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Fire drill

Bachy Soletanche considered using its recently developed CFA Screwsol soil displacement technique on the Broughton contract; but concluded that traditional CFA piles were more prudent on this fast tracked, 8,000 pile project.

The firm did however secure a separate, more modest, 170 pile Screwsol contract for the ground slab of Airbus UK's fire station serving the private runway alongside the factory.

The Screwsol technique centres on a smooth shaft auger with only the lower 1.5m fitted with auger flights.

This cutting head tapers to just 150mm at its tip and, as the auger is screwed in, soil is displaced and compacted.

The secret weapon is an angled protruding cutting fin attached to the bottom flight. During the auger's descent, the fin cuts only a narrow thread in the soil.

As the auger is extracted, it continues rotating clockwise so no soil is brought to the surface. But the cutting fin opens up a wedge-shaped slot around the pile shaft. This is immediately filled with concrete, along with the main auger bore, creating an integral sideways extension to the shaft, effectively increasing pile diameter from 350mm to 500mm.

For the fire station slab, project manager Steve Mallinson compared the performance of the 10m deep Screwsol piles to a conventional CFA alternative. He claims a saving on concrete and shorter pile lengths, with the major advantage of no pile spoil.

Had the ground been contaminated, the saving on spoil removal would have been even more significant.

The only drawback was having to use a large, somewhat cumbersome, high torque rig to sink piles at close centres on the very confined site. But the bottom line advantage was Mallinson's estimate that the Screwsol contract worked out at least 15% cheaper than conventional CFA operations.

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