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SLIP OF A THING

SLOPE ENGINEERING

The site of a new academy on the Kent coast features both a slope and a slip plane. Damon Schünmann reports.

The government's academies programme of sponsored schools is generating work, and challenges, for foundation contractors. One example is Kent County Council's £26M Folkestone Academy, sponsored by local businessman Roger de Haan.

The rectangular 160m by 50m site not only sloped at one end, but within this a slip plane crossed the west end in north-south direction.

So Jackson Civil Engineering's enabling works meant building shear keys to prevent slip plane movement as well as modifying the slope using cut and fill.

'The worry was the existing ground would fail if we put a building on top, ' says Jackson site manager Jonathan Stewart. 'The slope was slipping and to prevent further movement we built two types of shear keys.'

For the fist kind about 30, 2m wide trenches were dug perpendicular to the slope down to the depth of the slip plane, about 3.5m. The trenches varied from 12-18m long.

'We excavated the existing clay and tested it to make sure it was classified as a 2a cohesive material and not too wet to go back into the trench, ' Stewart says. 'It was compacted to disrupt the slip plane and backlled over [to reprole the slope].'

Reinforced concrete provided the second shear key type. Site workers dug about 14, 3m long, 1m wide and 6m deep holes at 5m centres and lowered in reinforcement before lling the holes with concrete.

The site's geology was known to be Gault Clay overlying Folkstone Beds. But the possibility of the odd piece of unexploded ordnance meant piling for the school could not begin without probing.

Mike Cowan, project manager for piling subcontractor May Gurney, says: 'Because Folkestone was heavily bombed during both World Wars and was also shelled from the French coast in the Second, every pile cap position must be probed.'

Foreman Richard Noel adds: 'The probe goes into the centre of the pile cap position and detects in a 1.5m radius.' Although the probes detected metallic objects in two areas, main contractor Wates was able to give the go-ahead to pile in all areas.

Enabling works have not completely removed the problem of the slip plane and there is still the possibility of forces acting on the building's foundations.

'We expect a combination of compression and tension loads on the piles, which are normal, as well as horizontal loads and bending moments, ' Cowan says. 'So we have put in different cages with all sorts of combinations in the piles which range from 6m and 11.5m and found in the Gault Clay.'

Because of the different forces that will act on the foundations, different aspects of their design was divided between two companies.

Buro Happold was responsible for pile design to resist any forces generated by the slip plane while May Gurney handled the building load design element. The consultant speci ed a minimum reinforcement of 1% of the piles' cross sectional area.

'This is unusual as we don't normally split the design responsibility. But Buro Happold wanted part of the responsibility as its geotechnical engineers are specialists in that area, ' Cowan says.

This has meant 60t of reinforcement for the 500, 600mm diameter CFA piles. The weight is accounted for by the almost full length cages in the piles, which can resist up to 700kN compression, 150kN horizontal, 175kN tension loads and 75kNm moments.

'It's very hard to get cages into concrete deeper than about 6m.

They are just about going in ok here using the excavator bucket to push them down, ' Cowan says.

Engineers used two preliminary sacrificial anchor pile tests to confirm designs and site workers are now using five working test piles to confirm installation techniques.

May Gurney's £300,000 piling contract was due to run for eight weeks from 20 February.

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