Contractors working on the 55 km (34 mile) Brenner Base Tunnel linking Austria and Italy will continue under the Isarco River in Fortezza, northern Italy following the ground freezing operations.

The Brenner Base Tunnel, the longest railway tunnel in the world, is a twin-tube tunnel that will emerge under the Brenner Pass. Contractor BBT SE said 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 tunneling 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.

This procedure solidified the loose material below the riverbed and excavation work began on Oct. 5.