Images Courtesy Of BBT-SE Unless Otherwise Stated
For the best part of 20 years, the excavations for the Brenner Base Tunnel project in the Alps have been advancing, first with mobilisations and then small access and the early exploratory tunnel works before moving to open up the long, twin main rail tubes and various caverns, in different contract packages and to various scheduled timelines.
Now the hard rock excavations are more than 90% completed on the joint Austrian-Italian high-speed rail project. Main excavations continue on only one Lot while final lining works proceed elsewhere.
PROJECT PLAN
The full Brenner project is 64km in length, most of which is the principal underground structure – the 55km-long Brenner Base Tunnel. An extra rail tunnel portion at the north end of the Austrian-Italian project will make the full Brenner link the world’s longest underground rail connection, says bi-national project developer BBT-SE.
The Brenner Base Tunnel project layout comprises twin 8.1m-wide, single-track main tunnels, running mostly parallel at 40m-70m apart, under a variable Alpine overburden of up to 1720m. The main tubes are connected every 333m by cross passages. In addition, the long rail tunnel has three larger tunnel complexes that will be Emergency Stops along the long underground link.
The Brenner tunnel configuration also includes a narrower third tunnel, generally positioned between and about 12m below the main tubes. The exploratory tunnel helped to investigate geology along the route during planning and early construction phases of the main works; in future, during the operational phase of the rail link, the tunnel will provide drainage capacity and access/service capabilities.
Aligned almost North-South, Brenner Base Tunnel will be generally straight. Geology has large variations along the route and includes: quartz phyllite; Bunder slates (containing dolomites, quartzites, anhydrites, greywacke sandstone and other slates); gneiss; and, Brixner granites. There are also some fault zones, and the high overburden along the route presented some risk of squeezing ground conditions. At the ends, near the portals, the geology is variable again and there is much existing transport infrastructure.
Brenner is a strategic link in the expanding European network of high-speed rail links (TEN-T) and its design speeds for freight and passenger trains are 120km/h and 250km/h, respectively. The tunnel sits in the Scandinavian-Mediterranean Corridor of TEN-T, specifically in the Munich-Verona portion. As such, the bi-national rail transport project also has a wider European dimension, which brought long-term EU funding support from the outset.
The estimated cost of the project, in January 2023 prices, is almost Euro10.54 billion, says BBT-SE – with construction costs, including fit-out, accounting for 81% of that budget. The total budget also includes about Euro1.1 billion for risk provision, which raises the accounted-for share to almost 92%. The balance is allowance for monetary and inflation adjustment, according to BBT-SE.
The total budget was revised and approved in 2023 following inflationary increases caused by higher energy and materials costs, in part caused by covid pandemic difficulties. The new total was calculated by a joint Austrian-Italian inflationary method, established specifically for the project. Previously, budget estimates were calculated nationally and therefore differed – Euro9.6 billion (Austrian method, 2021) versus Euro 8.8 billion (Italian, 2021); in 2017 the corresponding figures were Euro9.3 billion and Euro 8.4 billion.
DREAM AND PROGRESS
A vision for some kind of major tunnel at the Brenner Pass was aired in the mid-19th Century. By the early 1970s the initial dream was being etched into modern reality. The International Union of Railways commissioned a study for a rail link that would having a long base tunnel through the Alps. A few feasibility studies had been performed by the late 1980s. The early 1990s saw the EU looking at supporting the development of a network of rail corridors across Europe, and some priority projects were listed. Soon, feasibility studies and preliminary plans got underway on what would be Phase 1 of the Brenner project. The completion of Phase 1 brought Austria and Italy to sign a treaty to build the project.
With the greenlight, Phase 2 activities for the Brenner project were undertaken over 2003-2010, focused on finalising the plans and performed the Environmental Impact Assessment (EIA). Before that stage had ended though, Phase 2a activities had begun, leading to early works for the exploratory tunnel, performed over 2007/8-2013. Before those were completed, Phase 3 started in 2011, involving early construction activities such as large and long access tunnels in different locations to enable tunnelling for the main tubes and further works on additional sections of the exploratory tunnel.
