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Pushing back the boundaries of particle physics

The hunt for the most elusive bits of matter in the universe uses the biggest instruments imaginable - particle collision detectors - huge loops of wires, screens, inert gases, glass, magnets, cables and computers, built in giant underground caverns.

Two such caverns are currently under construction below ground at the CERN physics centre. Sprayed concrete is in use as tunnellers gingerly excavate the soft, variable rock.

Caverns housing two particle collision detectors are part of a complete restructuring of the 26.7km ring accelerator at the multinational CERN physics facility. The existing electron-positron accelerator, which sits in a 100m deep circular tunnel straddling the Swiss-French border, is being replaced with the Large Hadron Collider.

While the magnets, and controls for the new, more powerful accelerator fit into the existing 3.5m diameter tunnel space, the new Atlas detector and an even bigger Compact Muon Solenoid, need more space at key points around the tunnel. They also require large shafts to surface and above ground facilities.

According to Steven Heard of consultant Knight Piesold, which is working on the project with the design section of France's electricity utility EDF, the design analysis needed for the Atlas chamber and shafts would have been virtually impossible a few years ago.

'The software before simply was not good enough,' he says. Even now, he adds, the three dimensional programme used for the design work, FLAC 3D, has been 'pushed to the limit' on the huge 30m span, 35m high and 50m long chamber now under construction.

Creating a space of this size in fairly soft layered rock is hard enough but the cavern is also intersected at right angles by a second 62m long cavern with a 20m span and 13m height.

The work is the first of three phases, this first worth pounds30M. In total some pounds150M will be spent on civil works, says Tim Watson, head of CERN's civil engineering group.

For the consulting team on the pounds48M contract 2, a group led by Gibb, Switzerland's SGI Ingnieure SA and GeoConsult of Austria, there are also major geometrical complexities, explains project director Pip Squires.

'We are building two parallel tunnel shape chambers, close enough that they act as a system.' Whereas the first contract has only a few metres of waterlogged glacial moraine above them, he adds, here there is 50m with just 20m of soft rock cover to the caverns.

These chambers span 26.5m and 20m, the bigger on the tunnel axis being 50m long and 35m high to house the CMS. The second houses scientists and the huge banks of computers and equipment.

The third contract is designed and supervised by a joint venture of Brown & Root Projects, Hidrotecnica Portuguesa and Intecsa from Spain. It is worth just under


'This comprises a series of works scattered across the ring area,' explains project manager Mike Lepper, 'including a number of smaller chambers and a 2.8km length of tunnel.'

Construction has begun on all three contracts, though only the Atlas project is beyond initial site preparation. Excavation in the main will be by a form of NATM with roadheaders, and spoil will be removed by truck into France.

During the period when the existing particle accelerator is still operating, monitoring will be crucial both for the NATM and to ensure excessive rock movement is restrained. The accelerator can be adjusted, but disturbance of the electron beam at nearly the speed of light is not wanted.

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