While drill and blast excavation makes steady progress from the north end of the exploratory tube for the 55km long Brenner Base Tunnel to narrow the gap with the TBM-bored southern end, bids are currently being assessed for a central section that will pass through a major fault zone with high overburden. To add to the scope of work on the Periadriatic Line package, the contract will also include some minor preparatory underground works for the main rail tunnels construction on the Austrian-Italian rail link.
The greenlight for main construction on the trans-Alpine rail project was given earlier this year to the developer and owner Brenner Base Tunnel (BBT), following the formal funding commitment of the Austrian Government which came a few months after those from Italy and the EU. Austria and Italy are each covering just over a third of the cost of slightly more than EUR 8bn (USD 10.8bn), while the EU has committed to meet 30 per cent.
However, it will not be until about 2015 that tender calls are issued for the main tunnel works, which will be the biggest contracts on the scheme. Three contract packages are anticipated at this stage, and construction work is programmed to start in 2016 to enable the scheme to be finished by 2025-26.
Until the procurement of the main works are underway a series of further tunnelling contracts will be let for the various access adits along the alignment, such as the Wolf-Two package, the Ampass adit, the Mauls (Mules) branch adit, and the works to take the exploratory tunnel through the tectonic zone. There are also, potentially, two lots at either end of the twin-bore running tunnels to establish the portals and excavate 1km along each 8.1m i.d. tube. In parallel, if not before, there are also the current works to complete – the Innsbruck-Ahrental works and the Wolf-One package.
Route and geology
The route of the Brenner Base Tunnel will be generally straight, almost north-south, below the rugged Alps to link the Innsbruck bypass in Austria with Franzensfeste (Fortezza) station in northern Italy. The link will be 64km long, including the existing bypass which will join the under construction Lower Inn Valley rail project.
To run as single-track tubes about 70m apart, and with a service tunnel positioned between and below them, the main rail bores will be linked by three cross passages per kilometre. Near the ends of the main tubes there will also be evacuation tunnels. The scheme also includes three multi-function underground stations, one at Innsbruck bypass, as mentioned, and two others to the south, at about 20km intervals – St. Jodok (near Steinach in the middle of Brenner Base Tunnel) and Trens (close to the Mauls [Mules]).
Constructing this triple tunnel layout, stations and access adits calls for difficult Alpine geology to be negotiated, not least the Periadratic Line which is a complex fault zone where the European and Adriatic (African) tectonic plates crush together.
Besides hydrogeological challenges in the zone and elsewhere along the alignment, the route also presents large variations in strata – Quartz phyllite, Bunder slates (containing dolomites, quartzites, anhydrites, greywacke sandstone and other slates), gneiss and Brixner granites. With such geology and overburden extending up to approximately 1,600m, there is also potential for squeezing conditions.
Presently, BBT expects almost three-quarters of excavation for the main tunnels to be performed by TBM, the balance by drill and blast – which will also be used for the stations and adits.
Excavation progress
The Brenner Base Tunnel is a project of many phases, and parts, one of which has already been completed – excavation of the southern end of the exploratory tunnel, in the Aicha-Mauls (Aica-Mules) package.
As an extension of the earlier site investigation phase, the exploratory tube will provide key data and experience for design of the main tunnels. BBT is also in discussions to learn lessons from tunnelling experience on the 57km long Gotthard Base Tunnel, in Switzerland.
Once completed, the Brenner rail link will use the exploratory tunnel as a service tube, also helping with drainage as possibly carrying power and data cables.
Aicha-Mauls
The 10.4km long, southernmost section of the 5.6m i.d. exploratory tunnel is the Aicha-Mauls (Aica-Mules) package, which was bored by TBM.
The 1.8km long Mauls lateral access adit helped to retrieve the TBM but is also needed for further exploratory, and later main, tunnel work as well as allowing a branch to be excavated to support future construction of Trens station.
Launched in early 2008, the 6.3m diameter double shield machine completed the almost 32m2 cross-section tunnel by November 2010. The excavation was performed by Seli as part of a JV with Pizzarotti, Bilfinger Berger, Alpine, Beton –und Monierbau, Jaeger, Collini Impresa Costruzioni, and Societa Italiana per Condotte d’Acqua.
The advance of the northward bore was successful but it did experience some higher rock strengths (up to UCS 220MPa) and resulting cutter wear, which drastically slowed progress. Then the TBM encountered high pressure groundwater up to 27 bar in a fault zone, which damaged a stretch of already built concrete segmental lining. Recovery works included ground stabilisation and replacement steel rings along the section affected by the unanticipated anisotropic stresses.
The contract package also included a 400m long spoil transport tunnel.