Over 2014-2018, at different times and locations, the scheduled tunnelling works for the main construction lots on Brenner Base Tunnel got underway. Employing both hard rock TBM and Drill & Blast tunnelling methods, by 2020 more than half of all excavation work for the project had been completed, including excavations for access and logistics tunnels, and initial sections of main and exploratory tunnels.
By mid-2025 more than 200km of tunnels had been excavated for access and supporting tunnels as well as for the two main and the exploratory tubes along the alignment.
While some of the rock excavations finished a few years ago others were completed more recently, including a flurry of finishes in 2025. In September, final breakthrough on the exploratory tunnel created the project’s first continuous, full-length tube on the Brenner Base Tunnel.
Most of the Brenner Base Tunnel project is now built. Relatively, there is not so much tunnelling left to do, at least in terms of the overall project – but the remaining large excavations are both substantial and extensive in their own right, and focused on one Lot, H53, Pfons- Brenner, on the Austrian side, closest to the border with Italy.
Elsewhere, final tunnel lining works and some cross passages excavations are underway.
Long sought after, the enhanced Brenner link is set to be completed in the near future. High-speed rail services are planned to start running through the long Alpine base tunnel in the early 2030s.
PROJECT – CONTRACT ORGANISATION
The tunnelling packages on the full Brenner project are organised in the following Lots, in sequence from north to south along the alignment (Austria to Italy):
For the TEN-T route link tunnel:
- H33 – Tulfes to Pfons
And, for the Brenner Base Tunnel:
- H21 – Sill Gorge
- H41 – Sill Gorge to Pfons
- H53 – Pfons to Brenner
- H52 – Hochstegen
- H61 – Mules 2-3
- H71 – Isarco River underpass
In Austria, a long connecting tunnel (Lot H33) allows the TEN-T high-speed route to join the Brenner Base Tunnel while bypassing the northern portal, at Innsbruck. The tunnels are not aligned and the connection is a relatively sharp turn. The connecting tunnel’s own portal is farther north, at Tulfes. This connecting link utilises a prior rail bypass tunnel while adding various extra tunnels plus the necessary tie-ins to the northern end of Brenner Base Tunnel.
The Brenner Base Tunnel itself extends from the Innsbruck portal southwards to Italy, to the Fortezza portal (in Italian, or Franzensfeste in German). It is far longer than the connecting tunnel and, as such, its Lots hold the majority of the underground works needed for the entire Brenner link project, including H21 to H71 as listed, plus other tunnels.
Some of the Lots are near the portals, which provided them with access for the works, but most of the excavations for Brenner Base Tunnel are along the alignment and required intermediate adits to get underground for main tunnelling activities.
These access and logistics tunnels are (north to south): Connecting Tunnel:
- Austria: Ampass Access Tunnel; part of H33
And, for the Brenner Base Tunnel:
- Austria: Ahrental Access Tunnel;
- Austria: Wolf Access Tunnel;
- Italy: Mules (Italian) or Mauls (German) Access Tunnel;
The alignment also includes construction of three large tunnels complexes as Emergency Stop, at:
- Innsbruck;
- St Jodok;
- Trens
In addition, to help prepare for all these works, the planners strategically invested in an exploratory tunnel to run the full length of the Brenner Base Tunnel.
With early excavations in different sections along the alignment, the main tube and cavern works were prepared for, and the tunnel will have a future in the operational stage of Brenner Base Tunnel.
The Lots, the tunnelling works involved and their status – main excavations completed or works still underway – are discussed in the following sections.
CONNECTING TUNNEL
H33 – Tulfes to Pfons
Off-line from the main Brenner Base Tunnel, the Connecting Tunnel is, as noted, vital to the TEN-T highspeed rail corridor that passes Innsbruck. It is tied-in to the Brenner Base Tunnel and the necessary and complex package of tunnel works involved have been executed under contract Lot H33.
