The Brenner Base Tunnel (BBT) is a 55km-long rail tunnel being built beneath the Brenner Pass, an important north-south connection over the Alps in Europe. It will become one of the world’s longest tunnels, upon completion.
The tunnel will connect Italy and Austria, starting in Fortezza, Italy, and ending in Innsbruck, Austria. It is the core element of the new 425km-long Brenner railway line running from Munich, Germany, to Verona, Italy.
The project is being developed by BBT, a European public limited company. Austrian railway company Österreichische Bundesbahnen (ÖBB) holds 50% share in BBT, while the remaining 50% is owned by Tunnel Ferroviario del Brennero (TFB) on behalf of Italy.
Excavations have been completed along 152km of the 230km of tunnels of the project. Construction of the BBT project is expected to be completed by 2032.
According to the final configuration estimates, the Brenner section will have the capacity to withstand a weight of 400 trains (222 cargo trains and 42 passenger trains in the base tunnel).
Initially, 22 cargo trains will use the existing lines and 182 will pass through the tunnel. The number of cargo trains travelling through the tunnel is expected to increase to 201 within five years.
The BBT is a 55km-long straight, flat railway tunnel, which will run through the Eastern Alps. The Inn Valley Tunnel, located south of Innsbruck, connects to the BBT. The two tunnels will have a combined length of 64km, forming one of the world’s longest underground railway connections.
The BBT system will consist of about 230km of tunnels, .
will form the central link of the Scandinavian-Mediterranean TEN Corridor between from Helsinki, Finland, and La Valletta, Malta.
The tunnel is expected to have a design speed of 120km/h for freight traffic and 250km/h for passenger traffic.
Design and features of tunnels
The tunnel infrastructure will comprise two main tubes, exploratory tunnel, four lateral access tunnels, side tunnels, and connecting tunnels. Each tube will have a single track running through it.
The Brenner tunnel will be located at an altitude of 790m, below the lowest alpine pass at 1,370m. It will have a slope between 4% and 7%.
Side tunnels will be used to link the two main tunnel tubes every 333m metres to provide escape routes in the event of emergencies.
The two single-track tubes of the main tunnel system will have a diameter of 8.1m each and will be 40m-70m apart.
An exploratory tunnel with a diameter of 5m will be located 12m below the two tunnel tubes to test the rock. The test results are used for the main tunnel construction in order to reduce construction risk and save costs and time.
The spoil will be taken out through the exploratory tunnel while building the main tubes to avoid any interruptions to construction. The exploratory tunnel is also expected to function as a drainage tube for underground water that accumulates across tunnels.
The tunnel system will be installed with a 25kV, 50Hz traction system.
Furthermore, the project will have three underground emergency stops in Innsbruck, St, Jodok, and Trens.
The four lateral access tunnels in Mules, Wolf, Ahrental, and Ampass will provide connectivity from the underground stations to the outside.
About 50% of the tunnel excavation will be completed using tunnel boring machines (TBMs) and the remaining 50% through drilling and blasting.
Equipment used for digging comprises TBMs, explosives, mechanical diggers with hydraulic hammers and drilling equipment.
Prospection boreholes totalling 35,000m were drilled as part of project planning to understand geological conditions of the mountain.
Equipment to be installed in the tunnel system will include rails and vibration/shock absorbers, ventilation and air conditioning, telecommunications and surveillance systems, command and control systems, electrical traction systems, energy supply systems, machinery, and emergency systems.
Construction is currently taking place at five sites across Austria and Italy.
The H21 Sill Gorge construction lot, which is expected to be completed by the end of 2024, will provide connection between the BBT and the central station in Innsbruck. It includes a 600m section and other works. The lot covers the area through the narrow Sill Gorge and involves mostly above-ground construction.
Construction works in the lot include a 200m-long supporting wall, two railroad bridges over the Sill, a 130m-long cut-and-cover tunnel, the northern portal of the BBT, and a .
Excavation works for the western part of the main tube commenced in October 2021, followed by drill-and-blast excavation at the northern portal site in November 2021.
The H41 construction site is located in the northern portion of the BBT, between Pfons in the south and the Sill Gorge in the north. The site was set up in January 2022, while excavation activities in the direction of Innsbruck began in July 2022. The lot will involve the excavation of 22.5km of main tunnel tubes and 38 cross passages by Summer 2028.
All work on the H33 Tulfes-Pfons lot was completed in September 2021. It included the construction of the Tulfes emergency tunnel, main tunnel tubes, connecting tunnels, Innsbruck emergency stop, and Ahrental-Pfons exploration tunnel.
