Koralm Tunnel, Construction section 3

The Koralm Tunnel (KAT), scheduled to open in 2025, will establish a link between two Austrian state capitals: Graz and Klagenfurt. The project can be listed alongside the Brenner Base Tunnel and Vienna Central Station as a key element of the Baltic-Adriatic corridor, which links Danzig in Poland with Ravenna, Italy. The Koralm Tunnel has two tunnel tubes lying 25m to 50m apart and connected by cross passages every 500m. In 2013, Austrian railway company ÖBB Infrastructure AG awarded the contract for the third and final construction section, KAT3, to PORR Bau GmbH. This major project involves construction of the two tunnel tubes with a total bore length of 21km. With an overall order volume of 297 million euros, KAT3 currently represents PORR’s largest construction site.

From a geological point of view, the construction section can be divided into three distinct zones. For the first 4km or so, the PORR miners bored through sedimentary rock from the Neogene period, alternating between clay, silt and gravel. This was followed by a 400m stretch through the main Lavanttal fault before reaching a crystalline section of fine-grained gneiss, mica schist and marble that extended to the end of the construction section.

Work has been ongoing on both tunnels since November 2013. The team began in the Lavanttal in Carinthia and is boring through the Koralpe Massif towards Styria. The north tunnel is being bored throughout its 10,474m length by the “KORA” tunnel boring machine (TBM). The conventional drilling & blasting and excavating technique was used for the south tunnel. There was an existing exploratory tunnel around 7,000m long here; this has been extended downwards, so that the exploratory tunnel forms the upper two-thirds of the final cross-section, i.e. the tunnel dome. The bottom third, the side walls and tunnel floor, are the result of this widening process. For the remaining 2,700m to reach the edge of the construction section, the full cross-section was bored. The cross passages between the north and south tunnels are being created at 500m intervals via cyclic advance: excavation/drilling & blasting.

In the next section of advance, the existing exploratory tunnel running from TM 4,400 to TM 7,098 was widened, once again using drilling & blasting. The excavated material was then transported out via the crusher and conveyor belt system.

The side wall and tunnel floor advance then continued from the portal to TM 4,400. Due to the challenging geology, preliminary drainage measures were necessary in some sections: boreholes in the existing dome cross-section were extended in advance and placed under a vacuum. In some areas, up to 570,000l was shifted within 24 hours.

In the Neogene zone, a Jetcrete dome invert had been installed along the full length of the exploratory tunnel. This invert was removed using an extra tunnel excavator with a cutting wheel, an innovative solution which represented a significant optimisation for the tunnel boring process, as it meant that cutting could be largely carried out independently of the time-critical path.

The passages linking the two tunnels were constructed in parallel with the cyclic advance through the south tunnel. The lining segments through the Neogene zone were shored up with steel reinforcement. In order to take the forces until the in-situ concrete inner lining had been installed, the upper and lower reinforcement traverses were pre-tensioned using presses over the supports; these were retained until the solid reinforced lining block had reached the required strength. The steel reinforcement was then removed, transported to the next cross passage, and reassembled.