The H33 package has taken an existing, wide, bypass rail tunnel and alongside built a parallel tube (emergency tunnel), and linked them with cross passages. In addition, the existing main bypass tube is linked across (via split tunnels) to the main tubes of the Brenner Base Tunnel, in the Ahrental area. The complex Lot also then built most of an Emergency Stop’s large tunnels at Ahrental on the Base Tunnel, and from there also bored south a portion of the exploratory tunnel.
The contractor was a JV of Strabag and Webuild (formerly Salini-Impregilo), which was awarded the works in mid-2014. By Q3-2021 the excavations in the package of underground works had been completed, involving a total of 43.3km of tunnels.
While Brenner Base Tunnel has two main tubes, the Connecting Tunnel will take both lines of trains in a single tunnel. As such, for emergency support and safety purposes, the parallel 9.7km-long tunnel was needed, of smaller width (35m2). This is Tulfes Emergency Tunnel, excavations for which were performed by Drill and Blast method, with three advancing faces: the portal at Tulfes; and, from the Ampass intermediate adit (built previously, over 2011-13, following early exploratory works under Lot 32) from where faces were blasted in opposite directions.
The Tulfes Emergency Tunnel was completed in mid-2017. That Summer also saw completion of the Connecting Tunnel’s tie-in tunnel to the Brenner Base Tunnel, at Ahrental.
The package of works also included a stretch of main tunnels, along with part of the Innsbruck Emergency Stop large tunnels, as noted, also at Ahrental.
At the southern end of the H33 package of tunnelling works there was a portion of the exploratory tunnel bored southwards from Ahrental towards Pfons, and beyond. This was not the first part of the exploratory tunnel at the north end of the project. Previously, a portion was built at the north end, in the Sill Gorge area, over 2009-2013, and it was constructed south as far as Ahrental. Therefore, the H33 package was extending the exploratory tunnel farther south from Ahrental.
The southern extension of the exploratory tunnel was performed by an approx. 8m-diameter gripper TBM, which bored almost 17km southwards – beyond Pfons and almost to the next Emergency Stop (St Jodok, in Lot H53). It was bored through quartz phyllite and slate, starting in Q3-2015 and finishing in mid-2019.
Like the other sections of exploratory tunnel works, undertaken earlier, the purposes was to gather geological data in advance of the main tunnel works. In this case the works that would come later, at Ahrental and for the main tubes either side of the exploratory tunnel, would be performed in a separate Lot (H41). The TBM met some challenges with collapsing ground that had to be repaired but the works also saw adjacent portions of the main tubes also constructed, ahead of the TBM drives to be done later, on H41.
BRENNER BASE TUNNEL
H21 – Sill Gorge & Portals
At the north end of the Brenner Base Tunnel, just south of Innsbruck, saw complex works performed and completed for Lot H21 Sill Gorge (Sillschlucht in German, or Gola del Sill in Italian). The mixed package of construction activities included bridges, river engineering works, roads and the Innsbruck Portal structures and initial, some tunnels – up to 130m lengths of the two main tunnels (Viller Berg section), leading to cut-and-cover stretches (Silltal section).
The contract was led by Porr Bau. All excavation works on the lot were completed by mid-2022. Most of the main works on the Lot, though, were above ground and involved a wide range of civil engineering structures over a relatively short distance involved in the narrow gorge, already packed with infrastructure.
All works were completed at the end of 2024. It would be a few months later that the short lengths of main tunnels would see their first connection with a portion of longer main tube tunnel, constructed as part of the neighbouring Lot H41 Sill Gorge-Pfons.
Prior to these. Works, though, more extensive tunnelling works were undertaken in the Sill Gorge area for the Brenner project, focused on excavating the north end of the exploratory tunnel. Excavation of that 5.4kmlong portion of the exploratory tunnel began off the main tunnel alignment in late 2009. Construction of the 26m2 tube finished four years later, in late 2013, within the alignment and near Ahrental.
