Unlike thousands of years of human history, the fastest route between north and south no longer leads over rocky cliffs and muddy trails, but directly through the massive mountains. Through underground tunnels for roads and railways that were long considered utopian. The Gotthard railroad tunnel, built in 1872, was the first of these. Opened in 1882 after ten years of construction, it was the longest tunnel in the world at the time with a length of around 15 kilometers.

The challenge could hardly have been greater. Such tunnels under entire mountain massifs and with tunnelling lengths of dozens of kilometers are among the most difficult projects in infrastructure construction and place enormous demands on men and machine. They are like an endurance test lasting years under extreme conditions, where everything has to be completed as quickly as possible and yet with maximum safety.

Superlatives of construction

In record-breaking projects, Herrenknecht has proven that almost nothing is impossible in alpine tunnel construction and that even the longest and most difficult tunnel drives can be successfully mastered with boldness, persistence and the best technology. As different as the projects are, they all have something in common: they are all superlative infrastructures. They all cross under the Alps and connect Italy with its northern neighbors.

The current world record holder as the longest railroad tunnel in the world is the Gotthard Base Tunnel with a length of around 57 kilometers per tube. The largest part of this – over 85 kilometers of the two main tubes – was driven by Herrenknecht Gripper tunnel boring machines. On June 1, 2016, the world’s longest railway tunnel at the time went into operation and has been transporting goods and passengers quickly and safely through the Swiss Alps ever since.

Challenge accepted

However, the title of the world’s longest railroad tunnel is already being challenged by new superlatives. Upon commissioning, the Brenner Base Tunnel (BBT) between Austria and Italy will be the longest underground rail link in the world at 64 kilometers. Once again, tunnel boring machines from Herrenknecht are involved, boring a total of 90 kilometers of the three-tube tunnel system through the hard rock of the Alps.

In order to assess the challenges involved in driving the two main tubes of the BBT as accurately as possible, an exploration and service tunnel was constructed first, running centrally 12 meters below the main tubes. An open Gripper TBM with typical rock support measures was used for tunnelling on the Austrian side. On the Italian side, however, a Double Shield TBM with segmental lining was chosen. The reason for using the two hard rock machines was to gather as much information as possible about the predicted geological fault zones and how to deal with them.

Unlike in the construction of the Gotthard Base Tunnel, the decision was made to use shielded TBMs with segmental lining for the main drives. However, the choice of machine type also varied for the main lots: two Single Shield TBMs for lot H41 Sillschlucht-Pfons (Ø 10,250 mm) and two Double Shield TBMs each for lots H61 Mauls 2-3 (Ø 10,650 mm) and H53 Pfons-Brenner (Ø 10,330 mm).

Meanwhile, the BBT project is progressing rapidly. In May 2025, Herrenknecht’s third Double Shield TBM reached its destination after traveling 14.3 kilometers north below the Italian-Austrian border. This marks the successful completion of all tunnelling operations started on the Italian side. In the neighboring country, the miners had reason to celebrate in August 2025: TBM “Ida” was the first tunnel boring machine to break through the main tube – after 8.4 kilometers and 26 months of tireless work.

Another milestone followed in October: TBM “Lilia” successfully broke through the eastern main tube – completing the main tube east in the “H41 Sillschlucht-Pfons” construction lot.  Two additional Herrenknecht hard rock machines are currently advancing the two main tubes of the BBT from Austria towards the south in lot H53 Pfons-Brenner.

Complex system for top logistical performance

The challenges involved in building such gigantic underground infrastructures are far from limited to long-distance tunnelling through difficult terrain. The necessary logistics are also a Herculean task. This includes assembling the TBM in launch caverns, transporting materials required for tunnelling, as well as providing rock reinforcement materials in the caverns. For the BBT project as a whole, around 21.5 million cubic meters of excavated rock must also be moved out of the mountain and transported to its storage location. This is equivalent to the volume of 13,700 Airbus A380 airplanes. Absolutely impossible without a sophisticated logistics concept.

Together with its subsidiary H+E Logistik, Herrenknecht has developed and installed an extremely powerful conveyor belt system for the Brenner project that is unique in its dimensions. At the Wolf construction lot alone, near the Tyrolean municipality of Steinach, the conveyor system will grow to a length of around 60 kilometers over the course of the project – made up of 42 interconnected conveyor belts. The highlight: the entire system can be controlled from a central control station on the construction site. 70 cameras monitor the smooth running of each module, and the control center team can intervene at any time by remote control.

