CERN (HL-LHC): the construction experience for the underground works at Point 5

High-Luminosity LHC (HL-LHC) is the new project aiming to upgrade the underground particle accelerator Large Hadron Collider (LHC), at Point 1 (ATLAS in Switzerland) and Point 5 (CMS in France). This paper describes the construction experience for the new underground works at Point 5, located near the existing LHC tunnel. From a geological point of view, the site lies in an area covered by Quaternary moraine with the presence of two independent aquifers. The outcrop consists of Molasse, sub-horizontal lenses of heterogeneous sedimentary rock, known in this area to locally retain hydrocarbons and to have potential swelling behaviour. The main challenges of the project were related to the limitation of vibrations and settlements induced from the excavation activities, which might degrade the operation of the in-use LHC machines. This paper presents the local geology, the monitoring data recorded and the optimizations performed during the excavations together with the contractual management aspects. The completion of the works is scheduled for September 2022.