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Sliding over the border

Highways - A steel bridge slide over the river Esk will help complete the M6 project lling in a missing link. Report and pictures by Adrian Greeman.

To the consternation of desperate eloping couples, the last few kilometres of motorway between the Scottish border and Gretna Green have long been missing. The M6 drops to dual carriageway for the 9km 'Cumberland Gap' before the M74 continues on to Glasgow.

But soon elopers will be able to make their getaway a little quicker. Contractor Carillion is widening the dual carriageway to a full three-lane motorway completing the long ribbon of 'high road' from London to the north.

Key elements in the £175M project are two new bridges, one a concrete portal across the East Coast Main Line and another for the river Esk crossing - a slide construction £5.2M steel structure placed into position last week.

'The steel bridge sits alongside the existing crossing and will eventually take the southbound traffic lanes, ' explains Andrew Watson, operations director for consultant Grontmij, which has designed the two structures as subconsultant to the project's main consultant Capita Symonds.

'For the immediate future however it will carry all the twoway traffic once it is ready.' The transfer will allow the existing bridge, known locally as the 'metal bridge', to be cleared for renovation work.

Leaking joints over bearings will be replaced and the road reconstructed with new deck for the three lanes of the motorway and a local road crossing to one side.

The slide construction was chosen for safety reasons; to keep construction at height and over water to a minimum. At this point the River Esk's estuary comes off the Solway Firth and it has a substantial tidal range, explains Grontmij site design representative Amanda Henton.

'The flow in and out is very fast. It was suggested jokingly that if someone fell in we should phone the coastguard 15km down the coast rather than bother chasing them with the safety boat, ' she says.

'At an early stage we looked at using craneage to lift in the bridge but there was also a major safety issue using cranes in the river's ood plain, ' adds Carillion structure's agent John McNiffe. 'On top of that the marshes downstream are a European protected environmental area.' A substantial amount of equipment was required for the pier construction which included earthmoving to create temporary access causeways for constructing cofferdams.

The new bridge will have four spans rather than the eight of the original crossing, but although that keeps work in the river to a minimum it still means that three piers are required.

Watson says a smaller number was ruled out with a longer span because the steel beams would have needed to be deeper and that would have interfered with clearances for the river.

Fortunately the foundations could be relatively simple spread footings because the river runs across bedrock - the red sandstone used on Carlisle Castle.

'Even on the north abutment where there is 3 to 4m of cover we went for digging out and inlling with concrete rather than piles, ' says Henton. The decision was inuenced by construction issues raised by the contractor.

In the river, sheet piled cofferdams 15m long and about 5m wide were used to build the pier bases. The 7.5m long sheet piles were keyed into the rock to protect against the 2.5m tidal range of the river 'which can rise 1.5m in 20 minutes, ' says McNiffe. A 1:100 ood event had to be catered for as well.

Building the pier bases was a major challenge.

Some 6,500t of 40mm fill was required to make a causeway to access the two river pier locations - the north pier was on the flood plain - and another 2,000t of protective rip-rap.

'We also used high density bulk fill in bags as protection 'We were only allowed one causeway at a time, taking the other out before we could go ahead from the other side, ' McNiffe says.

Water ingress caused further problems because the 250mm key into the rock for the piles tended to cause cracking.

Underwater concrete with a Sika additive was used to seal the base and even then substantial pumping was needed, says McNiffe.

Reinforced concrete leaf piers rise 7m from the foundation slabs. The slender shape widens out at the top for aesthetic reasons, Watson explains.

But it did also allow enough width for positioning hydraulic jacks for the installation operation; they are being used to take the load from the new steel deck in position and to remove the temporary bearings, dropping the beams 350mm onto new pot bearings.

The temporary bearings were rocker skids rather than roller bearings used for some pushing operations. Five of these were positioned on each pier top to catch the five steel beams making up the bridge deck structure, or at least the tapered launch nose. The rocker is selfadjusting and tilts as the bridge passes over.

Fairfield Mabey, the subcontractor who fabricated and assembled the deck, also carried out the launch operation from a position on the north bank. The operation came in two halves, the first two-span length arrived in mid-May and the remainder came on 13 June, to complete the 180m length.

'It was easier to do the work in two halves for assembly purposes and because we were using a restricted working area, ' explains Watson.

The first push was a little lighter than the second because it was only half the length and because the leading span was left as a bare steel frame, to reduce its weight. It had to sustain a 50m cantilever at the maximum position during the launch.

The remaining spans were preassembled with infill permanent formwork panels, not the usual Omniaplanks but EMJ plastic coasted steel beams 'which are a little lighter' explains Watson.

Some 30% of the reinforcement for the deck was installed in advance. Total weight of the entire deck in the second phase was about 1,300t.

Fairfield used two 180t capacity jacks for the push, though in the event only about 20t of the capacity was needed for the slide over the PTFE surface pads on the tilting bearings. The top of these was further coated with a 'soap' which is 'a little bit like Swarfega' says McNiffe.

The launch went well he says, not least because of the meticulous planning work put in by the whole team.

Once the bearings are installed there remains deck work accessed from the banks, with installation of the string courses and diaphragms, he says.

When the basic composite concrete deck is completed, scheduled for sometime around November, the waterproofing and blacktop can be done, ready for the onslaught of traffic in spring next year.

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