The work and the technique: Brenner Base tunnel, Austria and Italy
The Brenner Base Tunnel is a cutting-edge construction project to improve transport in central Europe, and one of the most challenging in engineering terms. Once completed, the tunnel will pass below the mountain, with an incline from 4 to 7%.
The Isarco Underpass lot is in the southern section of the tunnel, about 1 km north of the village of Fortezza (near Prà di Sopra, Bolzano). The project involves the construction of 5.8 km of single-track railway tunnels (2 main tunnels and 2 interconnection passages) starting from Fortezza, the Italian terminal point, and passing, for about 240 meters, under the bed of the Isarco River and below the A22 motorway, the SS12 state highway and the Verona-Brenner railway line.
To ensure excavation safety and sustainability, and avoid deviating the river from its natural course, use was made of an eco-compatible technology to freeze a section of soil between the tunnel to be excavated and the river above, to make the excavation impermeable and guarantee stability.
In the first stage, liquid nitrogen (at -196°C) is circulated through special freezer tubes inserted around the section to be excavated. This causes a sudden drop in the temperature of the soil, freezing the water it holds. Subsequently, during the temperature maintenance stage, a liquid refrigerant (-40°C) is circulated through the same tubes; the liquid is cooled using special refrigerators and put back into circulation through a closed circuit. This solution makes it possible to create an impermeable and resistant shell of ice inside which the excavation work can be carried out in dry conditions, below the riverbed. The technique is an undeniable improvement in terms of environmental impact, since there is no need to divert the river.
The Mules 2-3 lot, the largest of the project lots, involves the construction of 65 km of tunnels of varying cross-sections (an exploratory tunnel, 2 main line tunnels, cross tunnels and an emergency stop with an access tunnel) excavated in part with traditional methods, in part with TBMs.
The Tulfes-Pfons lot is located in Innsbruck, in the northern section of the Tunnel, and consists of more than 42 km of tunnels, of which 15 km were built using mechanized excavation and the remaining 27 km with conventional excavation using the New Austrian Method (NATM), which installs the final linings after the excavation has been completed.
The construction of this lot, guaranteed by use of a TBM equipped with state-of-the-art monitoring and control systems, involves the construction of an almost 10 km emergency tunnel along Innsbruck’s existing railway, updating the working tunnel to comply with the latest railway safety regulations. During excavation, the monitoring systems process the photographs taken by industrial cameras installed on the head of the TBM, and provide 2D and 3D images of the excavation face for geological studies, with no need to stop the TBM.