At the European Centre for Nuclear Research (CERN), the well-known deep particle accelerator is hosted in a circular underground facility excavated in the weak sedimentary rock mass, in Geneva basin. Aged concrete tunnels commonly exhibit tunnel lining deterioration with time, enhancing long-term tunnel distress and, hence, compromising the performance of physical experiments. Recent changes in the ground water flow regime around the tunnel have brought challenges to the operation of an existing horseshoe concrete-lined tunnel housed within the underground facility at CERN. Due to extreme weather conditions, large amounts of water flowed towards the tunnel, resulting in the long-term hydraulic deterioration of the drainage system, such as the clogging of the drains. As a result, some tunnel cross-sections are experiencing a vertical tunnel elongation as a mechanism of deformation. This paper presents the investigation carried out for CERN TT10 tunnel that underwent damage many years after construction. Field monitoring instrumentation has been conducted to assess the tunnel lining behavior, using both conventional total stations data and advanced distributed fibre optic strain sensing (DFOS). The observational data results show that the tunnel lining experiences compressive stresses at the tunnel crown and tension cracks at the tunnel shoulder, whereas the tunnel floor exhibits heaving. Furthermore, more significant changes in tunnel diameter were observed for certain tunnel sections where the very-weak swelling-potential marl layer is encountered.