Months of preparation preceded the work to excavate under the river.
The Brenner Base Tunnel (BBT) will be a 55km railway link between Innsbruck in Austria and Fortezza in Italy. The specialist tunnelling works are taking place in the southernmost section of the BBT construction site near Fortezza.
The groundwater and alluvial material under the river bed was frozen, starting from one of four shafts on the Isarco construction site. Liquid nitrogen was first injected into a closed circuit of ‘freezing pipes’ below the river. The -196°C nitrogen in the freezing pipes drew heat from the surrounding soil, freezing the water in the soil below the river and causing the soil temperature to drop to -35°C.
The temperature must be maintained for the tunnelling and so salt brine is now circulating in a cooling circuit.
Use of the freezing technique has avoided any need to alter the river’s course and has reduced the impact on the environment, said the project team.