Brenner Base Tunnel: A Masterpiece of Engineering in Europe

Prof. Konrad Bergmeister, Austrian CEO of Brenner Base Tunnel SE since 2006, gives an overview of the state of construction as well as insights of the project, and TBM Manager Sebastian Grüllich, Joint Venture Strabag (Technical Leadership)/Salini Impregilo of construction lot Tulfes-Pfons, explains details of the construction work in his lot.

Both were interviewed in Innsbruck and on the Tulfes-Pfons job site, Austria, by International Freelancing Journalist Roland Herr from Germany/Thailand, E-Mail herrroland@t-online.de.

The Brenner Base Tunnel (BBT) is a straight, flat railway tunnel connecting the two European countries Austria and Italy. It runs for 55 km between Innsbruck portal (in Austria) and Fortezza portal (in Italy) through the Alps and is – from the Innsbruck bypass (the Inn Valley Tunnel) to Fortezza – with altogether 64 km the longest railway tunnel in the world.

“This tunnel is a masterpiece along the North-South-Corridor (Scan-Med-Corridor) in Europe, belonging to the TEN (Trans-European Network for transport, energy and telecommunications) of the European Union,” explains Prof. Konrad Bergmeister. “The tunnel itself has a very high relevance for the three European countries Germany, Austria and Italy since the Scan-Med-Corridor with a total length of 9,000 km – starting in Finland, running through the three countries and ending at the island of Malta – is the longest of the nine Trans European Network corridors connecting all parts of Europe. ”

Due to the fact that the European Union considers the Brenner Base Tunnel a high priority infrastructure project, 50% of the costs for the exploratory tunnel and 40% of the costs for both main tunnels will be financed separately. The remaining 50 to 60% of the costs will be covered by Austria and Italy each.

In May 1994, a railway bypass was opened south of Innsbruck, known as the Inn Valley Tunnel. This 12.7-km tunnel links to the Brenner Base Tunnel. Passenger and freight trains along this stretch will therefore not only travel through the Brenner Base Tunnel, but for a few kilometers, through the Inn Valley Tunnel as well. After being connected with the BBT, this line will be in total 64 km long.

The BBT is a complex tunnel system, composed of two main tubes, each 8.1 m wide, running 40 to 70 m apart from one another. These tubes are each equipped with a single track for one-way train traffic in each tube. In the case of unforeseen events, there are three emergency stopping areas in the Brenner Base Tunnel where the trains can halt underground in the tunnel. The two tubes are linked every 333 m by connecting side tunnels, which can be used in the case of emergency as escape routes. This configuration conforms to the highest security standards for tunnels.

The slope in the base tunnel is 6.7% on the northern side and 4% on the southern flank of the Brenner. The apex height is 790 m above sea level, lying 580 m below the Brenner Pass itself (1,371 m).

Ongoing Construction Work

Four construction sites are currently in operation, two in Austria and two in Italy. (Updated information on the progress of the work is posted on the project website: www.bbt-se.com/en/tunnel/construction-progress. The excavation progress reports of the individual tunnel structures in the various construction lots are updated weekly and a work progress is shown on a graph.

Depending on the geological conditions, the Brenner Base Tunnel is being excavated through rock with 30% by drill-and-blast and 70% by mechanized tunneling, including at least six tunnel boring machines at the same time.

Construction Lot Tulfes-Pfons

The Tulfes-Pfons lot, worth 380 million Euro, was awarded to the Strabag/Salini Impregilo bidding consortium in Summer 2014. Excavation work began in September 2014 and will last until Spring 2019.

“The construction lot includes about 40 km of tunnel excavation with Tulfes emergency tunnel, Innsbruck emergency stop, main tunnel tubes, connecting tunnels and the Ahrental-Pfons exploratory tunnel. About 25 km of that, we excavate with drill-and-blast and 15 km with mechanized tunnelling. The excavated volume will be about 2.3 million m3 and about 430,000 m3 structural concrete for the inner lining will be used,” explained TBM manager Sebastian Grüllich from Strabag, the technical leading company.

The main tubes have been excavated by drill-and-blast according to the New Austrian Tunnel Method (also known as Sequential Excavation Method) and are completed. The ceremonial first volley was set off in mid March 2015 and the construction included 3 km of main tunnels with an excavated cross-section of about 70 m2. Construction of the connecting, safety and logistic tunnels between the BBT and the existing Innsbruck bypass started in Summer 2015. These tunnels were excavated by drill-and-blast with a cross-section of about 115 m2 and are also completed. The construction lot of the exploratory tunnel includes a 15 km stretch of that tunnel between the Ahrental junction point and the town of Pfons. The open gripper TBM with about 8 m diameter and a length of 200 m started at the end of September 2015 and will excavate the 15 km of the exploratory tunnel in southward until Spring 2019.

Construction Lot Mules 2 to 3

The construction lot “Periadriatic seam,” carried out between October 2011 and Summer 2015, comprised the excavation of 3.7 km of the main tubes and 1.5 km of the exploratory tunnel. The crossing of the Periadriatic fault line, one of the biggest geological fault lines in the Alps, was accomplished without a hitch.
In May 2016 Mules 2 to 3, the biggest construction lot of the Brenner Base Tunnel project, was awarded to the bidding consortium Astaldi S.p.A., Ghella S.p.A., Oberosler Cav Pietro S.r.l., Cogeis S.p.A. and PAC S.p.A with a contract volume of 993 million Euro, starting with construction in September 2016. This lot reaches from the construction lot “Isarco River Underpass” up to the Austrian border. In the course of a 7-year construction period, 39.8 km of the main tubes and 14.8 km of the exploratory tunnel will be excavated, including the emergency stop in Trens and its access tunnel, as well as the bypasses which connect the main tubes every 333 m. A total of about 65 km of tunnels will be excavated. Once Mules 2 to 3 is finished, all excavation activities in the Italian project area will be complete.

