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Rail roaded

Conversion of a Scottish rail bridge into a road crossing demanded extensive pre-assembly to beat the Highlands winter. David Hayward reports from Oban to start our repair and refurbishment special report. Photos by Jim Mackintosh Photography.

Next month's re-opening of the long-abandoned Creagan rail bridge offers mixed relief to the shinty team from nearby Oban.

The 12 men used to disembark from their team bus at one end of the bridge on their way back from playing their wild version of hockey against rivals North Appin. While the coach ground its way along the 9km of twisting road around the head of Loch Creran, the shinty team would stroll along the old trackbed to the far side. A discreet pause in the middle of the bridge to answer nature's call, and they would be ready to be picked up again when the coach finally arrived.

This ritual will disappear forever when the bridge reopens as a shortcut for road vehicles. Balfour Beatty Construction has spent this last harsh Highland winter re-equipping the exposed structure with a novel new deck and faithfully reconstructed masonry piers.

Deck design was just one of the challenges facing the contractor when, last April, it won the year-long £3.4M design and build contract for a unique conversion of the 97 year old structure.

The original crossing, with its two main spans formed of heavy through trusses, lost its rail tracks when the Highland line between Oban and Fort William closed in 1966. But it still stood proud and structurally secure across the upper reaches of Loch Creran, 20km north of Oban.

A decade ago, the 130m long crossing was identified as an ideal road shortcut, eliminating a problem section of the busy A828 tourist route between Oban and Fort William.

Early designs by Scottish Office consultant Bullen outlined a 6km scheme utilising both the old bridge and the abandoned rail route either side. The £10M price tag killed the scheme until it was revived a year ago as a design and build option.

Bullen's conceptual bridge design, worked up in conjunction with Scotland's Royal Fine Arts Commission, provided only guidelines.

'We could have demolished all the old bridge but we were told its replacement had to have the same spans and pier positions,' recalls Balfour Beatty structural consultant Carl Bro project director Ewan Angus. 'The high tidal range and fast current at pier sites made new build an expensive option.' Instead, the consultant chose to retain two of the three piers and rebuild only the third, which had inadequate caisson foundations deemed beyond economic repair.

The long term lean and upper cracking on this southern pier has triggered a colourful - though dubious - anecdote of operational maintenance during the structure's railway days.

So obvious were the cracks to train guards as they slowly passed over the bridge, it is said, that bags of cement were occasionally poured down into the cracks from the moving guards' van directly above.

Road alignment for the new plate girder deck meant the concrete-filled masonry piers had to be lowered 2m. Granite blocks were individually removed and a large capping slab across the top remodelled and repositioned lower down, complete with decorative stonework.

The new side pier was formed in concrete over simple steel H piles and clad with original granite blocks. Strengthening the foundations for the retained central pier proved less straightforward.

This more elaborate mid river pier was founded on twin 3.8m diameter steel caissons, filled with concrete and keyed into bedrock. 'Both were so rusted you could push your hand through the holes and the friable concrete inside could easily fall out,' says Balfour Beatty agent Alistair Henderson.

The solution was to surround each caisson with a protective steel sleeve and fill the 200mm annulus with concrete. But installing the sleeves underwater, in the sea loch's 6 knot currents and against a 3.8m tidal range, took near double its programmed seven weeks.

The 7m long sleeves were formed in five horizontal slices, each with a hinged opening to help slot it around the caisson. Each section was positioned and bolted above water at low tide, with the growing sleeve gradually lowered down around the submerged caisson.

'We only had two hours of slack water in any twelve to do most of the work in,' Henderson recalls. 'The current was so strong that our moored working barge created its own hydraulic jump and we had to abandon it several times.'

Horizontal Highland blizzards, rather than fast tidal flow, were the dominant weather challenges at superstructure level. Here craneage demanded its own weather windows to remove railway trusses and replace them with twin 800mm wide by 2.4m deep plate girders.

Calm winds last June allowed just enough time for the 250t Mersey Mammoth floating crane to make its now customary appearance at such large bridge lifts and remove the two 130t railway trusses. But it was much harsher late November weather before the seven lifts of plate girder beams, fabricated and erected by Fairfield Mabey, were ready to be positioned using a land based 800t capacity Demag crane.

Located close to the shoreline, this counterbalanced mobile needed its maximum 72m reach to place the 2.4m deep plate girders half way across the bridge. It was then trucked round the loch for a repeat operation from the other side.

But at each location the crane also installed half the precast deck units on to the newly placed plate girders. All these 20t units had been precast on site close to the bridge.

'We effectively erected all the superstructure in just 17 days,' claims Henderson. 'With a traditional insitu deck it could have taken two months longer.'

This saving was down to the innovative state of the art superstructure design developed by Carl Bro. A conventional all-insitu concrete deck composite with steel beams beneath was rejected in favour of a two section deck design - transverse precast beams overlaid by an insitu topping.

All 43 slender 150mm thick, 3.6m wide concrete beams, spanning full carriageway width, and precast complete with side parapet upstands, are now in place over the twin plate girder superstructure. Longitudinal reinforcement has been laid on top and the 100mm thick insitu upper layer is now being concreted.

The design problem with such two section decks is matching the speed of erection advantages of precast planks with the need to provide enough longitudinal steel to accommodate the bridge's bending moments.

'We have yet to achieve a fully precast deck design, and even partial precasting is rare,' explains Carl Bro's Angus. 'We reckon these beams are the largest yet used and, on this exposed site, they offer increased safety, near factory quality and at least eight weeks' time saving.'

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