Webuild completes under-river section of 64km tunnel

Work on the Isarco Rover Underpass Tunnel section of the tunnel, which will be the longest railway tunnel in the world, was begun with the construction of two bridges over the Isarco River and Rio Bianco, and a temporary build of a cargo loading area.

Two sub-river tunnels, each 4.5km long, were built as well as two interconnecting tunnels of 1.7km to connect with the existing railway line. 

According to Webuild, construction was complex technically because both main and interconnecting tunnels pass under the river, two motorways and a railway line, with minimal leeway.

Liquid nitrogen was used to freeze the ground along the second of four tunnels being excavated for the underpass, which meant workers could excavate the second tunnel unhindered by water infiltration. 

“In order to carry out the excavations safely and sustainably, avoiding the displacement of the river from its natural course, Webuild has adopted the technique of consolidating the land by freezing it with liquid nitrogen,” the group says. “The Group has used this particular technology for the construction of the four tunnels because the freezing of the shell of soil around the shape of the tunnels – which makes the excavated land white – has allowed the construction of the tunnels where there is a strong presence of aquifers and in full respect of the environmental context, preserving flora and fauna.

“More specifically, the freezing technique guarantees the freezing of a portion of the ground, between the tunnel to be excavated and the river above, sufficient to make the excavation impermeable to water and useful for ensuring stability.

“Its application included a pre-consolidation phase, in which the soil was drilled in the section below the river and its permeability was reduced by cement injections. Subsequently, the freezing phase took place with the introduction of liquid nitrogen at a temperature of about -196°C: the coolant circulates in the holes made around the excavation perimeter to create a shell one metre thick.

“The last phase is the one in which the preservation of the ice shell has been guaranteed through the use of brine brought to a temperature between 30°C and 35°C below zero.”

Now that tunnelling beneath the Isarco River part of the tunnel has been completed, the next steps will be to dig under the Alps.

The project is estimated to cost a total of €15.7 billion and should be open to high-speed trains for cargo and passengers by 2032.