Despite collapses and unpredictable fault zones, top-heading breakthrough is expected later this month on the 1.92km long Herzoberg NATM road tunnel in the Styrian town of Modriach, Austria.

Tunnelling on the US$45M project was not easy, Gerald Pischinger, Gruppe Geotechnik Graz (3G) engineering geologist, told T&TI: “Special difficulties arose from water bearing fault zones (up to 20l/s) and flat faults which accompanied the heading over long stretches.” On one occasion they were faced with a collapse of 220m3.

The strong-weathered metamorphic rock – gneiss, micaschist, amphibolite, eklogite and pegmatitegneiss – further complicated the drilling works for the three grouted 12m pipe umbrellas to overcome this fault zone. The project was delayed by over two months.

Drill+blast using Atlas Copco drill rigs was used in fair rock conditions, while Liebherr excavators with Volvo loaders were used in faulted rock. The shotcrete lining is being applied by two Putzmeister mobiles.

Herzobergtunnel JV is the contractor, comprising Strabag, Hinteregger & Söhne, Porras-dag Tunnelbau, and Östu-stettin. 3G provided consulting services.

The 10.6m diameter tunnel will provide a second route for the A2 – an important link between Vienna and Southern Europe.