Tunnel vision. Drilling through rock is "child's play" compared with the challenging conditions construction workers are facing as they extend the tram line from Linz to Leonding in Austria.

Daily traffic jams are causing chaos for commuters struggling to get around the North Austrian city of Linz, with work on the extension of one of its main tram lines turning the city into a construction site. The deadline for the completion of the tunnel linking Linz railway station to Harter Plateau in Leonding is autumn 2011 - but there is still much to be done before then. Once completed, traffic woes will be a thing of the past as it will take just 10 minutes to travel from Harter Plateau to the centre of Linz on tram line 3. A further extension is planned to take place after 2011, which would allow commuters to use the tram to travel as far as Traun, Ansfelden and Nettingsdorf.

Sand, gravel and sediment
Construction work started on the 1.3km tunnel in March 2009 and it has been a painstaking process since. Drilling the tunnel in a built-up area through geologically and geotechnically difficult loose soil has proven an immense challenge for the manpower and technology employed to get the job done. Linz Linien GmbH commissioned the construction firms G. Hinteregger & Söhne, Östu-Stettin and Dywidag to form the consortium responsible for the construction work.

As project manager, Manfred Heimgartner is in charge of all construction work above and below ground. Heimgartner has a wealth of experience in transport construction, having also been responsible for the construction of Linz's local traffic hub from 2001 to 2003. "Our biggest challenge is that the construction work is being carried out in a highly built-up part of the city, which therefore involves many restrictions," says Heimgartner. "When all is said and done, such extensive work cannot be carried out unnoticed. Added to this are the difficult geological conditions. Under the gravel washed up by the Danube and Traun in the interglacial period, there is sediment known as schlier deposited by the sea around 25 million years ago. The tunnel construction goes through both the water-filled gravel and the fine-grained schlier sediment."

Two parallel tunnel shafts - 953m and 938m long with a cross-section area of 35m² join a 370m long tunnel and 70m long access ramp that is being built using the 'cut and cover' technique. Around 600m of the single-track tunnel shafts have been bored through the schlier, which is geologically difficult to work with. "A few weeks ago, tunnelling experts from Norway visited the site," says Heimgartner. "They were amazed to see the conditions which we face and said that tunnelling through rock, as they do in Norway, is child's play in comparison." The actual tunnelling and excavation work should be completed by May 2010. Once the concrete lining has been applied to the inner shell of the tunnel and the raw shell of the service building been built using the 'cut and cover' method, the handover to the rail construction firm is planned for October 2010. Tram line 3 should finally become operational in the autumn of 2011. The tunnel's safety concept complies with modern standards, with cross-cut walkways every 250m, sprinkler systems and fire alarms. Some 100 workers have been employed to deliver the project, working in three shifts. The cost of the initial construction is about €45 million, and the total project cost is €150 million.

Creative transport logistics
Bernegger GmbH was appointed subcontractor for the project because of its experience in building and demolition activities. As it was not authorised to transport the excavated material through residential areas, a new solution had to be found to be able to work as rationally and effectively as possible. A cable excavator would traditionally have been used in this situation, but a special attachment designed by the company, Wimmer, allowed Bernegger to carry out the work with a Volvo EC460C crawler excavator. "There were clear advantages to using this type of excavator," says Kurt Bernegger of Bernegger GmbH. "In addition to its high resistance, the hydraulic excavator with its kinematics is far superior to a cable excavator." Bernegger GmbH Managing Director Helmut Lugmayr Dipl-Ing and engineer Christian Fröhlich explain further: "We have to remove around 200,000 tonnes of material excavated from the tunnel by June 2010. In addition to the advantage of being able to work faster and more efficiently, the excavator will still look like a well-oiled new machine after this operation and will be able to provide further services either in our Pfaffendoben production site or on large earthmoving sites, despite having been in operation for quite a number of hours up until the completion of this job."

A delicate situation
The tram line extension project poses an unusual challenge for the tunnel construction workers. Constant attention must be paid to any surface subsidence or movement, especially as the minimum cover above the tunnel ridge is only about 5m. The groundwater lies at a depth of five to seven metres in the construction site area. The schlier in which most of the tunnelling work is being carried out serves to block the water. A total of more than 80 boreholes are required to pump the groundwater out (approximately 50,000m³ a month) in order to advance with the tunnel. According to ÖBB regulations, the rails of the west railway line which lie just a few metres above the construction site must not sink more than 20mm. So far, the maximum average subsidence has been 18mm. Heimgartner has at his disposal a 24 hour hotline directly to the ÖBB rail headquarters to raise the alarm for the immediate closure of the line in the event of an emergency - in this case if the rails sink two millimetres more. But he is hoping it won't come to that...

Austrian tunnel

Text: Elizabeth Tilley