Numerical models for the design and construction of new underground structures at CERN (HL-LHC), Point 5

The Large Hadron Collider (LHC) is the latest, most powerful, world’s largest underground particle accelerator realized on the CERN site. High-Luminosity LHC (HL-LHC) is a new project aimed to upgrade the LHC, at Point 1 (ATLAS in Switzerland) and Point 5 (CMS in France) in order to enhance scientific progress. This paper describes the design and construction issues developed at the Point 5 for the new underground structures, located near the existing LHC tunnel. The project requires new technical infrastructure: an additional shaft with a 12 m-diameter and 60 m-height, cavern with 270 m2 cross-section, approximately 500 meters of tunnels connected to the LHC tunnel, vertical linkage cores and additional technical buildings at the surface. The geological ground model of this site lies in an area covered by Quaternary moraine with two independent aquifers. The bedrock of Molasse comprises sub-horizontal lenses of heterogeneous sedimentary rock, that is known to locally retain hydrocarbons and to have a swelling behaviour. In order to investigate the heterogeneous behaviour of the rock mass composed of several layers with different strengths, numerical calculations have been performed, under a 2D plane strain condition with RS2 9.0 FEM-software.. The purpose of using the software was to design both the rock-supports and the concrete inner lining for the tunnels and the shaft. Data from a comprehensive monitoring system with pre-defined threshold values was compared to the 2D FEM results, confirming the importance of the observational method to verify the assumptions used in the numerical modelling. The execution of the underground works started in April 2018. The excavation of the main un-derground works has been successfully completed without any critical impact on the nearby ex-isting underground structures. The completion of the works is scheduled for September 2022.