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Paradise regained

MILLENNIUM PROJECTS

Extensive use of geosynthetics allows rapid construction of Cornish hothouses.

When it opens in spring 2001, the Eden project in Cornwall will house more than 10,000 exotic plant species and, it is hoped, become one of the world's leading botanical gardens.

The £77M enterprise is funded by a £37M Millennium Commission grant and money from the European Regional Development Fund and English Partnerships.

A charitable trust, the Eden Project, was formed to develop the scheme which involves building two domed hothouses and redeveloping a disused China Clay pit near St Austell in Cornwall.

The hothouses or 'biomes' will display hot tropical and warm temperate plants in walk-through and apparently natural ecosystems. The larger hothouse is composed of five intersecting geodesic domes while the smaller consists of four domes. These are made up of steel frames with ultra-lightweight ethylene tetrafluoroethylene panels.

Consultant Anthony Hunt Associates and architect Nicholas Grimshaw & Partners developed the concept for the lightweight geodesic structures to meet the horticulturists' demand for column-free open space on a giant scale.

The two hothouses, joined by a cedar shingle-clad and earth roofed 'link building', will cover 1.5ha and 0.7ha respectively, without any need for internal support. German specialist subcontractor Mero carried out the detailed design of the hothouses and is building the steel work which is expected to be finished next month.

To ensure they get enough natural sunlight as possible, the hothouses are being built part way up the steep sloping northern side of the quarry. The main contractor, a joint venture between Sir Robert McAlpine and Alfred McAlpine, had to construct a bund between 14m and 20m high on the clay pit floor on the structure's southern edge before the 890m long continuous weaving ground beam could be installed.

Because the project is due to open early next year, rapid consolidation of the earthworks was essential to ensure construction remained on schedule. But the bund material is variable clay fill from the quarry, so to ensure consolidation occurred evenly and in the shortest possible time, geosynthetics had to be incorporated into earthworks design.

More than 170,000m2 of Colbond Geosynthetics Enkadrain Wide supplied by MMG Civil Engineering Systems was installed at 1m intervals throughout the 95% compacted clay fill and was also used between the fill and the quarry face to provide a permanent near vertical drainage layer to prevent a build-up of hydrostatic pressure and ensure long term stability.

Of the larger hothouse's 550m long foundation, only 100m is being built on granite bedrock. The rest, and all of the 340m foundation of the smaller dome, sits on soft fill reclaimed from the quarry.

Installed by contractor Dean & Dyball, the ground beam is 2m wide and 1.4m thick and rests on a 2m wide base slab on top of the bund, anchored at between 2.5m and 5m centres with 10m long ground anchors. These were tensioned to between 200kN and 300kN and will prevent movements during construction, becoming all but redundant once the domes are built. Steel dowels are used where the beam passes over competent granite.

In all, the larger of the hothouses touches down on 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 contouring of the ground means that forces are different throughout the length of the beam. To simplify design, the beam is a constant width and thickness, even though this meant it was larger than necessary for some of the smaller domes.

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