ABSTRACT
The Brenner Base Tunnel (BBT) is a double-tube, high-speed railway line currently under construction between Italy and Austria across the Alpine chain divide. The underground section is 55 km long, is composed of 230 km total tunnels and has a maximum overburden of 1.800m. As from design programs, the realization of main tubes (MT) was continuously preceded by the excavation of an exploration/drainage tunnel (EDT) 27 km long. On the Italian side, the EDT construction is now completed, as well as large portions of MT. The local geological context consists of superposed metamorphic tectonic units having strongly variable lithology, including gneiss, phyllites, micaschists, marbles and gypsum/anhydrites. The hydrogeological conditions observed during the EDT construction are heterogeneous, owing to variable lithology and fracturation state. The relevance of water inflows and the related possible impacts on springs was one of the main concerns in the design phase. The peak of cumulative water discharges during excavation is currently in the order of 250 l/s. Appropriate forward monitoring and investigations were designed to prevent the unexpected drainage of springs aquifers. These procedures, based on the execution of forward boreholes equipped with blowout preventers, had to couple with the double shield TBM operation. The EDT completion provided relevant know-how feedback, in terms of knowledge of groundwater flow systems in Alpine domains and of capability to operate forward hydrogeological monitoring while excavating with a double shield TBM. This experience is here summarized.
