Excavation works start on new stretch of record-setting Brenner Base Tunnel

Just a few kilometres from Innsbruck, tucked away amidst Austria’s green valleys, colorful church towers and the sheer rock faces of the Alps, excavation has begun on another section of the Brenner Base Tunnel. This high-speed train line tunnel will reach a length of 64 kilometres (40 miles), and is on track to set a new record for the longest rail tunnel in the world.  

For years now, work crews have been tunneling through mountains nearly 2,000 metres (1.2 miles) high to provide a fast, modern and sustainable transport link that will help relieve congestion on the Brenner freeway, one of Europe’s busiest traffic arteries, thus securing another high-speed link in Europe’s interconnected TEN-T network, among the most extensive rail networks in the world. 

On July 12, another chapter in this engineering history was written in the Sill-Pfons Gorge, where excavation began on Lot H41, which will connect the city of Innsbruck to the north with the city of Pfons further south through the construction of 22.5 kilometres (14 miles) of parallel main tunnels and 38 connecting cross passages. This important milestone was marked by a ceremony attended by Austrian Federal Minister for Climate Action and Environment Leonore Gewessler and Pat Cox, European Coordinator for the Scandinavian-Mediterranean Corridor of the TEN-T network.  

The start of excavation on the Austrian side opens a new front in the construction of a strategic work for European transport, which stands out for its high level of engineering complexity as well as adherence to some of the most demanding construction standards in the world. 

In Lot H41, for example, special attention has been paid. All site logistics have been set up to reduce the travel time required to transport materials. Likewise, a number of innovative techniques are planned to reduce the noise impact of the works, as well as the production of dust. Even the water used by the construction site will then be treated before being returned to the surrounding environment. 

About 400 people will take part directly in this construction phase, and about 1,000 people will be employed in the allied industries. Their task will be to complete the excavation in one of the most complex hydrogeological contexts in the world, a challenge that the Webuild Group (which was awarded the contract together with its subsidiary CSC Costruzioni and partner Implenia) has already successfully completed in the other finished lots the Tulfes-Pfons Lot in Austria, and in those under construction on the Italian side. 

It is almost “mission accomplished” for work on the first lot of the Brenner Base Tunnel where it passes beneath the Eisack River in South Tyrol (northern Italy). In this section of the tunnel, work progress has reached 93%. Here, too, the Webuild Group counted on the support of about 300 people, including technicians and workers, at peak times; today 84 people are working on the construction sites and 34 in the offices.  

The task was a very complex one, because carrying out an excavation under a river called for particularly innovative techniques such as freezing. This technique is used when the subsoil is rich in water, which risks making the excavation uncontrollable. The technique involves drilling the ground beneath the river and injecting cement mixtures necessary to slow the velocity of the water. Second, liquid nitrogen at a temperature of 196 degrees below zero is injected into the holes. The nitrogen at that point changes from a liquid to a gaseous state, and absorbs heat from the surrounding soil and freezes a portion of it equal to about one meter around the tunnel excavation. The freezing makes the excavation impermeable to water, and provides strength to the material placed around the excavation itself. 

“On the construction site we made use of the most innovative techniques,” explains Antonio Gassirà, Project Manager for Lot H71. “Freezing was one of them, as well as a series of techniques for soil consolidation that — precisely because they are so innovative – have been presented several times at conferences organized by the Italian Tunnelling Society and the Swiss Tunneling Society.”  

The lots Mules 2 and Mules 3 of the Brenner Base Tunnel are one of the largest in terms of the length of tunnels built. Overall, the section is 22 kilometres (13.6 miles) long, 14 kms of which are being built by mechanized excavators (TBM, or tunnel boring machines) and the rest by conventional excavation. The project overall involves stupefying numbers: 65 kilometres (40 miles) of total tunnels, 42 kms of which are being built by mechanized excavation. The numbers tell the story of the size of this lot, which for long stretches runs under hundreds and hundreds of meters of rock in the mountains overlooking the Brenner valleys. 

“To understand the logistical complexity of this lot, consider that — precisely because of the area’s inaccessibility — there is only one entrance for all the underground work: the tunnels start from the Mules window for about 5 km to the south and 17 km to the north,” explains Andrea Fossati, Webuild Project Manager for Lot H61. “The excavation involves the parallel construction of an exploratory tunnel tracing the path of the tunnels where the trains will run. The exploratory tunnel is necessary to understand what we will be facing, what kind of rock, and what kind of soils. And very often, just based on what we find, we have to revise our excavation techniques.” 

Despite these complexities, work is moving forward. In November 2021 the exploratory tunnel reached the Austrian border, while today the completion of the two line tunnels to the Austrian border is 1.9 km (1.1 miles) and 4.4 km away, respectively.  

“The work on this lot requires constant experimentation because it is impossible to have certainty working under 1,700 metres (1 mile) of rock,” said Fossati.  

This is yet another engineering frontier for major infrastructure projects; a frontier of technology being put at the service of people’s needs, pursuing the common goal of delivering a record-setting tunnel network to Europe that will allow goods and people to run under the Dolomites.