The H52 Hochstegen construction lot is situated in the Steinach am Brenner township. The ground-breaking event for the construction lot was conducted in May 2022.
The lot H53, one of the longest stretches in the Austrian side of the project, will be located between the townships of Gries am Brenner to the south and Pfons to the north. The tender process for the lot was started in January 2022.
The construction lot H61, named Mules 2-3, is one of the biggest lots of the project, extending from the Isarco river underpass construction lot area to the Austrian state border.
The H71 Isarco river underpass construction lot is the southernmost construction lot of the BBT project. It will connect the BBT to the current Brenner line and the Fortezza train station.
The BBT is a top-priority infrastructure project for the European Union (EU), which is contributing up to 50% of its expenses. The remaining expenses will be divided equally between Austria and Italy.
The EU co-financing for the project between 2016 and 2023 is equal to 50% of the costs for the exploratory tunnel and 40% of the costs for the main tunnels.
The estimated cost of the project is more than €8bn ($11.53bn).
The new tunnel is estimated to reduce the travel time between Innsbruck and Fortezza from 80 minutes to 25 minutes. It will reduce traffic congestion on the road network.
Signalling and communication
The BBT will use ERTMS/ETCS Level 2 and GSM-R systems. ERTMS (European rail traffic management systems) comprise two components, the European Train Control System (ETCS) and Global System for Mobile Communications-Railway (GSM-R).
The tunnel will have the capacity to withstand up to 1,300°C. A fire extinguisher will be installed within the tunnel.
The project includes conducting surveys to determine flow rate, electrical conductivity and temperature as well as chemical analysis of water levels and tests on drinking water for the presence of bacteria.
Herrenknecht, a German manufacturer of tunnel boring machines, provided the 200m-long, 1,800t Gripper TBM for the excavation of the exploratory tunnel.
The company also supplied three Double Shield TBMs for the excavation of the main tunnel tubes and the exploratory tunnel A belt conveyor system developed by the company in collaboration with its subsidiary H+E Logistik was installed at the Wolf access tunnel.
HeidelbergCement’s subsidiary Calcestruzzi is providing ready-mix concrete for the Italian side of the tunnel project.
GEODATA, a provider of sustainable and value-added solutions for underground projects, was engaged to review the final design and develop guidelines for future design phases between 2011 and 2013.
The instrumentation and monitoring of the BBT were handled by Encardio-rite, a real-time safety monitoring solutions provider.
CHRYSO, a supplier of admixtures and additives for construction materials, received a contract to perform soil consolidation operations using the jet grouting technology and freezing technique.
Amberg Group, an above and underground design and construction services company, was contracted to provide design services for the project, from initial design to final design. The company also conducted geotechnical site supervision on the Austrian side of the project.
Getzner was contracted to provide vibration protection and sound control services for the BBT.
A joint venture (JV) comprising engineering services and consulting company Lombardi was selected to provide planning, tendering, design details, and geotechnical monitoring services for the construction lots H21, H41, and H51.
SYSTRA SWS provided services such as TBM selection, detailed design based on building information modelling (BIM), mechanical, electrical and plumbing (MEP) design, and technical assistance during construction for the Mules 2-3 lot of the BBT.
RK Safetec, part of Rhomberg Sersa Rail Group, was contracted to provide access control, video surveillance, wireless communication system, alarm systems, and staff location for the Wolf access tunnel.
Akron received a contract to perform tasks associated with structural and geotechnical monitoring during tunnel excavation, as well as acoustic emission monitoring for rockburst risk management and geophysical surveys carried out as excavation progresses on the Mules 2-3 lot.
A consortium between Strabag and Webuild, formerly known as Salini-Impregilo, was awarded the contract for the construction of the H33 lot in 2014.
A contract worth €651m ($739m) for the H41 construction lot was awarded to a consortium comprising Webuild, CSC Costruzioni, Implenia Österreich, and Implenia Schweiz in November 2021.
Porr Bau, a subsidiary of Porr, is responsible for the 600m stretch of the H21 Sill Gorge lot.
Isarco, a consortium of Webuild, Strabag, Consorzio Integra, and Collini Lavori, received a contract for the H71 construction lot in October 2014.
Webuild along with its consortium partners Ghella Cogeis, Oberosler Cav Pietro, and PAC received the Mules 2-3 construction lot in May 2016, while Swietelsky Tunnelbau was selected for the H52-Hochstegen construction lot in December 2021.