The Brenner Base tunnel: Geological issues from the exploratory tunnel to the final design

This paper deals with the geological issues involved in the Brenner Base Tunnel Project, which crosses one of the main tectonic alignments of the alpine area, namely the Periadriatic Fault. The paper first describes the tunnel system, the geological setting in which it develops, and the related excavation support and monitoring systems. Then, the geological and geotechnical investigations are described, pointing out the differences in the results obtained from the different scales of investigations: from laboratory tests, to drillings and ending with the exploratory tunnel. All these data are discussed in order to define the ground-tunnel conceptual model, afterwards modelled with a 3D finite element numerical code. Finally, numerical results and monitoring data are discussed especially with regard to the scale effect, showing that for the Brenner Base Tunnel the amplifications of the studied phenomena (deformations, instabilities, water inflows, etc.) due to the enlargement are smaller than expected. Such a result is quite different compared to the evidence obtained in previous studies on the scale effect in other tunnels. It may be justified by the high compressive strength acting along the Periadriatic fault, which brings about high levels of confinement stress.