In the municipality of Fortezza works on the Isarco River Underpass construction site, which is the southernmost construction lot of the Brenner Base Tunnel, are proceeding at full speed. The complex freezing procedure already produced successful results in autumn 2020. The following step was to proceed with the excavation works for the first connecting tunnel below the Isarco river. In mid-March 2021, the ground under South Tyrol's second longest river was again artificially frozen in order to start the excavation work for the first main tunnel under the Isarco river.
Freezing Procedure for the Isarco River Underpass
As the Isarco river is located on the new tunnel route, two shafts have been excavated at the Isarco River Underpass construction site, on each side of the river. The four shafts are about 25 m deep. In order to excavate the tunnels below the river, the groundwater is frozen starting from the shafts by circulating a refrigerant fluid inside the soil through special heat exchangers. As a first step, liquid nitrogen is injected to this effect in a closed pipe circuit below the river bed. The refrigerant substance has a temperature of -196 °C and is eventually dispersed into the atmosphere as a gas. The following step is about maintaining the temperature between -30 and -35 °C by using saline brine, so that the ground stays frozen, allowing the excavation works for the underpass to proceed.
This highly complex procedure was deemed necessary to avoid relocating the Isarco river from its original river bed. The significant advantage of this freezing procedure is the lack of environmental impact. This ensures that the diversity of the river’s flora and fish fauna is preserved.
Construction Progress at the Brenner Base Tunnel
In March 2021, four main construction lots were under construction:
Mules 2–3 and, Isarco undercrossing (in Italy)
Tulfes–Pfons and Sill Gorget (in Austria)
To date, 138 km of the total 230 km planned in the project have been built. Of these, 47 km are main tunnels, 51 km are exploratory tunnels and 40 km are other tunnel structures, such as access tunnels or logistics tunnels.