Patrick Reynolds, Freelance Reporter

A green light for the Brenner Base Tunnel to push ahead into the main building phase has come from Austria's approval of finance at the beginning of this month. Italy, its partner, gave its commitment to funding for the bi-national rail link through the Alps late last year. With challenging geology facing tunnellers, an exploratory tunnel is already under construction - with one end finished and excavation progressing at the other - and the project is gearing up for procurement of more contracts in the near future. Packages for the main running tunnels are due to be let for a start of their construction in 2016. Patrick Reynolds reports on the latest developments on one of Europe's most important and far-reaching transport infrastructure projects.

Fig 1. Planned design and configuration of the project

Last week the Austrian Government gave its financial commitment to the next phase of developing the 55km long Brenner Base Tunnel. With the backing of Italy and the EU already in place, the finance is now assured to at least 2015 and ushers in a burst of procurement activity that, over the coming months, will see calls issued to contractors for bids to build more access adits as well as the third, and possibly the most challenging, section of the exploratory tunnel.

In addition, it is anticipated that bids could be sought this year for the portals and initial short stretches of the twin main tubes of the bi-national rail scheme. The largest tunnel packages, for some of the main running tunnel reaches will be put to tender until in about 2015 for construction to start in 2016.

Getting funding agreed for procurement of all works needed to that point was vital and is now secure. While the approach of Austria and the EU is to give periodic assessments and assurances on funding for the trans-Alpine link, the Italian Government approved its contribution for the full duration of the project, in November 2010. The latest budget for the entire underground rail link, which is to be completed in 2025-6, is slightly more than €8 billion (US$10.9 billion), at 2010 calculation. About 30% of the total budget is to be met by the EU with the balance

Fig 2. Planned alignment of the North-South link of Europe

funded equally between Austria and Italy. The Brenner Base Tunnel is a strategic connection in the Trans-European Network (TEN) of transport infrastructure priority projects.

Last November also saw completion of the first tunnelling works on the project. On the Italian side of the border a TBM drive for the initial southern section of the exploratory tunnel at Aicha-Mauls was completed by Seli, working as part of a JV. The JV's contract included the Mauls intermediate adit, which is one of four accesses in the mountains from which sections of the exploratory and main tunnels will be excavated. Earlier last year excavation of the exploratory tunnel started at the opposite north end in Innsbruck, Austria.

End of the exploratory tunnel celebrations - Nov 2010

Construction progress has been reasonably good so far on the exploratory tunnel, although there was a short section of difficulty on the southern bore, and a major boulder blocked the portal at the outset of the northern end drill+blast works. The geology, including groundwater, has been, overall, slightly better than expected in the underground works performed to date, TunnelTalk was told by a spokesman for the massive project's client, the Brenner Basis Tunnel SE (BBT).

But building the rail link will be no simple task. Besides the challenges of variable rock types, with groundwater and high overburdens towards the centre of the alignment presenting the risk of potential squeezing conditions, the route also crosses a major fault zone where the European and Adriatic (African) tectonic plates press together. These anticipated conditions gave rise to the practical need for an exploratory tunnel, which will provide additional design and programming benefits to the excavation of the main tubes.

The next section of the exploratory tunnel, to be tendered this year, will be for a drill+blast drive through the fault zone.

The Project
The Brenner Base Tunnel through the European Alps is one of the TEN transport routes across the heart of Europe that runs the 2,200km between Berlin and Palermo in Sicily. Just to the north of the Alpine crossing in Austria is another key section of the scheme, the Lower Inn Valley railway improvement project that comprises a chain of tunnels to place a major of the line underground.

Drill+blast of the Innsbruck-Ahrental exploratory tunnel package

While the Brenner Base Tunnel is to be 55km long, the total south-to-north alignment will stretch from Franzensfeste (Fortezza) station in north Italy to join with the Innsbruck bypass in Austria, forming an underground connection with a total length of 62.5km. The bypass will then join the Lower Inn Valley section of the TEN route.

Innsbruck and Franzensfeste (Fortezza), at Ch.0.00km and Ch.55.00km respectively, will be linked by twin, single-track running tunnels of at least 8m i.d. (cross section, 64-79m²), set about 70m apart, and linked by three cross passages per kilometre.

A service tunnel will run between and about 12m below the main tubes. The scheme is to include three multi-function stations (MFS), approximately 20km apart, at the Innsbruck bypass; St Jodok, south of Steinach; and Trens, north of Mauls/Mules. Evacuation tunnels will run of the main tubes before each end of the Brenner Base Tunnel (Figs 1 and 2).

Geology along the alignment comprises zones of quartz phyllite, Bundner slates (containing dolomites, quartzites, anhydrites, greywacke sandstone and other slates), gneiss, and Brixner granites. Slightly south of the mid-point the overburden will rise to the highest point of about 1,600m (Fig 3). There are also faults and hydrogeological challenges, not least of which is at the so-called Periadriatic Line or tectonic plate boundary at about Ch. 47-48km and just north of the Brixner granites.

Fig 3. Geological section through the Brenner Massive

In addition to site investigation studies and geological borings taken over many preceding years, it was determined that the project would need an exploratory tunnel to help better determine the geological challenges. The exploratory tunnel will be used for drainage and eventually as the service tunnel. It is also planned to carry power and data cables.

