Builders freeze ground to start digging world’s longest rail tunnel under the Alps

Thanks to freezing liquid nitrogen, contractors have been able to start digging under the Isarco River in Fortezza, northern Italy to create the southern portal of the longest railway tunnel in the world.

The site is where the 55km, twin-tube Brenner Base Tunnel will emerge from under the Alpine Brenner Pass, having begun its subterranean journey across the border with Austria at Innsbruck.

After months of preparation, the first tunnel metres have now been bored under the river, project company BBT SE said today. 

But they couldn’t begin until groundwater and loose alluvial material under the river bed had been solidified.

To access the rock face of the Isarco River Underpass, contractors built four shafts, up to 25m deep (BBT SE)

The only other approach would have been to shift the course of the river.

So engineers installed a complex system of pipes in the ground and circulated liquid nitrogen at -196°C until the soil chilled to -35°C, freezing solid.

The ground must stay frozen until the tunnels’ structures are complete under the riverbed, so salt brine is now circulating in a cooling circuit.

Night-time view of the Isarco River Underpass work site (BBT SE)

The southern construction lot of the Brenner Base Tunnel, worth €301m, was awarded in October 2014 to the Isarco consortium composed of Webuild (formerly Salini Impregilo), Strabag AG, Strabag S.p.A., Consorzio Integra and Collini Lavori.

This lot links the Brenner Base Tunnel with the existing Brenner line and the railway station in Fortezza. The work is scheduled to be completed in 2022.

To access the rock face of the Isarco River Underpass, contractors built four shafts, up to 25m deep.

Top image: Thanks to the ground-freezing technique, the first few metres of tunnel under the Isarco river have now been bored (BBT SE)