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Rock art

Geotechnics - Rock engineering

A giant cavern 'sculpture' to be hewn from a mountain in the Canary Islands will push rock engineering to its limits. Paul Wheeler reports.

Engineers responsible for turning late sculptor Eduardo Chillida's audacious proposal for giant cubic-shaped cavern artwork into reality face one problem in particular.

The cuboid space, hollowed from a hill on the Spanish island of Fuerteventura, will be 65m long by 50m wide and 45m high. So the challenge could reasonably be expected to be preventing wholesale collapse of the void's perfectly flat roof. In fact, the engineers' main concern is to prevent tiny wedges of rock falling.

A recently-completed feasibility study led by consultant Arup indicates it is technically possible to create the unique structure within Mount Tindaya in the Canaries. Not only will it be one of the largest unsupported rockspans in the world but, unlike any existing large-span rock cavern, Chillida's cuboid will have a flat roof rather than a structurally more efficient arch. It will be pierced by two light wells, providing a view of the sun and stars from inside the artwork. In addition its walls will be absolutely vertical.

Even more unusually, Chillida's concept was for the cut rock faces to be smooth with no visible signs of support. The desired finish is close to the appearance of a clad building.

Put simply, this is rock engineering beyond anything attempted before.

Much of the work of Chillida, one of Spain's most celebrated artists, has explored perceptions and definitions of space and scale.

The cavern is unquestionably his ultimate work.

Chillida conceived the project a long time before he died in 2002, and spent a great deal of time searching for the 'perfect' mountain for his proposed work.

Arup became involved in the scheme about five years ago.

It is now being driven by Madridbased architect Lorenzo Fernandez Ordonez, whose structural engineer father was a close friend of Chillida. Ordonez senior has now also died, but it was he who brought Arup on board.

Mount Tindaya is made of trachyte, an extrusive igneous rock, which is typically lightcoloured and has a porphyritic texture with abundant, large, well-formed crystals.

Arup associate and project leader Richard Bickers says Ground characterisation to date is based on surface mapping, geophysical investigations and remote sensing. Information has been put into a GIS model, enabling interpretation in three dimensions of important geological features on the mountain, notably dykes, ahead of the borehole investigation.

the project team has already discounted on philosophical grounds the idea of constructing an arch and then filling it in to create the flat roof. 'It's about the integrity of the art. We want to create a structure that is not lying.'

Fortunately the team does not consider countersunk rock bolts to be an artistic compromise.

Surface mapping indicates the trachyte is 'massive', meaning it is relatively unbroken by joints and bedding, and should be a very good material to work with. It is, however, cross cut by sheet-like igneous intrusions known as dykes, which can be traced over the surface of the mountain. The condition of these within the mountain will determine the complexity of the project.

Some structural analysis is being carried out by Scott Wilson Piesold, brought into the project team by Arup because of its cavern engineering experience, most notably the CERN super collidor cavern near Geneva. The project is complemented by leading international rock mechanics experts professors Evert Hoek, Alcibiades Serrano and Claudio Olalla Marano.

Tony Miller, Scott Wilson's technical director for tunnels and dams, explains that because the cavern is located at the top of the mountain, it is in a low stress environment. So although the cavern is large, excavation will not create massively high stresses in the surrounding rock.

Miller says, 'Numerical modelling to date shows stresses resulting from the excavation are manageable and that the unusual shape can be made to work under a range of assumed rock and joint conditions. The excavation won't require a great leap in terms of rock support systems.'

The Spanish government wanted proof that the project was technically feasible before allowing any intrusive site investigation into the mountain. With this task complete a 14-borehole site investigation is now imminent.

Miller says that, whatever this reveals, 'we have an increasingly complicated scenario for the support - but we feel confident we can engineer a solution'. His main concern lies with public safety issues - namely, how to stop small wedges of rock falling. It may well come down to dentistry, in which blocks are individually pinned into place, he says.

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