The new equipment for the High-Luminosity LHC requires new large civil engineering structures on the sites of the ATLAS experiment in Meyrin, Switzerland (LHC Point 1) and the CMS experiment in Cessy, France (LHC Point 5).

On each site, the underground constructions consist of:

- A vertical shaft around 80 m deep and 10 m in diameter

- An underground service cavern (16 m in diameter and 46 m long) that will notably house cryogenics, cooling and ventilation equipment

- A 300-metre-long gallery for accelerator equipment and infrastructures, including power converters, protection systems, electrical distribution boxes, beam instrumentation and accelerator controls

- Four galleries measuring around 50 metres in length, connecting the new structures to the accelerator tunnel. These will house specific hardware, such as radiofrequency equipment for the crab cavities, superconducting links, and cryogenic distribution lines.

The connection to the LHC tunnel will be made via 12 vertical cores (1 m in diameter and 7 m deep), 6 on either side of the Interaction Point (IP), which will be drilled later and completed during Long-Shutdown 3 (2026-2028).

On each site, the surface structure consists of five new buildings, representing a total surface area of 2800 m2. These will house the cooling and ventilation equipment, cryogenic equipment, as well as electrical equipment.

The HL-LHC civil engineering project is based on four main contracts. Two consultancy service contracts are dedicated to the design and to the construction administration: SETEC–CSD–ROCKSOIL (ORIGIN) at Point 1 and LOMBARDI–ARTELIA–PINI (LAP) at Point 5. Two supply contracts are dedicated to the construction of both the underground and surface structures: MARTI TUNNELBAU - MARTI ÖSTERREICH - MARTI DEUTSCHLAND (JVMM) at Point 1 and IMPLENIA SCHWEIZ - BARESEL - IMPLENIA CONSTRUCTION (CIB) at Point 5.

The HL-LHC civil engineering works began in May 2018 and lasted five years. All foreseen underground and surface works were successfully completed in September 2022 at Point 1 and in December 2022 at Point 5.

In total, 92,000m³ of spoil were excavated from the underground structures on both sites. The excavated material was analysed on the surface to check its quality and the spoil polluted by hydrocarbon was managed accordingly. On the Meyrin site, much of the excavated material will be reused to create a platform on which to erect the surface buildings. The rest will be taken to a treatment centre. At Cessy, almost all the excavated material will be taken to inert waste storage facilities located less than 20 km from the worksite, to limit the transport distance. On both sites, the topsoil will be reused for landscaping.

• Road traffic

During the excavation period, lorries will transport the spoil to treatment or storage centres. A maximum of 10 lorries will come and go on the Meyrin site each day, and 10 to 15 on the Cessy site. Spoil will be transported only during working days (Monday to Friday) and, in Meyrin, during off-peak hours (9.30 a.m. to 12.00 p.m. and 1.00 p.m. to 4.30. p.m.).

• Footpaths and cycle paths

In Meyrin, the footpath linking the Maisonnex sports complex to the Chemin de la Berne (north-north-easterly direction) will be closed while the work is taking place; access to the complex will be possible via the Route de Meyrin and the path along the border.

In Cessy, the pedestrian and cycle path around the site (Chemin du Milieu and Chemin de Mouillets) will stay open and will be sheltered from the worksite.

The contracts concluded with the two civil engineering consortia imposed environmental restrictions and, notably, the hiring of experts to perform environmental monitoring on the worksite.

• Noise

The noise generated by the work was limited to respect French and Swiss regulations. An acoustic system monitored noise levels at various times. Measures were taken to limit noise, such as the construction of a temporary building with noise barriers above each shaft to minimise noise pollution from the excavation, or the installation of mufflers on the ventilation systems for the underground work. 

• Light pollution

To limit light pollution at Point 5, it was decided to limit during the nights the worksite lighting to the minimum compatible with the safety of the workers.

• Air

Host State regulations govern the release of dust into the air while the work is taking place. Measures were implemented, such as a wheel-washing system for lorries leaving the sites, vehicle speed restrictions and a sprinkler system in dry weather.

• Water

A worksite water management plan was established to prevent pollution. The excavation areas were protected from rainwater, as far as possible. A water treatment plant was installed on each site to treat all water coming from the worksite before it was discharged. The water quality was monitored.

• Traffic

Road traffic will not be affected. When the work is complete, the footpath in Meyrin linking the Maisonnex sports complex to the Chemin de la Berne will be reopened, with the route modified to pass around the new buildings. The footpath in Cessy will remain unchanged.

• Landscaping

Both sites will be landscaped and planted with around twenty local tree species.

• Noise

An acoustic survey of the future facilities has been performed and forms part of the planning permission requirements. Noise emissions will remain below the legal limits. All the new equipment for the HL-LHC will be installed either in surface buildings with soundproofing or underground.

• Water

In Meyrin, a new water retention tank was installed to regulate the flow of rainwater into the Nant d’Avril stream. A new monitoring station was set up (CERN has 27 water monitoring stations on its sites and the surrounding areas). A similar tank is already in operation in Cessy, regulating the water released into the Oudar. In Cessy, two water tables are located beneath the experiment site. The work will penetrate only the surface water table: a watertight, circular wall will first be installed to a depth of 15 metres to isolate the water table and make it possible to excavate inside it. The work will not penetrate the deeper water table.