we sit down, someone approaches: “When’s the viaduct due to be finished?” Within a few minutes, the next query: “What’s that crane for?” The 160-tonne crawler crane sits a few yards beyond the temporary bufferstops at the station, ready to offload deck units arriving by road.
Townley recounts comments from the open days. He explained that the bridge would be significantly quieter as it would have continuous welded rail fixed to vibration-damping Pandrol VIPA baseplates.
One resident, having clarified that this meant there would be no more ‘clickety-clack’, expressed dismay – “when he heard the 06:03 to Barrow go over then he knew it was time to get up”!
A roar surrounds us as a fighter plane banks and levels, the 475m straight of the viaduct forming an irresistible training target. Within seconds it is gone, the sound taking longer to fade. The jet returns minutes later, lower, for a second pass.
Out in the estuary, work proceeds undisturbed on the bridge. The red gantry continues its production line task. The new walkways and parapets, key to the gantry method, are appropriately enough painted a close match to Heron grey.
