Breakthrough of the first main tunnel under the Isarco river

excavation worksBBT SE has completed the excavation works for the first main tunnel below the riverbed of the Isarco river for the future Brenner Base Tunnel railway traffic.

The excavation works for the west main tunnel below the Isarco river started on 22nd March and are about 57 metres long.

The excavation below the river was only possible after freezing the soil with liquid nitrogen and then maintaining low temperatures by circulating brine. With the ground freezing technique, a frozen shell is created around the tunnel perimeter, which allows safe excavation works to take place below the river without having to re-route the bed of the river under which the Brenner Base Tunnel runs.

This reduces the environmental impact on the fish fauna and flora of the Isarco river. Before the start of the excavation and until the final lining is in place, it is necessary to monitor soil temperatures on a daily basis in order to check the status of the frozen shell using special “thermometers” placed inside the ground, as well as to verify there is no water inflow inside the tunnel.

Once the soil has been consolidated, works are carried out with the usual excavation methods, in this case with a percussion hammer, which breaks up the soil reaching an average daily production of about 1.5 metres.

After the excavation works have been completed, the following step is the final concrete lining of the tunnel.

One of the tunnels has already been excavated with this soil consolidation method throughout 2020 and two other tunnels are yet to be excavated.

“A very important goal was achieved with the first main tunnel breakthrough, which is a testament to the excellent work that has been done by BBT SE, the contractor and the construction management, as well as to the common goal of completing the work,” Matteo Stelitano, geologist at the Isarco Underpass construction site said.

The southernmost construction lot of the Brenner Base Tunnel, worth EUR 301 million was awarded to the consortium Isarco s.c.a.r.l. comprising Webuild, Strabag AG, Strabag S.p.A., Consorzio Integra and Collini Lavori S.p.A. in October 2014. This lot links the Brenner Base Tunnel with the existing Brenner line and the railway station in Fortezza. The Isarco River Underpass section includes civil works for two main tunnels totalling 4.5 km as well as two interconnecting tunnels with a total length of 7.5 km that connect with the existing rail line.

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