Innsbruck-Ahrental
While the TBM drive was coming to a finish in the south, at the opposite end of the exploratory tunnel – in the Innsbruck- Ahrental package – the initial excavation work on the 26m2 excavated tube was getting underway. The Ahrental adit will provide access for construction of the Innsbruck station and main tunnels.
Finding a large, obstructive boulder at the portal was not the best beginning but progress using drill and blast along the 5.6km long section has been steady for the JV of Strabag and Porr Tunnelbau. The minimum finished width of the exploratory tunnel is 5m.
Blasting got underway in earnest by February 2010, and in the first year approximately 2.2km was excavated. By the beginning of September 2011, a further seven months on, another 1.1km has been blasted, completing almost 60 per cent of the length under the contract. BBT says the daily advance is 8m-10m with up to five blasts.
BBT adds that, based on surface geology, it is predicted that several fault zones will be met but none have been encountered so far though there have been thin shear bands, along which quartz phyllite has been ‘crushed to a clay-like state’. Groundwater conditions have been much less of a problem than anticipated, with flows of less than 0.1l/s.
The package also includes the 2.4km long Ahrental adit with a 10 per cent Slope and 90m2 cross-section. Located close to Wipp valley fault, the rock mass is ‘remarkably fractured’ and further complications in the geology came from gravel layers and loose rock in the upper part of the profile. Proceeding in relatively short, 1.3m advances, the face is being opened in crown-bench-slab stages, and in just over a year, half of the adit has been built.
The design and planning consultant is ILF, and construction supervision is by a JV of Bernard Ingenieure, Bernd Gebaur and Intergeo Basustellenkoordinator.
Wolf-One
The Wolf system of lateral access adits is in the middle of the project, and will create access for construction of St. Jodok station and future main tunnel works. The scale of the access works has led to them being split into two packages – Wolf-One and Wolf-Two. Planning and design of the Wolf sections has been handled by IC Consulenten, and construction supervision is provided by a JV of HBPM Ingenieure and BWB.
Initial tunnelling work on the Wolf-One package began in April and has involved multiple parallel excavations totalling approximately 1.9km by drill and blast. The package involves three different tunnels – the 200m long Wolf adit, the 702m long Padaster tunnel and the 1,000m long Saxener tunnel.
Wolf adit has a gradient of 10 per cent and cross-section of 104m2. Padaster is a spoil conveyor tunnel with eight per cent slope and cross-section of 84m2, and leads from the Wolf adit to the deposit site. The Saxener tunnel, which has a gradient of 10 per cent and cross-section of 65m2, will take traffic onto the construction site. Construction of the Wolf adit and Padaster tunnel is complete, and by early September more than a third of the Saxener tube had been blasted. The contractor is a JV of Swietelsky Tunnelbau and Swietelsky Baugesellschaft.
Excavation Plans
Of the packages coming up, that of the Periadriatic Line on the exploratory tunnel is the shortest, at a mere 1,350m long, but the geological conditions will be the most challenging. The tunnel is long enough to bridge the tectonic fault zone. While the package also includes some main tunnel work it is only preparatory, and will not involve construction of parallel short stretches of main tunnel through the weak, high stress zone.
Tunnelling will be done by drill and blast as well as mechanical excavation, and the section will link to that built by the TBM which stopped, as planned, short of the fault zone to be dismantled in a cavern carved at the end of the Mauls (Mules) adit.
There will be further packages for works on the exploratory tunnel, but BBT says the tube will not be fully built before excavation starts on the main tunnel contracts in 2016.
Development of the main tunnels is to commence with packages at each end to create the portals and about 1km of each tube. Tenders for the packages are to be issued around 2012.
No plan has been given for how the stations are to be built – if part of main tunnel packages or separate contracts.
In the meantime, the most immediate other tunnelling works coming up are the excavations just starting at the Ampass section, and those to come at the Wolf-Two and Mauls branch tunnel.
For the Ampass section, at the north end of the Brenner scheme and off the main north-south axis, a JV of Strabag and Porr Tunnelbau began work last month to build the 1.4km long adit.
The tunnel is an access tube with a cross section of 35m2, and is needed for excavation of the rescue tunnel to parallel to the Innsbruck bypass.
The Wolf-Two package will create a 3.5km tunnel by drill and blast to complete the local adit system started by Wolf-One. Planning and design is being performed by a group of consultants – IC Consulenten Ziviltechniker, GWU Geologie – Wasser Umwelt, BWB Ingenieurburo, Lombardi- Reico Srl, and Hbpm Ingenieure.
The Brenner Base Tunnel runs north-south from Innsbruck to Fortezza Brenner Base Tunnel stations, portals and adits
