Grinding rock and freezing soil is all in a day’s work when it comes to building the Brenner Base Tunnel. The project also leads the way when it comes to sustainability.
After more than three years of grinding its way under the Alps, a tunnel-boring machine (TBM) named Serena recently completed its work on an exploratory tunnel for the Brenner Base Tunnel, an ambitious project to build the longest underground railway in the world.
The 1,500-tonne TBM was the first of three to arrive at their shared destination at the Italian border with Austria hundreds of metres below the peaks of the mountain range. The other two – Virginia and Flavia – will need some time to catch up. With more than two and four kilometres, respectively, to go along a 14-km route, they are scheduled to arrive at the border in 2022.
The project is a massive undertaking, and Italian infrastructure group Webuild is responsible for a good part of it. It has overseen four sections of the tunnel that will eventually be fitted with a twin-bore double track for high-speed/high-capacity trains.
Although the Brenner Base Tunnel will be 64 kilometres long, the entire network of tunnels that comprise it will total a combined 230 km. At its deepest point, the tunnel will run 1,700 m below the peak of the mountain.
The depth demonstrates Webuild’s expertise in tunnelling, an expertise that is also on display on Australia’s Snowy 2.0, the largest hydropower project in the country. In the heart of the Snowy Mountains, the group has begun using the most technologically advanced TBMs to reach a depth of nearly one kilometre underground, where it will dig a cavern to house the project’s power station. The cavern will be the deepest of its kind in the world.
Connecting Europe
Upon completion, the Brenner Base Tunnel will become an important part of the Scandinavian-Mediterranean Corridor of the Trans-European Transport Network (TEN-T). This network is designed to improve connections among the region’s transport systems and shift more people and cargo from road to rail in a push towards sustainable mobility.
This corridor will facilitate transport between Helsinki in Finland and La Valletta in Malta. The vision behind the TEN-T – let alone its scale – is simitar to Inland Rail, Australia’s ambitious project to improve connections between the ports of Melbourne and Brisbane across 1,700 km.
The Brenner Base Tunnel will traverse below the 1,371 m high Brenner Pass at an altitude of 794 m, a major improvement to the existing line that is full of tight curves and gradients as steep as 3 per cent, limiting speeds to 70 km per hour.
As it follows a straighter trajectory with maximum gradients of up to 0.7 per cent, the new line will enable trains to travel up to 250 km per hour for passengers and up to 160 km per hour for freight. It will reduce travel times between the two population centres at either end of the tunnel – the Italian town of Fortezza in Italy and the city of Innsbruck in Austria – by nearly 79 percent, from 80 to 25 minutes.
A number of construction companies are working on the Brenner Base Tunnel, but Webuild is overseeing the most sections. Having completed the Tulfes-Pfons section on the Austrian side of the Alps, the group is busy with the Mules 2-3 and Isarco River Underpass sections. It recently won a fourth section, known as Gola del Sill-Pfons.
Mules 2-3, where TBMs Virginia and Flavia are busy at work, is the biggest section of the entire project, comprising a combined total of 65 km of tunnels under excavation.
The three sections – Mules 2-3, Isarco and Gola del Sill-Pfons – employ many workers and a supply chain of more than 1,000 direct suppliers, nearly all of them local.
Webuild Project Manager Andrea Fossati says tunnelling through the Alps is a complex endeavour because it is being conducted where the geological fault known as the Periadriatic Seam is located.
“It is one of the most critical areas of the entire project,” he says, referencing how tunnellers had to adjust their work according to the different rock formations they encountered.
Ground freezing
Webuild’s innovative approach to overcoming challenges posed by projects of this size is also being deployed on the Italian side of the Alps.
On the Isarco River Underpass section, rather than diverting the natural course of the river and disrupting the environment, the group is excavating four tunnels under the river to allow for future trains to travel south after exiting the Brenner Base Tunnel. Its workers are using a technique that freezes the water in the ground under the riverbed in order to reduce the risk of water infiltration and prevent the subsequent collapse of the tunnels under construction.
To get a sense of the scope of the project, watch the below video.
In order to do this, they need to keep the ground at temperatures of least -10°C. That requires the initial use of liquid nitrogen. It works like this: freezing probes are inserted around the outline of each tunnel to be excavated. There are 150 probes for each tunnel. A gyro instrument keeps the probes aligned as they are inserted halfway across the river (35 m). The probes consist of steel tubes that contain two coaxial stainless steel pipes. In the gaps between the tube and the pipes, a special cement mixture is inserted to maintain thermal conductivity. This prevents any water from seeping through and blocking the system as it freezes against the nitrogen.
Once the probes are installed, 18,000 litres of liquid nitrogen are pumped through them every hour. The liquid nitrogen leaves the system via exhaust chimneys in the form of gaseous nitrogen at -80°C and -120°C. For each tunnel outline, it takes eight to nine days to freeze the ground. That means trucks carrying the liquid nitrogen need to arrive almost constantly to avoid any delays. In order to keep the ground frozen after the initial freezing, brine is then pumped through the pipes at -35°C.
The technique is as sustainable as it is complex. The only gas emission to come from the system is the nitrogen – an innocuous by-product considering air is made up mostly of nitrogen. As for the brine, it keeps being recirculated through the pipes in a closed circuit. As a result, no harmful chemicals seep into the ground nor enter the air.
Want to learn more? Visit Webuild to learn more about its presence in Australia, its approach to smart mobility and sustainable construction.