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Eden prepares for visitors

THOUSANDS OF visitors have been flocking to the Eden Project in Cornwall since the futuristic scheme began taking shape last May, even though it is not due to open until next month.

Work began in early 1999, when 800,000m3 of spoil from the edge of the site, a 60m deep disused China Clay pit near St Austell, was moved into its centre.This was used to form a level surface for the two massive geodesic biomes that will house more than 10,000 exotic plant species, and create one of the world's leading botanical gardens (Ground Engineering January 2000).

The £80M project is being built by a McAlpine joint venture.Consultant The John Grimes Partnership carried out the geotechnical assessment and designed the slope stabilisation of the quarry faces and the foundation ring beam for the biomes.

Site geology is kaolinised phenocrystic biotite granite, varying from fresh to completely kaolinised, and is intersected by a large number of tourmaline veins.

Contractor Saxton Deep Drillers carried out the geotechnical work to stabilise the quarry faces using more than 2000 Dywidag rock bolts and soil nails. Where the ground was harder, the firm installed conventional GewiSteel rock bolts using open-hole drilling.

A mixture of bolting and nailing was carried out using self-drilling MAI Hollow Bar soil nails in the softer kaolinised granite. The system enables simultaneous drilling and grouting, ensuring placement of grout to the full depth of the borehole in loose or collapsing soils.

Sprayed concrete was also used on unstable slopes, tied back by rock bolts.

All the rock bolts were proof tested and then locked-off at the required working load.Access to the rock bolts on the higher faces for stressing and testing was carried out using abseiling and staged access.

The foundation ring beam was built on up to 20m of fill in various stages of settlement.To alleviate any potential for differential settlement, Saxton installed permanent double corrosion protection ground anchors through the ring beam.

Holes were cased through the fill and pressure grouting was also needed at a few locations to develop sufficient bond stress over the anchor's fixed length. Once installed, the anchors were progressively stressed in series to induce primary settlement in the ring beam.

Main construction finished at the end of last year and the site is now closed to the public while planting and finishing touches to the attractions are carried out. The centre will open in mid-March.

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