Civil engineering at CERN

The Large Hadron Collider (LHC)

The LHC is housed in a tunnel of 27km in circumference, buried about 100m underground, and tilted with a gradient of 1.4%, dictated by the geology of the Geneva basin.

Challenging geology

During the last ice age, the Jura mountains that rise to some 1700m above sea level, or 1200m above the city of Geneva, were almost entirely covered in ice. They were sandwiched between mountain ranges thrown up millions of years ago as the Eurasian and African tectonic plates collided.

As a result, the geology is challenging. The sedimentary rock, which is stable and largely impermeable, makes a good tunnelling medium, but the porous limestone karst under the Jura mountains is another story.

The tunnel has an average depth of around 100m crossing the Franco-Swiss border six times, with eight access points, four of which lead to experimental caverns.

How the tunnel was built

Civil engineering for the 27km tunnel got underway in 1983. It was the largest civil engineering project in Europe, a mammoth task, and it was accomplished at a time before there were GPS systems to help.

The tunnel was mapped out on the surface thanks to a series of geodesics (the shortest distance between two points on a sphere) in the form of yellow pillars dotted around the countryside, all connected by line of sight.

The measurements were then transferred underground through a series of boreholes to guide the mammoth tunnelling machines below.

When the breakthrough came and the tunnelling teams met up deep under the Jura mountains on 8 February 1988, the massive ring had been completed to a precision of just 3mm.

Unearthing Roman remains

When works started for new large caverns, civil engineers faced some challenges, but they were rapidly overcome.

At the site of the new cavern for the compact muon solenoid (CMS) experiment (a general-purpose detector), the remains of a fourth century Gallo-Roman villa were discovered in 1998. The site had to be handed over to archaeologists while its secrets were revealed.

“We had big earth movers, and they had toothbrushes,” says CERN senior civil engineer John Osborne. “It was quite interesting to see the difference in excavation.”

Coins minted as far afield as London and Rome were found at the site.

‘Cool’ new techniques

When work resumed, new techniques had to be developed to build the large caverns the LHC required in such difficult terrain.

To excavate the shaft descending to the CMS cavern, the ground had to be frozen first to give it the necessary stability and prevent water leaking in.

CMS required two caverns, one for the detector and another for its services, separated by a 7m-thick wall. In order to do that, the separating wall had to be built before the caverns were excavated.

Across the LHC from CMS is the ATLAS cavern, where a different technique was deployed.

The roof space was excavated first, and then the roof constructed and suspended by cables from the overlying rock.

That gave the stability for the rest of the cavern to be excavated and built, before de-tensioning the cables so the roof could be supported by the walls.

No caverns of this scale had ever been constructed in rock of this nature, and the civil engineers had to be innovative.

Tunnelling to the future

CERN is currently studying the possibility of building the Future Circular Collider (FCC).

This proposed project would constitute one of the largest tunnelling projects ever undertaken.

With a circumference of 90.7km, it would weave through the molasse and limestone beneath Lake Geneva and around Mont Salève.

It would constitute the largest tunnel ever constructed at CERN and be considered a major global civil engineering project in its own right.

Other projects under study include the 91km Future Circular Collider (FCC) at CERN, Muon Collider, Compact Linear Collider (CLIC), the International Linear Collider (ILC) and the Einstein Telescope.

How would the FCC tunnel be built?

Should the FCC be approved, civil engineering will be the first major onsite activity to take place.

The FCC feasibility study schedules ground-breaking for the first shafts to begin in 2033. After that, it would take between six and eight years for each underground sector to become available for the installation of the technical infrastructure, the machine and the experiments.

The tunnel would be constructed using up to eight tunnel boring machines, which are able to excavate and install the tunnel lining in a single-pass operation.

Desktop studies show that the geology would be favourable, since the molasse rock is usually watertight and can be easily supported.

The main beam tunnel will, however, need to pass through about 9km of limestone, which may require the drill-and-blast method.

These geological assumptions need to be confirmed via a major ongoing, in-situ site investigation campaign planned to finish by the end of 2025.

Sustainability

With a total of around 15 million tonnes of rock and soil to be excavated, sustainability is a major focus of the FCC civil engineering studies.

To this end, in the framework of the European Union co-funded FCC Innovation Study, CERN launched an international challenge-based competition, Mining the Future, in 2021. The aim is to identify credible and innovative ways to reuse the molasse.

In addition, the FCC feasibility study is working towards a full assessment to minimise the carbon footprint during construction.