From the Grand Paris Express to the Brenner Tunnel: Tunneling Is Key to Green Cities

In an increasingly urbanized world, where surface space is precious, building large underground infrastructures – road or high speed rail tunnels, subways, and hydroelectric plants – has become a concrete solution to the issue of land consumption and the environmental and urban impacts.

Today, in the construction industry, underground tunnelling is not just an engineering option, but a conscious choice for urban sustainability. It means not sacrificing green spaces, agricultural areas, or already congested urban environments. Moreover, tunnel construction helps limit the noise and visual impact of construction sites and completed structures.

Large cities like London, Paris, Milan, and Copenhagen are investing heavily in underground infrastructure projects, especially concerning public mobility. The same is happening in the United States, Latin America, Asia, and the Middle East, where large urban subways are developing underground, supporting the transition toward more sustainable, people-friendly cities.

All of this is thanks to the incredible evolution of excavation techniques. In tunnelling, in addition to traditional mechanical methods (still widely used), new mechanized excavation techniques are employed through the use of Tunnel Boring Machines, also known as mechanical moles, which have radically transformed the way large infrastructure projects are conceived and built.

To trace the origins of large underground construction works, one must go far back in time, even to 2200 B.C. in Babylon, where a tunnel was excavated to connect the royal palace to the temple of Belos.

In the 6th century B.C. in Samos, the Aegean Sea island where the mathematician Pythagoras was born, the Eupalinos Tunnel was built — an underground aqueduct over a kilometer long, one of the greatest technical achievements of its time and considered by many as the eighth wonder of the ancient world.

During the Industrial Revolution, the Thames Tunnel in London, completed in 1843, was the first tunnel built under a navigable river, marking the beginning of the modern tunnelling era. In the United States, the Hoosac Tunnel, begun in 1853, represented a milestone for engineering techniques, being the first case where explosives and pneumatic drills were used.

According to the latest Global Tunnel Construction Market Report, the tunnelling market value will grow from $174 billion in 2024 to $330 billion by 2031, with an annual growth rate of 9.5%. This increase is driven by investments in transportation infrastructure projects.

A key contributor to this growth is the use of Tunnel Boring Machines, with a global market value estimated at $6.71 billion for 2025, projected to reach $9.51 billion by 2032.

The adoption of advanced TBMs — according to the report — has improved efficiency, safety, and reduced construction times, making tunnelling the preferred solution for large-scale projects.

One of the most emblematic projects in the tunnelling sector is the Brenner Base Tunnel, the new high-speed/high-capacity rail line that will connect Fortezza in Italy with Innsbruck in Austria, running beneath the Alps.

With a total length of 64 kilometers, the Brenner Base Tunnel will become the longest railway tunnel in the world — a colossal project, now in advanced construction stages, that will drastically reduce freight traffic by road, lowering emissions and environmental impact along the Brenner corridor, one of the busiest in Northern Italy.

As part of the Mules 2-3 Lot of the project, built by the consortium of companies led by Webuild on behalf of BBT SE, the TBM Flavia recently completed excavation on the Italian side after a journey of over 14 km. In another lot, named Isarco River Underpass, an extraordinary excavation is planned: about 6 kilometers of tunnels located at 240 meters below the Isarco riverbed.

To build these tunnels, an innovative and cutting-edge technique was used – the soil consolidation through freezing – applied specifically in tunnel construction when there is a strong presence of groundwater. This technique precedes excavation and involves three operations:

1. First, the soil is drilled to allow cement injections that make it water-impermeable and ensure its stability.
2. Next is the freezing phase, injecting liquid nitrogen at about -196 degrees Celsius, creating an ice shell around the ground one meter thick.
3. The final step is maintaining the ice shell using brine cooled to between -30°C and -35°C.

The freezing technique was used in many excavations of Milan’s M4, the metro line built by Webuild that connects Linate airport with the city center in just 12 minutes. In this case, this innovative technology was essential due to the extreme proximity of the foundations of numerous historic buildings to the tunnels under excavation.

As in the case of Milan’s M4, where 20 kilometers of tunnels were dug at a depth of 30 meters, tunnelling for metro line construction is one of the most complex fields due to the high-density urban environment in which these tunnels are built.

This is also happening in France, where the Grand Paris Express is under construction — an ambitious project that will revolutionize the mobility of the French capital, connecting nearly all municipalities in the Île-de-France region. As part of this mega metro network, 200 kilometers long, Webuild, along with its French partner NGE, will build Lot 2 of the Western Segment of Line 15, is currently constructing Lot 2 of Line 16, and has already completed Lot 4 of Line 14 South.

Back in Italy, another strategic and complex infrastructure project in the tunnelling sector is the Third Giovi Pass, currently under construction, which crosses the Ligurian-Piedmont Apennines, linking Genoa to Milan.

Here, the complexity lies also in the project’s numbers. The Valico Tunnel is 27 kilometers long — the longest railway section in Italy. Including all secondary interconnections, the project involves 90 kilometers of tunnels, 32 of which are excavated using TBMs and 58 using traditional methods.

Designed to improve the railway connection between the port of Genoa and Northern Italy, the Third Giovi Pass is a key project for the future of Italian and European logistics, set to transform the freight transport sector in a large part of Europe.

Building large mobility infrastructures underground without affecting people’s quality of life is not just a technical solution but a cultural paradigm shift in urban planning. Developing new transport systems centered on tunnelling means imagining multi-layered sustainable cities, where surface life is freed from invasive infrastructure, while increasingly complex and invisible networks develop underground.

In this way, tunnels become silent arteries that reduce traffic, pollution, and surface land use, contributing to community well-being and urban resilience.