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23Aug91 UK: TARMAC ON TIME WITH M80 CONTRACT NEAR GLASGOW DESPITE PROBLEMS FROM VANDALS. (2 OF 2)

One of the most interesting structures along the route from Mr Laidlaw's point of view is a four-span service footbridge which replaces an existing railway bridge over the Glasgow to Cumbernauld line.Standing next to one of the holes ripped in the chicken wire, originally put up to prevent kids clambering over the scaffolding, Mr Laidlaw said: 'This bridge is being built in situ over a railway line. I've never come across that before. But if we had used pre-cast sections we would have needed something like a 1,000 tonne crane to lift them into place.'It would have taken about a week to assemble the crane and during that time we couldn't guarantee a failsafe situation because the site is so tight here. So it had to be in situ.'But building the bridge over a series of weekend possessions was not only a logistical problem. The twin constraints of height and width restrictions imposed by Scotrail meant that if the gap across the line met the width restriction the beams needed to support the span would be too deep to meet the minimum height requirement.As a result the contractor had to cast a substantial block of concrete which acts as both a crash barrier for the trains and as a falsework support.With a crash barrier in place Scotrail allowed the contractor to close the gap to 12 m which left just enough room to squeeze in the 900 mm deep I beams above the line and still meet the height restriction.Elsewhere the majority of the bridges are either complete or approaching deck slab level. However to the west there is still some complicated work to be done at the M8 tie-in where a tight radius and a 7 per cent superelevation have produced a complex design for the two viaducts that form the on and off ramps.In the short term it seems likely that the new road will do little more than relocate the existing traffic congestion that currently exists around the M8/A80 junction at peak hours because the new road ties back into the existing A80 a few kilometres away at Crow Wood.But once the whole route to Stirling is opened the benefits should be significant.As Mr Daville pointed out though, few of these benefits will extend to the residents of the Blackhill estate.'I don't want to go overboard about how bad Blackhill is, though it certainly merits mention because it has caused us problems,' he says.'But what you've got to remember is that the people round here are more concerned with getting more buses and better footpaths. They don't get anything out of this motorway except noise and disruption.'However as another police van swept into the Tarmac compound it was difficult not to wonder how long that sympathy would last.