H41 – SILL GORGE TO PFONS
The H41 Lot was the first, and most northerly, package of extensive tunnelling works on the Brenner Base Tunnel itself. While H33 Lot was mostly offline for the Connecting Tunnel and tied-in to and started to open up the Ahrental area, and the H21 Lot prepared the north tip of the Brenner Base Tunnel, the works then for H41 were mostly major excavations that linked back to H21 while completing the Ahrental tunnels (including the Innsbruck Emergency Stop) and then advanced southwards with the main tubes.
The tunnelling package was executed by a JV of Implenia, Webuild and CSC Costruzioni. Contract award was in late 2021 and ground-breaking took place in mid-2022.
Excavations in Lot H41 involved both TBM and Drill and Blast tunnelling, primarily, for the two main tubes.
The underground works involved constructing a total of approx. 22.5km of main tunnels plus 38 cross passages (approx 2.3km in total), and completion of the Emergency Stop (mostly built in Lot H33). Access underground was via Ahrental adit – itself built earlier, in the E41 Exploratory Lot, over 2010-12.
The Ahrental Access Tunnel and Innsbruck Emergency Stop arrangement created a T-junction, in effect, on either side of which different tunnelling methods were employed: to the south side of the Emergency Stop the main tubes had TBM launch chambers constructed, from which two shields bored southwards, towards Lot H53; to the north side of the Emergency Stop the main tubes only would be blasted towards the Innsbruck portal, and tie-in to the H21 works.
The Ahrental adit allowed short, initial portions of the main tubes to be blasted, in 2015. This readied the location for tie-in of other tunnels, such as from H33, and to proceed later from the area.
The JV contractor launched the two 10.4m-diameter single-shield TBMs on their drives south from Ahrental starting in May 2023. TBM ‘Lilia’ would drive 8.1km southwards for the East main tube, and sister shield ‘Ida’ would bore 8.4km for the West main tube. The TBMs very recently completed their drives – ‘Ida’ in late August 2025, and ‘Lilia’ in early October.
For the Drill and Blast works, the JV excavated distances of 2.3km and 3.4km for the East and West main tubes, respectively. Tunnel blasting for the East tube finished in September 2024 and for the West tube the excavations were completed in May 2025.
The H41 package did not include much exploratory tunnel works. The northern portion had been built already, and the H33 Lot built the stretch at Ahrental and extended it much farther south.
H53 – PFONS TO BRENNER
The remaining large-scale tunnelling works in Austria for the Brenner Base Tunnel are almost totally in the H53 Lot (Pfons to Brenner), which runs to the border with Italy. The H53 contract package was won in early 2023 by a JV of Porr Bau, Marti and Marti Tunnel, and was the last major package of tunnelling works to be awarded by BBT-SE on the Brenner project. Excavations on the main tubes are well advanced. The Lot only has a relatively short portion (1.6km) of the exploratory tunnel and the tunnelling involved has been completed.
The main tunnelling works are taking place mostly from the Wolf Access Tunnel, with faces being advanced in opposite directions – northwards towards the Pfons area (H41) by TBM, and southwards by Drill and Blast to the border with Italy.
The large tunnels for the St Jodok Emergency Stop are located in the area where the Wolf adit meets the alignment of the Brenner Base Tunnel.
Construction works for the Wolf adit and associated access and logistics tunnels (Padaster, Saxen) were undertaken in 2011- 2015 (Lots E51 & E52), far earlier than the main works which began in 2019.
In late 2024, almost a year and a half after award of the H53 Lot, the two 10.4m-diameter TBMs were launched on their northward drives towards Pfons. The TBMs are ‘Wilma’ (West main tunnel) and ‘Olga’ East main tunnel. Each is to drive 7.6km-long sections of the main tubes, resulting in nearly 15.2km by mechanised excavation. The shields had completed nearly half of their total tunnelling by the end of October 2025.
The Drill and Blast works on the main tubes (80m2) are to construct almost 10km in total, advancing southwards, towards Brenner Pass. Blasting on the main tubes began early in 2024, before the TBMs were launched on their drives in the opposite direction. Nearly three-quarters of the total blasting for the main tubes has been completed. Final lining works are following.
On the Lot, more than half of excavations for the cross passages have been completed.