Two road tunnels through the mountain

Another important Alpine transit route for more than four decades has been the Gotthard Road Tunnel, which is used by around 16,000 vehicles every day. On peak days, the number is more than twice as high. Opened in 1980, this key infrastructure project is now showing its age and must therefore be closed for extensive renovation. To ensure that traffic can continue to flow, a second, parallel tunnel is currently being built. In the future, separate tunnel tubes without oncoming traffic will be available for northbound and southbound travel – a significant gain for traffic safety.

Due to the location of the second tube next to the existing Gotthard Road Tunnel and the experience gained during the construction of the Gotthard Base Tunnel in neighboring geology, the client and planners were able to draw on a wealth of experience. This led to the early development of a concept to open up the larger fault zone sections in advance with access tunnels and to drive these with a Gripper TBM in the north and a Single Shield TBM in the south. In February 2025, two Single Shield TBMs from Herrenknecht simultaneously began driving the second main tube through the hard rock. The huge machines are boring 7,755 meters from the south and 6,885 meters from the north into the rock and are expected to meet precisely at the end.

Real-time sensor technology for optimized maintenance

When tunnelling through hard rock, such as gneiss or granite, the excavation tools installed on the cutterhead must pass the absolute endurance test. For fast and economical long-distance tunnelling, it is not only the high quality and long service life of the excavation tools that are important. Monitoring systems specially developed by Herrenknecht on the cutterhead ensure that unavoidable wear is reliably detected. During the excavation of the exploratory tunnel for the BBT, the disc cutters were already equipped with DCRM (Disc Cutter Rotation Monitoring). The system monitors the rotation and temperature of the disc cutters, allowing maintenance intervals to be optimized. After all, any downtime on a project of this magnitude means costs that need to be minimized.

Another option for monitoring the disc cutters using sensors is face mapping, which precisely detects the loads on the disc cutters. The information generated in this way not only improves maintenance intervals, but also provides valuable insights into the rock mass at the tunnel face. For even greater transparency, project teams can digitally consolidate and evaluate all data from the sensors installed on the TBM using Herrenknecht.Connected. As an add-on developed specifically for cutter monitoring, Tools.ON documents disc cutter performance and maintenance information in real time.

Even though the technological trend in tunnel construction is clearly moving towards monitoring and transparency – when driving in hard rock with vibrations, dust, dirt, and water, a careful balance must always be struck between comprehensive sensor technology and simple, practical solutions in order to optimally reconcile both objectives.

Cross-border projects

Further west of the Gotthard, Herrenknecht’s tunnelling technology is also in demand. The Mont Cenis Base Tunnel forms the heart of the new high-speed rail link between Lyon and Turin – a central section of the European Mediterranean Corridor (TEN-T). At 57.5 kilometers in length, the new structure will exceed the Gotthard Base Tunnel by 500 meters. The tunnel construction consortia commissioned by the French-Italian project company “Tunnel Euralpin Lyon Turin (TELT)”, have so far ordered six tunnelling machines from Herrenknecht.

Three Single Shield TBMs are being used for tunnelling in the hard rock of lots CO6/CO7 Saint Martin – Villarodin/Modane, through sandstone, conglomerates, mica schist, and slate. The three TBMs underground are supplied on wheels by multi-service vehicles from Herrenknecht subsidiary TMS. The first TBM started tunnelling in September 2025.

In the neighboring lot CO5 “Villarodin/Modane – Val Clarea,” due to the complex geological conditions with varying geologies of marble, dolomite, limestone, anhydrite, quartzite, mica schist, conglomerates, and gneiss, the decision was made to use two Gripper TBMs, each of which will drive 18 kilometers. Unlike in lot CO6/CO7, the logistics for supplying the Gripper TBMs will be carried out using double-track train transport. To this end, a base element with a separate track slab is already being installed from the back-up, on which the tracks will be mounted and which is also part of the final tunnel lining. The excavated material will also be transported away by conveyor belt. The fleet will be supplemented by a Multi-mode TBM, which will drive 10.5 kilometers on the Italian side.

With these six machines, Herrenknecht is making a decisive contribution to one of Europe’s most important infrastructure projects. This marks another historic milestone for Alpine transport and for Herrenknecht’s tunnel construction experts.