ROCK TUNNELS | PROJECT
H71 - ISARCO RIVER UNDERPASS & PORTALS At the southern end of the Brenner Base Tunnel is Lot H71, Isarco River Underpass and portals, via which trains will, after a short distance, reach the first station in Italy - Fortezza (in Italian) or Franzensfeste (German). The Lot may be short in comparison to a number of
other packages on the major rail transport project but, like Sill Gorge at the north end of Brenner Base Tunnel, it has had many and varied complex civils works. The package was undertaken by a JV of Webuild Italia
(formerly Salini-Impregilo), Strabag, Consorzio Integra, and Collini Lavori. The contract package was awarded in late 2014 to connect the Base Tunnel with the existing Brenner rail line as well as Fortezza station. While most of the works in the Lot are on the river
valley floor, the package also included a total of 4.5km of main tunnels, cross passages, and 1.2km of connecting tunnels, plus shafts and an access tunnel. All excavations were completed by the end of 2022. Prior to the tunnelling works there was a need for
extensive preliminary ground engineering due to the fluvioglacial non-compact sediments in the river valley. Extensive jet grouting was performed. In the treated area four shafts were then sunk, up to 20m-25m depth, from which tunnel blasting then proceeded northwards. The tunnelling challenge also included rerouting a
state road plus works under a motorway, rail line and also the river bed. Artificial ground freezing (AGF) was also employed to enable the tunnel construction works. Construction works on the Lot were completed in late
2023.
BENEFITS OF EXPLORATORY TUNNEL While smaller in width, the strategic investment in an exploratory tunnel for the entire length of the Brenner Base Tunnel proved valuable before and during the excavations for the parallel main tubes but will also support operations and safety, long-term. This argues for the “usefulness and necessity” of an
exploratory tunnel in deep-lying tunnel systems (Marini, Lussu & Vendetti, 2024). On Brenner, it was constructed in five stretches (three
in Austria, two in Italy) and average advance rates of 10m/day-12m/day could be achieved, despite various challenges at different locations. The short portions constructed through the Periadriatic Seam, in Italy, and the Hochstegen water-bearing geology, in Austria, are not included in the total of major sections excavated, but that in no way diminishes their importance in locally solving the tunnelling challenges and, strategically, removing bottleneck risks from the schedule. The authors (Marini, Lussu & Vendetti, 2024) say the
exploratory tunnel works provided data and insights that supported procurement, advance redesign and relocation of some of structures (e.g., caverns, cross- passages). The investment also enabled tunnelling challenges to be anticipated in specific locations and to enable suitable construction measures to be prepared - including, at times, use of the exploratory tunnel to secure rock mass ahead of the main tunnels.
34 | December 2025
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