Distributed fiber optic sensing in harsh geotechnical environment like tunnels implies essential mechanical impacts for optical glass fibers, which must be reliably protected during installation and monitoring. In the presented application, pre. Distributed fiber optic sensing in harsh geotechnical environment like tunnels implies essential mechanical impacts for optical glass fibers, which must be reliably protected during installation and monitoring. In the presented application, pre-fabricated sensing cables from SOLIFOS AG (Switzerland) were used, which are specially designed for sensi. The installation of the sensing cables along the shotcrete lining was different compared to previous installations [12, 21] since the tunnel excavation in the area of interest was already performed and the initial shotcrete layers were already applied. For that reason, small stripes of welded wire mesh were mounted to the existing tunnel lining, wh. All installed sensing cables have been interrogated using the fTB 5020 from fibris Terre Systems GmbH (Germany). Based on the Brillouin optical frequency domain analysis (BOFDA) technique, this sensing unit enables distributed measurements up to 25 kilometers within a measurement time of several minutes, a spatial resolution of 0.5 m and a strain r. The instrumentation of the total DFOS network along both tunnel tubes (Fig. 5) was an ongoing process, carried out between December 2019 and November 2021, and strongly related to the construction work on-site. Initially, the shotcrete lining of the northern tunnel tube (cf. Fig. 1) was equipped with two continuous sensing cables (one for strain an.