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The greenhouse effect

Cover story; Eden project

A'wow factor' was included in the design brief for the £75M Eden project, and on first sight it does evoke this response. Nestled into the corner of a china clay crater in the unremarkable landscape around St Austel, the project's two 'biome' greenhouses look like a 1950's science fiction comic book vision of a moonbase station.

Much of the technology on the project has a 21st century bias. The biomes are glazed with inflated cushions of ethyltetraflouroethylene (ETFE), an extremely thin (50-200mm) film of extruded copolymer, which resembles and handles like transparent polythene, but is considerably stronger. According to Alan Jones, project director for structural engineers Anthony Hunt, ETFE is an ideal glazing material for the project, for many reasons. 'ETFE has allowed us to use glazing spans of up to 11m, giving an average glazing load of 15 kg/m2, which is a fifth of the weight of glass,' he says.

The cushions of ETFE are plumbed together in groups of six and kept inflated to 250 Pa with dried air. In the unlikely event of heavy snowfalls in Cornwall, the cushions can be pumped up to 400 Pa to resist snow loading. Each cushion consists of up to four layers of foil, which are welded together, with air permeable inner layers to allow pressure equalisation within the cushion.

'We were limited to a maximum foil thickness of 200mm,' says Hunt. 'Thicker foils suffer from embrittlement caused by the welding process.' The cushions are held in extruded aluminium frames using a luff and boltrope detail, derived from sailing and fabric structure technology. Although the foil is susceptible to punctures by bird or debris strike it can be patched by abseilers using ETFE tape.

Providing the largest glazing panels possible was also important to the horticulturists in order to allow maximum light into the biomes. 'ETFE transmits very high levels of visible light (up to 97%) and can be more thermally efficient when formed into cushions than conventional triple glazing,' says Hunt. 'The material also has high durability under UV - ETFE foil has been in place at Burgers Zoo in Holland for over 20 years.'

Large glazing spans were one of the reasons for locating the biomes in a sheltered quarry site. 'Normal wind loads in this area would have required significantly smaller cushions,' says Hunt. In fact, wind tunnel testing on models has revealed uplift to be the biggest windload, and the foundation anchors have been designed as tie-downs to stop the biomes taking off.

The Nicholas Grimshaw-designed project certainly has space. The greenhouses are the largest ever built, the biggest biome is 55m high, four times higher than the palm house at Kew. The greenhouses cover 22 hectares - the area of 29 football pitches - and have been designed to tell the story of man's relationship to plants to an expected 500,000 visitors per year.

Tourists who make the stop on the long drive down to west Cornwall's beaches will be able to visit two environments, humid tropics and warm temperate, each housed in its own biome, along with an extensive outdoor garden of cool temperate plants.

Each biome comprises four connected geodesic domes; each dome formed from a double layer galvanised steel spaceframe. The outer layer of the spaceframe is a true Buckminster Fuller dome composed of hexagons, while the semi-braced inner layer is formed from a combination of hexagons and triangular elements. Design, fabrication and erection of members has been carried out by specialist German contractor Mero, using elements of its proprietary spaceframe system. The geodesic domes are linked within each biome with a prefabricated trussed arch.

Erection of both biomes is nearing completion and glazing is well under way, with the humid tropics biome 25% glazed. The erection process was eased considerably by the bolt-together nature of the Mero system, with only 200 site welds required for the trussed arches. 'Our biggest task during erection has been the scaffolding,' says McAlpine Joint Venture Project Manager Barry Johnson. Made up of 46,000 poles weighing 1300t, the scaffold may qualify as the largest birdcage scaffolding structure in the world.

'It remains in place long after we need it because it takes so long to strike,' says Johnson. 'With hindsight, the design of the scaffold could have been done differently.'

China clay was extracted right up to the day before construction started. 'Our 3D ground model had to evolve as the pit contours changed,' says Hunt. What remains is a decomposed granite fill of varying quality. Triaxial testing has shown that the bulk of this material exhibits swelling behaviour.

Around 80,000m3 of fill had to be moved and compacted to form the final profile of the pit. Low permeability of the fill and high rainfall in the first few months on site created a quagmire. 'We were pumping up to our limit of 150 litres/sec,' says Johnson, 'but work was delayed for months'. A system of drainage channels has since been created to slow down and store runoff from the site, discharging an average 11 litres/ sec into surrounding watercourse, the rest being recycled.

The biome foundations straddle between embankments of fill and competent granite at the pit walls, and the strip footing and spaceframe structure will allow up to 15 mm of differential settle and competent granite at the pit walls, and the strip footing and spaceframe structure will allow up to 15 mm of differential settlement between adjacent node points. So far, this seems generous.

'We were anticipating problems with consolidation,' says Johnson, 'and programmed 60 days in for the embankments to consolidate'. Ground anchors were used to simulate a 20m overburden and there were fears about swelling, but horizontal inclinometers in the embankments revealed that the required consolidation was actually achieved in just over 40 days, and the anchors were released.

Other major works at the site have included a new 3.6km access road and cyclepath flanked by Cornish boulder hedges. The road construction, on a greenfield site, has created 15 acres of new wetland habitat. Paved parking areas for 2,000 cars have been provided on terraces, along with hydroseeded areas for overspill.

Before planting can begin, 85,00m3 of topsoil has to be manufactured, using fill from the site mixed with organic matter, such as mushroom compost. This will be landscaped by Land Use Consultants and planted to produce a number of habitats within each biome. Visitors from Earth will be allowed inside at Easter 2001.

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