The 64 km long Brenner Base Tunnel will be the longest railway tunnel in the world when complete. As part of this mega project, an exploratory tunnel is currently under construction with a tunnel boring machine (∅ 8 m). Even though a geological model along the tunnel route was generated beforehand from the results of geological field mapping and deep drilling campaigns from the surface, some uncertainties persist due to the high overburden of up to 1,300 m. In particular, several fault systems intersect the tunnel route, but their position and orientation is uncertain. The excavation work is being documented with geological and geotechnical data, continuously updating the current geological model. Continuous seismic prediction measurements are integrated into this revision process. The aim is the detection, location and characterization of (sub‐)perpendicular to (sub‐)parallel fault zones ahead and to the side of the tunnel, based on the principle of body wave propagation. In a case study of a seismic campaign, the smooth integration of the acquisition into the regular advance is presented and the obtained 3D seismic models are compared to the geological documentation of the site geologists. It will be shown how the seismic analysis gives an added value for the advance and how it benefits the geological forecast for the forthcoming main tunnels.

Approx. 42 km of tunnel were excavated on the contract Tulfes‐Pfons H33, including a 15 km long section of the exploratory tunnel, which has been excavated since October 2015 with a gripper TBM from the Ahrental access tunnel southward into the district of Steinach. The interaction of the tunnel boring machine with the rock mass is discussed here as a back analysis of the mechanized tunnelling drive. This article does compare the forecast with the actual geological/geotechnical conditions, but only considers the conditions actually encountered. The back analysis takes place analogously to the principle of geotechnical planning according to the ÖGG (Austrian Society of Geomechanics) guideline, in which, starting from the rock mass structure and the rock mass types, the encountered tunnel face and intrados behaviour are observed and an estimation of the system behaviour is carried out. A description of the tunnelling technology concept completes this paper together with two case studies and the lessons learned.

The Brenner Base Tunnel (BBT) is a straight, flat railway tunnel between Austria and Italy. It runs from Innsbruck to Fortezza (55 km), crossing the main Alpine crest with an overburden up to 1.7 km. Including the connection to the line around Innsbruck, which has already been built and which is the endpoint for the BBT, the total length of the tunnel will be about 64 km. Once finished, the BBT will be the longest underground rail link in the world. A peculiar feature of the BBT is the exploratory tunnel running from one end to the other. This tunnel lies between the two main tunnels and about 12 m below them and is noticeably smaller than the main tubes. So far, a total of 60 km of tunnels have already been excavated in Austria and Italy (access tunnels, exploratory tunnel, main tubes and chambers), driven both by blasting and by TBM. Crossing fault zones is a geological and geomechanical challenge, both for TBM and conventional excavation methods. The Periadriatic fault zone, with a total length of 1 km, has already been driven through. This first step of the project provides an interesting look at the comparison of the predicted rock mass conditions with those that were actually encountered, the rock mass behaviour in fault zones and investigation measures ahead of the tunnel face.