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Interchange elements

To link upper and lower canals to the £20M Falkirk Interchange site, Morrison-Bachy Soletanche had to build a 168m long tunnel, four span concrete aqueduct, three locks and new stretches of canal top and bottom totalling 2km.

The first new canal tunnel for a century, subcontracted to specialist contractor Spray Concrete, was routed beneath a main rail line and, more crucially, the Antonine Wall, once the Roman Empire's northernmost boundary.

This scheduled ancient monument, though in reality just a ditch cut into the hillside, commanded zero disturbance as the tunnel was excavated through sandstone and boulder clay just 2m beneath it. The 8m wide horseshoe shaped tunnel was driven in three headings; two adjacent quarter circles creating the upper half with the lower section and flat invert forming the third bench.

Upper headings, the second staggered about 15m, were worked by standard excavator with each drive immediately lined with 300mm thick sprayed concrete.

But the 2m deep invert section was cut by an adapted road planer in 100mm milled layers. This novel technique, estimated to be a fortnight quicker than conventional excavation, also worked out around 15% cheaper.

The tunnel portal opens onto a 104m long aqueduct supported on 24m high curved concrete columns matching exactly the profile of the wheel at the end.

With the aqueduct intended to form an integral part of the wheel's architectural impact, RMJM requested that its 8m wide concrete troughed deck should appear to 'float' though the columns hooped tops. For the JV's civils consultant Arup, satisfying the architect proved a significant structural and geometrical challenge.

'We had to support the deck on two narrow side sections of column and route 650t vertical loads through each, ' recalls Alan Richmond director of Arup Scotland.

'There was a lot going on in a very small area and our basic analysis model suggested it was unbuildable.'

But further finite element analysis, followed by full size test panels of this deck-pier interface cast at ground level, proved that the required high density of mainly 40mm bars was practical.

Previous occupants of the interchange's exposed hillside site have included an opencast clay mine, tar factory and deeper coal workings.

The resulting 20m depth of backfill, tipped loosely into the old quarry and containing large sandstone boulders, did not lend itself as a firm foundation for water containing structures demanding minimal settlement.

Everything is therefore deep founded with around 30, 22m concrete bored piles socketed into sandstone bedrock.

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