Progress
The first tunnelling works on the project called for a 10.5km long section of the exploratory tunnel at the south Italian end between Aicha and the 1.8km long Mauls (Mules) intermediate adit. This was awarded to the ATB Consortium Tunnel Brenner, which includes Pizzarotti, Bilfinger Berger, Alpine Meyreder, Beton-und Monierbau, Jaeger, Seli, Collini Impresa Costruzioni, and Societa Italiana per Condotte d'Acqua.

In a subcontract, Seli undertook the northward drive through granites and amphibolites with a 6.3m diameter double shield TBM. The drive was inaugurated in spring 2008 and was expected to finish in about 20 months in early 2010. Some faults and groundwater were expected in defined areas with more than half the drive in Class III rock, said Seli. Much of the 5.6m i.d. lining was to be shotcrete and rockbolts, the balance using 1.5m long 5+1 rings of 200mm thick precast concrete segments.

Segmentally lined TBM exploratory tunnel

Advance however was slow, with only a third of the distance completed at about half way through the contract time. Rock strengths of up to 220MPa created high cutter wear and low penetration rates. Then, in August 2009, a stretch of the tunnel lining through a fault zone more than 6km into the drive suffered damage from groundwater pressures of up to 27 bar. Seli said the client's geotechnical specification had not anticipated extreme anisotropic pressure on the lining.

Recovery works required ground stabilsation, removal of deformed and damaged rings, polymer injection for part of the affected stretch and installation of steel rings along the length and a little beyond the fault zone. The shielded TBM restarted in December 2009 and in just under a year, in September 2009, holed through into a dismantling cavern at the junction of tunnel and adit to successfully complete the works.

A few months after the TBM had re-started, a JV of Strabag and Porr was getting underway with drill+blast excavation on the second, 5.6km long exploratory tunnel section at the opposite end in Austria. First task, though, was to remove a large boulder sitting directly in the portal area in unconsolidated rock.

Since tunnelling began at the Innsbruck end in February/March 2010, the JV has blasted 2.2km, or about 40%, of the planned drive. It is also constructing the 2.4km long Ahrental adit, and has advanced about 400m on that heading.

Procurement
With the funding commitments in place, the client is now pushing forward with the next round of procurement. This focuses on two lateral access adits, Wolf and Ampass; the next section of the exploratory tunnel; and early works on the main tunnels. The level of tendering and contract award activity is set to jump this year.

The Wolf adits are located in the middle of the Base Tunnel alignment and are a multi-tunnel arrangement. Tenders have been called the first contract package is expected to be awarded within the coming weeks. The package is for a total 1.8km of drill+blast work on three headings: some 100m of the main adit; the 700m Padaster spoil conveyor tunnel; and the 1km Saxener truck access tunnel.

The second package at Wolf is to be tendered in the coming months. The package calls for 3.5km of drill+blast excavation to complete the adit-tunnel system in the local area and reach the alignment of the exploratory tunnel. The various adits on the projects have a standard cross section area of 92m².

A further tender planned for the near future is for the 1.4km long Ampass adit. The client expects both contract award and start of construction during this year.

Tenders are also due to be called in the coming months for the next section of the exploratory tunnel that will run north for about 1km from where Seli's TBM drive finished. Although not long, the work will blast through the Periadriatic Line, which is anticipated to be the most challenging stretch of tunnelling on the exploratory tube.

Further contracts on the exploratory tunnel are also to be tendered before long.

For programming reasons, the exploratory tunnel will not be fully completed by the time excavation on main running tunnel packages gets underway. While information and data from the exploratory drives are being used to update and refine the planning and designs for the main tunnels, discussions have also been underway to learn from the lessons and experience of tunnelling works on the 57km long Gotthard Base Tunnel, in Switzerland, which holed through to complete its first running tunnel late last year.

Winter start on the Innsbruck-Ahrental package in Austria

Currently, for Brenner Base Tunnel, the client sees the majority of the twin main running tunnels being completed as TBM operations. The relative proportions are expected to be about 70:30 in favour of TBM excavation – or about 77km of TBM advance and 33km of drill+blast work.

Three contract packages are presently planned for the largest of the main tunnel works. Details of their locations, boundaries and contents have yet to be finalised ahead of a planned call for tenders in 2015.

In addition, there are a number of preliminary lots to complete a mix of other, small tunnel works on the scheme. These are understood to include: a long branch adit off the existing Mauls access tunnel, to help with the logistics of constructing the Trens MFS; and initial short works on the main twin tunnels to establish the portals and the first kilometre of each tube at each end of the project. Tenders for the first short section of the running tunnels are due to be called this year or by 2012, for construction to get underway in about 10-18 months. This gearing up of the main Brenner Base Tunnel project through the Alps is thanks to the recent funding commitments made by Austria and Italy that have paved the way forward.

References

Exploratory bore for Brenner Baseline through - TunnelTalk, November 2010
Prelude to building Brenner Baseline - TunnelTalk, March 2009
Lower Inn Valley approach to Breen Base Tunnel - TunnelTalk, Feb 2008
Gotthard TBM safely across the Piora Mulda - TunnelTalk, Nov 2008
Epoch breakthrough on the Gotthard Baseline Tunnel - TunnelTalk, Oct 2010

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