In the town 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. Now the ground below the river will be once again artificially frozen to allow for a prompt start of the excavation works for the first main tunnel below the Isarco river.
Freezing procedure for the Isarco River Underpass
As the Isarco river is located on the new tunnel route, 2 shafts have been excavated at the Isarco River Underpass construction site, on both sides of the river - each shaft about 25 m deep. In order to build the tunnel tubes 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 then dispersed into the atmosphere as a gas. The following step is about maintaining the temperature between -30°C 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 on the second longest river in Alto Adige. This ensures that the diversity of flora and fish fauna is preserved.