Earlier tunnel works in the Pfons to Brenner area were performed under Lot H51, over 2018-2020, by a JV of Porr, Hinteregger, Condotte and Itinera. That contract was terminated by BBT-SE.
With some works completed and looking optimise the remaining main tunnelling works – and to risk manage dealing with a relatively short but geologically challenging section, at Hochstegen (investigated earlier, during 2016-18, in Lot E53) – the client restructured the package. The challenging section was put into its own Lot, H52, and executed in advance of the balance of remaining main tunnel works, put under H53.
H52 – HOCHSTEGEN
As noted, the H52 package involved undertaking works in a geologically challenging area over a relatively short distance, located in the Hochstegen area, close to the Austria-Italy border. The location extends over approx.
500m and the challenging geology involves mainly water-bearing rock layers. Risk management led to this section of Brenner Base Tunnel being handled separately from the balance of work remaining for the Pfons- Brenner package, in which had it had been initially included.
The contract was awarded to Swietelsky Tunnelbau in late 2021 and official ground-breaking took place before mid-2022.
The package of works in H52 included, in total: 860m of exploratory tunnel (60m2); 3km of main tunnels (60m2 – 80m2); 600m of connecting tunnels; and, 300m of cross passages. Significant injection works were performed to improve the rock mass, reduce inflows and the risk of changes to groundwater levels in surrounding protected areas.
All tunnelling works in the package were completed by late 2023.
H61 – MULES 2-3
Immediately south of the border, and dominating the Italian side of the tunnelling works for the Brenner Base Tunnel, is Lot H61, Mules 2-3 (Mauls 2-3, in German). It is also the largest construction package on the entire project. Started in late 2016, it has been undertaken by a JV of Webuild (which holds the previous contractor on the Lot, Astaldi (until 2021)), Ghella, Oberosler Cav Pietro (up to mid-2018), Cogeis and PAC.
The contract package includes the works for main tunnels, the Mules Emergency Stop, cross passages, a long section of exploratory tunnel, and access and logistics tunnels.
All excavations have been completed, except for some cross passages, and final lining works which are underway.
Well in advance of those works though was some early tunnel construction – for the Mules access tunnel over 2007-9; and, the southernmost portion of the exploratory tunnel, over 2008-10. Altogether, preparatory works took place over 2008-11, then more underground works for various tunnels, under Lot Mules 1/Mauls 1 (also called Exploratory Lot E62), were performed over 2011-15. Thereafter, Mules 2-3 would hold the main tunnelling works to be execute for the project.
Mules adit is in the area where two tectonic plates (European and African) press together as the Periadriatic Seam, making the locality one of the most important fault zones in the Alps, says BBT-SE. The adit was on one side of the fault zone and on the other was the future location of the Trens Emergency Stop’s large tunnels. Early works, therefore, were undertaken to carefully explore the fault zone and firmly open up the short stretches of the main tubes and the exploratory tunnel that were to pass through. Those excavations were performed by mechanical diggers over late 2011 to mid-2015.
The southern end of the exploratory tunnel was bored by a 6.3m-diameter double-shield TBM, launched from the south portal at Aicha (Italian) or Aica (German) which is off the alignment of the Brenner Base Tunnel.
The TBM bored northwards through Brixner granite for about 10.4km from mid-2008, coming into the main tunnel alignment and finishing, near the Mules/ Mauls Access Tunnel, in late 2010. The works were performed by Seli (now part of Webuild), as part of a JV with Pizzarotti, Bilfinger Berger, Alpine, Beton -und Monierbau, Jager, Collini, and Condotte.
With completion of those early works completed, preparations continued for the main tunnel and associated works to follow, in Lot H61.
In early 2017, soon after the Lot was awarded, one of the tasks was to build a 3.8km-long branch tunnel from Mules adit. The branch ran parallel to the main alignment of the Brenner Base Tunnel and then turned in to the location of the future Trens Emergency Stop.
