ABSTRACT
At the European Laboratory for Particle Physics (CERN), ageing tunnels are exhibiting tunnel lining distress and critical cracks many years after construction, raising the need for more maintenance activities and structural health monitoring. However, routine inspections of CERN facilities have inherent limitations due to accessibility, which is restricted by the radioactive environment.
This study presents the observed crack behaviour experienced by a concrete-lined tunnel section within the Super Proton Synchrotron (SPS) ring, which was constructed in the ‘70s and located 40 m deep in the molasse rock. The tunnel lining showed a critical mechanism of deformation: spalling at the tunnel crown and tensile cracks at the tunnel shoulder. To ensure safe running of CERN accelerators and the serviceability of CERN underground facilities, an emergency action plan was put in place. This consisted of on-site investigations and the implementation of both conventional and innovative monitoring techniques, enabling CERN engineers to validate the assumptions and plan mitigation measures. Results from geodetic measurements show that the tunnel is moving with time, suggesting a vertical tunnel elongation. Tunnel core drilling provided key information of the tunnel construction and the surrounding rock formation, enabling better planning for mitigation strategies and long-term structural health monitoring.
