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The Eden Project, Cornwall

Client: The Eden Project

The Eden Project is a unique redevelopment of a Cornish China Clay Pit into possibly the world’s largest greenhouse. The project took 2½ years to construct and opened to the public on 17 March 2001.

JGP were the Geotechnical Engineers for the project providing the geotechnical investigations, supervision of the on-site laboratory testing / monitoring and Resident Engineer duties with regard to the supervision and management of on-site geotechnical works.  JGP also designed the structural embankments, foundations, slope stabilisation, earthworks and surface water drainage routing.

The site was formerly the Bodelva Pit, which was due for decommissioning when JGP first became involved in the project in 1996.  JGP carried out the preliminary site investigations whilst the quarry was still operational, to inform the viability of reclaiming the site.

The Bodelva pit, which lies within the eastern end of the St Austell granite mass, had been worked for over a century.  The pit covered an area of about 22 hectares and varied in depth from 30 to 70 metres.  The geology comprises variably kaolinised, phenocrystic, biotite granite, which was emplaced during the Variscan Orogeny some 285 million years ago at the end of the Permian.

The ground investigation involved trial pitting and the drilling of both shell and auger boreholes and open-hole rotary percussive boreholes in order to ascertain the characteristics of the both existing fill material and the underlying granite.  Standpipes and pneumatic piezometers were installed to assess the groundwater regime.  Ground probing radar traverses were conducted around selected proposed foundations.

The granite was classified into four different kaolinisation grades, from Grade 1 (unaltered granite) through to Grade IV (where the feldspar had completely altered to a plastic mass and the material can be moulded by hand). Samples of the Grade II, III and IV granites were chemically analysed (for aluminium, sodium, calcium, magnesium and potassium oxide content), underwent physical testing (for index and particle size properties) and Scanning Electronic Microscopic analysis, which were all used to determine the engineering behaviour of the various granite grades and their potential for swelling under loads imposed by the redevelopment.

Work commenced on site in November 1998.  The original pit base level was to be raised by up to 17m by filling with site won material, which meant that the new biomes and other settlement sensitive structures would straddle the junction between the backfilled and cut areas of the pit.  An on-site laboratory was established to enable rapid grading of the fill material available on site into ‘acceptable’ fill (for use in structural embankments and roads) and ‘general’ fill (for areas of soft landscaping).  Approximately 800,000m3 of material were moved to create the final profiles around the whole site.

The structural embankments were constructed first to support the biome foundations in areas of the pit base being backfilled.  Band drainage was installed at 3.5m to 4m centres beneath the structural embankment areas to accelerate ground consolidation.  Once consolidation had reached an acceptable level, the biome foundation beams were constructed.  Ground anchors were installed through the foundation beam into the fill, so that the foundation could be preloaded before erection of the biome commenced, which removed any remaining short-term settlements within the embankments.

Slope stabilisation and re-profiling of the pit walls were engineered using a variety of techniques depending on the grade of the underlying granite and the steepness of the slope, from rock bolting, sprayed concrete retaining structures to re-profiling with hydroseeding to bind the surface.

The project won the Institution of Civil Engineers’ Merit Award 2002 for “An outstanding example of Civil Engineering Project completed in the South West Region in 2001”.  The design team that worked on the project included:

  • Civil & Structural Engineer:              Anthony Hunt Associates
  • Geotechnical Engineers:                    John Grimes Partnership Ltd.
  • Architect:                                                  Nicholas Grimshaw & Partners
  • Landscape Architects:                        Land Use Consultants
  • Mechanical & Electrical Engineers:    Ove Arup & Partners
  • Cost Consultant:                                  Davis Langdon & Everest
  • Project Manager:                                 Davis Langdon Management
  • Constructor:                                          McAlpine Joint Venture

Since 2001, JGP have continued to work with The Eden Project providing foundation advice for various new structures, such as their zip line and aerial walkway, as well as slope stabilisation monitoring and advice.