Brenner access: H7-1 Fritzens-Baumkirchen

The Fritzens-Baumkirchen underground valley route is part of the Brenner access section in conjunction with the Munich-Verona railway axis. Contract H7-1 is located approx. 15 km to the east of Innsbruck and is divided into various lots on account of the different construction methods that have to be applied. These are from east to west:

1,020 m by cut-and-cover, box cross-section and vault cross-section
520 m by the top cover method
430 m by mining means
2,000 m by top cover method, including 90 m starting pit
594 m by cut-and-cover with tub cross-section.

The entire tunnel is located in the soft soils of the Inn Valley. These are mainly varying sandy Inn gravels and heterogeneously composed fluvial sediments derived from brooks flowing into the Inn Valley, with the Inn gravels interfingering with the fluvial sediments. In addition, gravels from a glacial terrace were encountered as well as silty-sandy riverbank sediments and old landfill deposits contaminated by carbohydrons. The groundwater table fluctuates by roughly 2.5 m - with fluctuations of up to 0.5 m within 2 days and up to 1 m within 5 days being observed. The depth of the groundwater table of relevance for the construction work varies between 15 and 35 m. The layers located underneath form no groundwater barriers but are only slightly permeable. It was up to the contractor to decide on the type of excavation pit The JV plumped for a sheet pile wall with underwater concrete base. The sheet pile wall is to be secured with a layer of bolts. The underwater concrete base slab is back-anchored - at the same time, it must not be connected with the final structure constructively. The top cover is placed on overlapping drilled piles with a 1.2 m diameter and covered by up to 3 m. On account of the high groundwater table, the excavation is undertaken below the top cover and by mining means under compressed air. The maximum pressure amounts to 1.35 bar. The interior works follow up the drive continuously so that it is possible to minimise compressed air losses thanks to the bulkhead that is planned at a distance of 250 m. The crown of the mined tunnel will be tackled first followed by the bench and the floor. After completing the excavation the permanent lining will be installed. Owing to the shallow overburden and the subsoil, the drive will be executed protected by horizontal columns produced by jet grouting, which are created alternating with the drive. Quite apart from the geological and hydrogeological challenges, there is also the fact that a number of the excavation pits are located very close to the existing railway lines and the tunnel to be produced by mining means has to pass beneath Fritzens Station in shallow overburden. As a result, an extensive track monitoring measurement programme was installed and all operations are closely coordinated with the Austrian Railways.

Country: Austria
Region: Tyrol
Tunnel utilization: Traffic
Type of utilization: Main-line rail
Client: Brenner Eisenbahn GmbH BEG
Consulting Engineer: Obermeyer Planen & Beraten, IGT- Ingenieurgemeinschaft Geotechnik und Tunnelbau, Stella & Stengl ZT GmbH, IMM Prof. Dr.-lng. B. Maidl-Dipl.-lng. R. MaidI
Contractor: Strabag AG, Hochtief Construction AG, Ed. Züblin AG
Main construction method: Open
Type of excavation: Cut-and-cover
No. of tubes: 1
Tunnel total length: 5,285 m
Contract Volume: approx. € 138 million
Construction start/end: 2005 to 2009