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NATM 30 years ago

TUNNELLING

Porr Tunnelbau technical and contractual advisor for tunnelling, Harald Lauffer describes the original Tauern Tunnel project, which was his first job after leaving university.

'Tunnelling started in 1971 with the breakthrough in 1974. The big cross section and difficult geomechanical conditions that included heavily squeezing rock was a major challenge that triggered many innovative solutions.

The first was to drive through the talus deposits made up of cohesionless limestone gravel with blocks. The cross section had to be divided into top heading, bench one, bench two and invert. Mining of the top heading was done by hand supported by a small tracked loader while a bigger tracked loader excavated the benches and invert.

The second big challenge appeared when entering the rock part of the tunnel. Instead of the expected sound and stable limestone, tunnellers encountered heavily squeezing phyllites. The full face excavation was immediately stopped and a part face excavation organised.

We learned the hard way that it was necessary to install a great number of 6m to 12m anchors to control rock deformations. Big deformations destroyed the shotcrete shell and endangered miners and equipment. Against some opposition the team tried installing deformation joints in the shotcrete shell, which proved to be very successful. Afterwards, deformation joints became a state of the art method when driving in heavily squeezing rock conditions.

Innovative use of materials and equipment took place prior to and during tunnel driving, including the use of a predecessor to modern drill rigs. This had three booms mounted on a tracked undercarriage. Also, replacing part of the cement by fly ash in the concrete mix greatly improved the quality of the inner lining.

At first the plan was to excavate most of the 6.4km length from the northern portal using a rail transport system for mucking and supplies. But tunnel from the south began again to compensate for the delay caused by the problems with squeezing rock.

Despite the rail transport having relatively little diesel power the ventilation system was at the limit of its capacity when the breakthrough nally took place.'

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