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Decking out for winter

Lengthy main line track possessions are rare and several rail contractors had to sacrifice the Christmas break to take advantage of a lull in the traffic. David Hayward joined one of them.

Christmas was effectively cancelled last month for a construction team of about 80 workers housed in site huts beside the main trans-Pennine rail line near the West Yorkshire town of Mirfield.

A row of microwave ovens and Tupperware dishes provided a festive dinner of stew, carrots and mince pies in the bleak 3.00am darkness of Christmas Day morning. Even the combined warmth of triple pay and mugs of tea did little to compensate the team for the strict ban on alcohol during the nation's four day seasonal break.

To the occasional passerby, the eerie silence of a usually hectic commuter and inter city rail route, plus a gaping hole in the track embankment, provided clues to the construction challenge. But few outsiders witnessed the main event - the culmination of six months of planning, numerous 'what if' brainstorming sessions and a rare 58 hour track possession that had to be firmly booked back in summer 1997.

As most of the country was settling down to the Queen's Christmas message, a vast 850t section of Mirfeld station railway bridge was being painstakingly lowered into place. And fortunately, with the high winds fast building up to the notorious Boxing Day gales, there was as much an absence of craneage on site as there were observers.

'The 1,000t mobile we would have needed could not have operated in winds greater than 40km/h and, had we used cranes, we could well have overrun our possession time by a very expensive eight hours,' explains Peter Brack, project manager for main contractor May Gurney.

'As it was we installed the new deck without a hitch and handed back the tracks several hours early.'

The kit, which in hindsight turned a near certain costly delay into a genuine happy Christmas, was a 22 axle low loader. Replacing the conventional craneage operation that is still the norm for most bridge deck replacements, this increasingly favoured lifting gear alternative did the lot.

Specialist contractor Econo-freight's computer-operated self-propelled bogie lifted out two old wrought iron deck sections weighing more than 200t apiece and deposited them round the corner so they could be cut up and removed at leisure. The same vehicle then filled the gap with a single 25m wide near-complete steel and concrete replacement deck that had been pre-assembled down the road.

Cut through a deep rail embankment, the 60m wide single span bridge provides access for a local road beneath the three track main line between York and Manchester. The existing deck, concealed beneath 1m of track ballast, had been a hotch potch of several sections of varying ages.

Most of the deck was circa 1900 wrought iron but, at one side, an 11m length had been renewed five years ago by a steel beam replacement. At the other side, a skewed structurally separate accommodation bridge, also more than 90 years old, carried services rather than track but provided essential access to signal junction boxes along the embankment.

The challenge was to replace a 25m width of weak original decking, sandwiched between the accommodation bridge and the newer steel section.

May Gurney's chosen solution was first to lift out the old 120t accommodation bridge and store it temporarily on a nearby stillage. The weak wrought iron section was then cut in half and removed in two pieces.

The new deck, 560mm thick and formed of steel beams with pre-cast concrete planking and an insitu deck, was preassembled over trestled supports on a conveniently closed section of the road 200m from the bridge site. The 850t single section could then be lifted, moved down the road and installed before the accommodation bridge was replaced in front of it.

With a non-electrified rail line, there were no overhead wires in the way and the obvious method was to use cranes. But detailed risk analysis persuaded May Gurney to opt for Econofreight's wheeled plant, despite being about 30% more expensive.

'High wind is the one risk we could neither predict nor effectively manage,' Brack explains. 'The low loader method could go ahead in virtually any weather conditions, with only a blizzard that obstructed all vision likely to close us down.'

Christmas was the only time that client Railtrack was likely to grant a possession long enough to do the work, so Brack knew the site team had no room for mistakes or delay and every conceivable 'what if' scenario was rehearsed and catered for.

All important plant had backup duplicates stored on site; standard diesel was replaced by freeze-free fuel; excavators had snow ploughs fitted and rooms were reserved in a local hotel to ensure staff had no transport problems.

Most standby contingencies were not needed and the £886,000 operation went without a hitch. Fears that the old deck could have been pinned down to its abutments proved unfounded and the bridge was handed back to Railtrack some 12 hours ahead of schedule.

The only disruption throughout the 46 hour operation was the appearance on site of an inebriated well wisher on Christmas Day. Police quickly removed him but his condition was something of a red rag to the alcohol- free workforce.

Railtrack has a contractual rule on all its sites of no alcohol during work time and for 12 hours before. Every contractor must provide a box of breathalysers in case of spot checks by engineers or the client, though at Mirfield no one had cause to open it.

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