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Putting the serpent into Eden

Building a snaking continuous reinforced concrete beam is the latest challenge in the transformation of an abandoned quarry to a Cornish botanical nirvana. Andrew Mylius visited St Austell to see work in progress.

Contractors on the vast £57M Eden botanical garden project near St Austell in Cornwall are nearing completion of a 890m long continous foundation beam designed to support a series of domed greenhouses. The domes will form the focal point of the project when it opens in 2001 and will rest on the serpentine reinforced concrete beam which will weave around the greenhouse footprint.

The client, Eden Project, is a charitable trust formed to develop the project using a mixture of Millennium Commission funding and grants from the European Regional Development Fund and English Partnerships. The project involves building two domed hothouse areas in a disused China Clay pit. It comes with some unusual challenges for foundations engineers.

'Kaolinisation' of the granite bed rock in the area around St Austell has decayed much of it into a widely varied mix of clay and other soft materials. Only two bands of competent rock have been found on site, one at each end of the area where the greenhouses will stand.

Of the larger hothouse's 550m long perimeter foundation beam only 100m is being built on bed rock. The 340m perimeter of the smaller hothouse is mainly on fill and soft material.

To complicate things further, the hothouses are located part way up the steep sloping northern side of the quarry to ensure they get as much natural sunlight as possible. As a result the main contractor, a joint venture between Sir Robert McAlpine and Alfred McAlpine, has had to construct an embankment on the clay pit floor to raise the foot of the southern edge of the hothouse area before casting the foundation beam on top of it. The embankment is made of compacted earth, built on soft, relatively unstable ground and is between 14m and 20m deep. It will be landscaped to match the contours of the site when the foundation beam is complete.

Work on the embankment began in June with contractors using quarry spoil from other parts of the site and compacting it with vibrating compactors to Department of Transport load bearing standards. Foundations subcontractor Dean & Dyball then laid a 2m wide concrete base for the foundation beam atop the embankment. To stabilise it, 10m anchors at 2.5m-5m centres have been pushed through the base into injected epoxy 'plugs' to counter uplift and relaxation during construction. The anchors have been tensioned to between 200kN and 300kN. They will be left in-situ but will become all but redundant once the steel framed geodesic structures are in place.

Within the embankment full-depth vertical drains have been installed to remove rainwater and ground water squeezed from the 95% compacted soils. Water is pumped from the drains into a grey water collection pond which has been made into a feature on site. Eden Project design and development director Ronnie Murning says that during heavy rains last winter up to 40 litres/ second were being discharged.

Alan Jones, project director for Eden Project's consultant Anthony Hunt says that for the foundation beam to perform competently it had to be treated as a continuous entity. Its curvilinear geometry gives it stability against rotating moments, and must also perform under torsion, bending and shear.

The larger hothouse touches the foundation at 110 points and the smaller at 77 points; there are also four mountings for the structures' main arches. The complexity of the structures and site contours of the ground means that forces' vectors are different throughout the length of the beam. The ground swoops up and down slopes of as much as 60degrees, making for dramatic variations in forces transmitted.

For simplicity, the consultant kept the beam a consistent 2m wide by 1.4m deep. As a result it is larger than absolutely neccessary for supporting the hothouses' smaller domes, says Anthony Hunt senior designer Mike Purvis. 'Originally we designed weird and wonderful shapes, but it's difficult enough to build on Eden's terrain anyway.'

To prevent differential cracking during curing, concrete pours for the beam are being carried out in 10m sections and should be complete by the end of this month. There are five 'transition zones' - deliberately under- reinforced areas where top and bottom bars cross over - to allow controlled cracking and differential settlement where the beam crosses from one grade of foundation material onto another. No expansion joints are necessary. After completion the beam will be buried. 'The ground is a pretty good heat sink,' Purvis says.

Roughly half the beam snakes up and across the clay pit's northern slope. Where this section crosses competent granite, steel dowels are being used to key it into the rock. Where the beam passes through softer grade material, anchors are being installed to increase stabilty.

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