ÖBB Semmering base tunnel
Steinhaus, Province of Styria, AT
ÖBB Infrastruktur AG
IG Bilek + Krischner GmbH/Graz, Austria
INSITU Geotechnik ZT GmbH/Graz, Austria
G. Hinteregger & Söhne Bauges.m.b.H./
February 2013 to January 2014
Once completed, the new Semmering base tunnel between Mürzzuschlag and Gloggnitz in Austria will help to eliminate one of the most critical infrastructural bottlenecks along the Baltic-Adriatic Axis (BAA), one of Europe’s key north-south rail corridors. The BAA links the up-and-coming economic regions of Poland, the Czech Republic and the Slovak Republic with business centres in Austria and northern Italy. To dispose of the spoil from the drive for the approx. 27 km long Semmering base tunnel, a tip was created in rough, inaccessible terrain along the Longsgraben dell. Prior to the start of operations at the spoil tip, extensive earthwork and roadbuilding measures were needed in the surrounding area. One of the measures involved raising the Longsbach stream bed to a level some 50 m higher than the previous bed. With the new bed running along the western side of the Longsgraben dell over a length of more than one kilometre, extensive reinforcement works were performed to stabilise the adjoining steep slopes.
The up to 25 m tall steep slopes with an inclination of 75° were constructed using the cost effective, resource efficient geosynthetic-reinforced soil (GRS) system. Due to its flexible design principle, GRS also offers significant advantages over other systems in case of inhomogeneous subsoil conditions and difficult topographies. The steep slopes were built using the wrap-around method. The high product flexibility and favourable stress-strain behaviour of these geogrids make them ideal for use in technically challenging projects, as witnessed by their proven performance on countless construction schemes Worldwide. On this project, the geogrid reinforcement layers were installed at average intervals of approx. 55 cm. The fill material between these was placed and compacted in two layers. To prevent erosion of the fill material at the front slope surface, fine-meshed erosion control grids were installed on the inner side of the lost steel formwork.
The efficiency of site operations was vastly improved through the incorporation of preformed steel mesh as lost formwork, which also ensured the achievement of a perfectly level front face. The steep reinforced slopes were designed as a temporary measure (for 15 years) because, once the areas along the slopes have been filled with tunnel spoil, they will no longer need to fulfil a structural function. This limited service period allowed an economical sizing of the geosynthetic reinforcement. Despite the tight construction window, the works were completed on schedule.