Benefits of the branch would be two-fold: access during construction; and, backup in the operational life of the Base Tunnel. The branch tunnel was completed in mid- 2020 and works began to open up the Emergency Stop and other tunnels. Final lining works for the Emergency Stop were completed in mid-2025.
It was also in 2017 that excavations for two main tunnels began, with faces blasting in opposite directions from Mules adit. But blasting would only continue on the south side, as far as the boundary with the project’s southernmost Lot, H71 (Isarco River Underpass, and portals). Excavation of H61’s south tunnels finished in late 2020.
To the north side of the adit only a short distance of blasting was performed and double-shield TBMs were then launched to complete the main tunnels up to the border with Austria. The two northbound drives started in mid-2019. TBM ‘Virginia’ finished the 14.2km-long drive (East main tunnel) in early 2023, and ‘Flavia’ (West main tunnel), having had a brief difficulty at a fault zone, completed in May 2025 – marking the end of TBM tunnelling on the Italian side of the project.
The remaining tunnelling works in the Lot were for a portion of exploratory tunnel, extending northwards from Mules adit and Emergency Stop tunnels area. The tunnel was bored by 6.8m-diameter double-shield TBM ‘Serena’, which was launched on the 14km-long drive to in mid-2018 and finished in late 2021.
In September, the Lots H53 and H61 achieved final breakthrough on the exploratory tunnel, marking finally the end of excavation on the tube. It is the first tunnel where excavation has been completed fully on the Brenner Base Tunnel.
H71 – ISARCO RIVER UNDERPASS & PORTALS
At the southern end of the Brenner Base Tunnel is Lot H71, Isarco River Underpass and portals, via which trains will, after a short distance, reach the first station in Italy – Fortezza (in Italian) or Franzensfeste (German).
The Lot may be short in comparison to a number of other packages on the major rail transport project but, like Sill Gorge at the north end of Brenner Base Tunnel, it has had many and varied complex civils works.
The package was undertaken by a JV of Webuild Italia (formerly Salini-Impregilo), Strabag, Consorzio Integra, and Collini Lavori. The contract package was awarded in late 2014 to connect the Base Tunnel with the existing Brenner rail line as well as Fortezza station.
While most of the works in the Lot are on the river valley floor, the package also included a total of 4.5km of main tunnels, cross passages, and 1.2km of connecting tunnels, plus shafts and an access tunnel. All excavations were completed by the end of 2022.
Prior to the tunnelling works there was a need for extensive preliminary ground engineering due to the fluvioglacial non-compact sediments in the river valley. Extensive jet grouting was performed. In the treated area four shafts were then sunk, up to 20m-25m depth, from which tunnel blasting then proceeded northwards.
The tunnelling challenge also included rerouting a state road plus works under a motorway, rail line and also the river bed. Artificial ground freezing (AGF) was also employed to enable the tunnel construction works. Construction works on the Lot were completed in late 2023.
BENEFITS OF EXPLORATORY TUNNEL
While smaller in width, the strategic investment in an exploratory tunnel for the entire length of the Brenner Base Tunnel proved valuable before and during the excavations for the parallel main tubes but will also support operations and safety, long-term.
This argues for the “usefulness and necessity” of an exploratory tunnel in deep-lying tunnel systems (Marini, Lussu & Vendetti, 2024).
On Brenner, it was constructed in five stretches (three in Austria, two in Italy) and average advance rates of 10m/day-12m/day could be achieved, despite various challenges at different locations. The short portions constructed through the Periadriatic Seam, in Italy, and the Hochstegen water-bearing geology, in Austria, are not included in the total of major sections excavated, but that in no way diminishes their importance in locally solving the tunnelling challenges and, strategically, removing bottleneck risks from the schedule.
The authors (Marini, Lussu & Vendetti, 2024) say the exploratory tunnel works provided data and insights that supported procurement, advance redesign and relocation of some of structures (e.g., caverns, crosspassages).
The investment also enabled tunnelling challenges to be anticipated in specific locations and to enable suitable construction measures to be prepared – including, at times, use of the exploratory tunnel to secure rock mass ahead of the main tunnels.