The blasting work to connect the Mules access tunnel with the future emergency stop in Trens started in December 2016. The 3.8 km long tunnel with a cross-section of approximately 80 m2 will presumably be excavated by 2021.

The drill-and-blast excavation works on the exploratory tunnel northwards resumed northward in mid February 2017. After about 600 m, the rest of the stretch, all the way to the border, will be excavated with a tunnel boring machine (TBM). The driving operations are expected to be completed by the end of 2021.

The drift of the main tubes northward resumed mid March 2017. The first section of the stretch up to the future location of the Trens emergency stop, where new assembly chambers will be built, is going to be excavated by drill-and-blast in 2019. From here, TBMs will drive the remaining section of the tunnels northward up to the border of the construction lot, which should be reached during the first months of 2022. The first blast of the southward drift took place in April 2017. The southern end of the construction lot is expected to be reached in mid 2020.

Construction Lot Isarco River Underpass

The southernmost construction lot of the Brenner Base Tunnel was awarded for 303 million Euro to the RTI Salini-Impregilo S.p.A., Strabag AG, Strabag S.p.A., Consorzio Integra and Collini Lavori S.p.A. bidding consortium in October 2014. This lot links the Brenner Base Tunnel with the existing Brenner line and the railway station in Fortezza. The work is scheduled to be completed in 2022.

Since in this section the tunnel tubes are just a few meters below the surface, a portion of the activities pertaining to this construction lot will be carried out building artificial tunnels. Furthermore, as loose fluvioglacial materials and the groundwater layer will be crossed, it will be necessary to adopt specific ground consolidation procedures including ground freezing and jet grouting.

The current phase (main construction phase I) provides for the construction of four shafts, 20 to 25 m deep, to access the rock faces of the Isarco River Underpass. Furthermore, it includes the completed excavation of the access tunnel on the right side of the valley by using drill-and-blast, and of part of the main tubes and the connecting tunnels, to be built with the drill-and-blast method.

Exploratory Tunnel

A special feature of the Brenner Base Tunnel and often discussed is the exploratory tunnel running from one end to the other. This tunnel with 5 m diameter lies 12 m below and between the two main tunnels. Prof. Bergmeister explained the conception and why it was the right decision to do so:

“Having excavated more than 55% of the exploratory tunnel, we are now sure that it was the right decision to explore the unknown rock mass in the Alps with this tunnel before we start with the construction of the big main tunnels. To explain this statement, we have to identify four different issues.

“First of all we did in advance of the construction work a lot of vertical exploration drillings. But these are only very few points in a huge rock mass, identifying certain geological parameters without knowing exactly, how the rock will behave. We have to distinguish very clearly between a tunnel which runs very close to the surface or the construction of a Base Tunnel. In our specific case – and what we learned also from the construction of the other big Base Tunnels like the Lötschberg or Gotthard in Switzerland – was, that we are able through all the different investigations which are normally done in a so-called preparation phase, that there are only some estimations about the possible behavior and not more. With the exploratory tunnel, first of all, we get a much better identification of the minerals and the rock behavior.

“The water drainage system is a second issue. Because of the exploratory tunnel, we have a completely independent water drainage system which will be used not only during the construction phase but mainly also during the operational phase. And this allows us to clean the drainage system whenever it is needed, periodically, and we do not have to stop the operation for example during such a maintenance phase.

“A third issue came out while building the exploratory tunnel itself. When we run into a difficult zone it is possible – and we did that already – to inject directly from the exploratory tunnel into the vicinity of the main tunnel some specific injection mortar. Also we are able to construct vertical shafts or ramps directly to the position of the main tunnels and try to drill-and-blast, to excavate the main tunnel from the exploratory tunnel. That means that we have an independent logistic system in a completely independent connection tube to the surface, which is quite useful for safety purposes.

“A fourth issue, which was originally quite neglected, occurred now during the preparation phase for the rail equipment. Long railway tunnels always have the problem that all the rail equipment has to be placed in the so-called connection tunnels or directly in the main tunnels. This means, that you have to close one tunnel during an inspection or maintenance phase in order to do the intervention and to be able to go into the tunnel and to do some specific maintenance work. In our case we are studying now, which equipment can be placed in the exploratory tunnel for later use as a service tunnel. Then we will be completely independent from the operation issues in the main tunnels.

“The total costs of the exploratory tunnel in our specific case are about 12% of the total costs of the tunnel. This issue has been quite carefully discussed and repeatedly questioned. Nevertheless, we have been able to show, that – by using these advantages – we will have finally in terms of a life cycle management some savings and not additional costs. Since we will be able to use specifically this continuous tunnel as a third tunnel tube for maintenance and inspection purposes.

“Also, Switzerland came up with a new guideline for long tunnels, longer than 10 km, in 2018. Here they proposed the idea to have a third tunnel in terms of inspection, maintenance and exploring rock mass before construction of the main tunnel.

“Finally, it can be said that we took at that time the right decision.”