This paper introduces the successful implementation of a fibre-optic sensing system for direct and distributed strain measurement within the shotcrete lining of a conventional tunnel drive. The shotcrete lining of the top-heading and the invert are equipped with two layers (rock side and cavity side) of fibre-optic sensing cables installed in circumferential and longitudinal direction. All cables are measured autonomously for several weeks to capture the strain evolution inside the lining from the day of the construction to a posterior, well-hardened state. An additional follow-up measurement is conducted 2 months afterwards. The measurements enable an assessment of the strain distribution inside the lining with a spatial resolution in the range of some centimetres and a measurement resolution of up to 1 µm/m. Besides the conventional monitoring targets used for displacement recordings, measurement equipment like strain gages and pressure cells are also installed in the crosssection under investigation. Back-calculated strain from absolute displacements and the readings from the strain gages show good agreement with the results of the conducted fibre-optic measurements and verify the suitability of the used system.

In contract SBT1.1 of the Semmering Base Tunnel, the single‐track main tunnels are being driven from the portal in Gloggnitz and from a temporary intermediate starting point in Göstritz. The temporary intermediate starting point in Göstritz is being constructed in order to reduce the risk to the tunnel construction time for the Graßberg‐Schlagl fault system. During the construction phase, the contractor developed optimisations of the planned logistics concept for the intermediate access point in Göstritz, which led to a redesign of the shaft head cavern system. Shortly before the excavation of the shaft head cavern, an unexpected and geotechnically relevant fault zone was encountered in the access tunnel. In order to investigate the geological situation in the area of the shaft head, extensive additional investigation measures were carried out. Due to the updated geological forecast, the layout of the shaft head cavern and the support concept had to be changed. Thanks to the measures taken, the shaft head cavern system cold be successfully excavate without any damage to the support.

One of the major fault zones in the Semmering area runs along the Auebach Valley. In this fault zone a combination of soft fault rocks and water bearing hard rocks led to extremely challenging conditions in the design and execution of the tunnel. In difficult conditions, overlapping exploratory drillings in front of tunnel drive are essential to improve the geological‐hydrogeological model and to assess geotechnical risks. However, in the encountered geological‐hydrogeological conditions, the well‐established exploratory drilling methods reached their limits. Despite the ongoing risk analysis, a very large‐scale water and ground inflow with subsequent tunnel face collapse and development of a sinkhole on the surface occurred during the tunnelling. This paper describes the causes of the incident and the development of procedures to cope with the problem. In addition to the geological‐geotechnical analyses, the article also discusses the limitations of geotechnical surveillance in complex heterogeneous rock masses.