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Some of the project’s most demanding work occurred far from the final TBM drives in the Pfons–Brenner section — beneath 5 m to 8 m of cover under the Isarco River near Fortezza — where engineers advanced four shallow tunnels without diverting the river or lowering the groundwater table.

Excavating Beneath a Live River

The Isarco River underpass is the southernmost section before the base tunnel connects to the existing Verona–Brenner railway line. The route passes beneath the river, the A22 highway, State Road SS12 and an active railway corridor—all with minimal overburden.

Designers opted for underground construction instead of a cut-and-cover diversion to reduce environmental impact and prevent interference with surface infrastructure. This choice highlighted the challenge: excavation through saturated soils beneath active infrastructure without lowering the groundwater level.

A geological longitudinal section shows rock formations between Innsbruck and Fortezza along the alignment of the $11.5-billion Brenner Base Tunnel beneath the Alps. The 55-km rail link passes through quartz phyllite, Bündner schists and complex fault zones that shaped excavation strategy.

Image courtesy of BBT SE.

Geologically, the alignment shifts from solid granite to loose alluvial and fluvioglacial deposits—gravels, sands and silts that carry measurable groundwater flow. Excavation, therefore, had to proceed under hydrostatic conditions.

Once dewatering was eliminated, permeability reduction became the main structural tool. Engineers used jet-grouting to build a reinforced soil envelope around the excavation. Field tests showed column diameters nearing 2 m, with treated ground reaching compressive strengths above 5 MPa and permeability decreased by several orders of magnitude.

Pumping tests confirmed the seal before the headings advanced. When access was available, jet grouting was carried out from the surface, allowing ground improvement to continue independently of excavation and minimizing stoppages beneath the motorway and state road.

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Freezing the Ground, Not the River

The shallowest section—directly below the Isarco River—required artificial ground freezing. Jet grouting could reduce permeability and reinforce the soil envelope, but it could not provide the temporary structural stability needed under just a few meters of cover with live water above.

Each advance required dozens of grout and freezing boreholes. Liquid nitrogen was circulated through probe systems to freeze saturated soils into temporary structural cylinders, strong enough to support excavation. 

Once frozen ground reached target strength, excavation advanced in controlled 1-m increments with immediate support installation behind the face. Freezing then transitioned to a brine-based maintenance phase to sustain ground temperatures throughout.

The goal was clear: preserve groundwater levels and prevent interference with river flow. Under just a few meters of cover, even small settlements could have impacted the river channel or the infrastructure above. The margin for error was slim.

A defining feature of the larger tunnel system is a continuous exploratory tunnel situated roughly 12 m below and between the two main tubes. During construction, it supplied early geological data. When in use, it will serve as a permanent drainage conduit beneath Alpine overburden reaching up to 1,720 m—a long-term safety feature built into the design from the beginning.

From Excavation to Operations

When the TBMs reach their destinations, the project will move into commissioning and systems installation across the full 55-km alignment. This phase includes ballast-less track installation, traction power systems, safety cross-passage outfitting and the implementation of European Train Control System Level 2 signaling, along with ventilation, drainage and emergency infrastructure.

The tunnel's relatively flat longitudinal slope—approximately 0.4% to 0.7%—will allow longer and heavier freight trains to travel beneath the Alps without the steep climbs required on the current Brenner line. That profile is key to the project's goal of increasing rail capacity along the Scandinavian–Mediterranean corridor and shifting freight traffic from trucks to trains.

Systems integration will determine operational readiness—and ultimately whether the tunnel delivers its promised performance gains.

The Harder Engineering Test

Map shows the alignment of the Brenner Base Tunnel between Innsbruck, Austria, and Fortezza, Italy, forming the central Alpine link in the EU’s Scandinavian–Mediterranean rail corridor.

Image courtesy of BBT SE.

The Brenner Base Tunnel is the central element of the EU's Scandinavian–Mediterranean corridor. The February announcement showed steady progress.

The more consequential engineering test was solved years earlier: permeability was lowered by about five orders of magnitude in the treated zones; frozen soil cylinders were constructed beneath a flowing Alpine river; and excavation advanced under full hydrostatic pressure without draining the valley floor.

The greater challenge was never achieving the final breakthrough. Instead, it was to prove that a transalpine rail artery could be constructed beneath a live river, through saturated alluvium and under active infrastructure—without changing the groundwater regime that supports the valley above.

That test has already been passed.