WALTZING HOME AHEAD OF SCHEDULE

Austrian tunnellers beat the clock on town by-pass project

Austria and tunnels go together like Strauss and Viennese waltzes. The country's tunnellers, using innovative techniques and the best equipment, have a history of recording world-class performances - such as cruising to completion two months early on a recent project.

The New Austrian Tunnelling Method (NATM), developed between 1957 and 1965, put the country firmly at the forefront of tunnelling development.

The technique - largely based on exploiting the natural load-bearing capacity of rock and using a variety of swift and effective support measures, according to the type and quality of the rock - quickly became the standard for tunnelling excellence worldwide.

Now an increasing number of beautiful towns and villages in mountainous Austria are being by-passed by tunnels to reduce traffic congestion, noise, fumes and pollution of the alpine air.

These tunnels are built with the quick and sure expertise accumulated by experience. A current example is the Schönberg tunnel, about 40 minutes drive from Salzburg, which was started in February last year and completed in April this year - a comfortable two months ahead of its scheduled completion date.

Traffic diversion

Just under three kilometres long, the tunnel is part of a USD 60 million traffic-diversion project around the town of Schwarzach, on the road between Bischofshofen and Zell am See.

The companies involved in the joint venture, Vienna-based Stuag and Universale Bau of Salzburg, are two of the best-known and experienced long-distance tunnelling operators in Austria.

Site Manager Helmut Lutz, of the Stuag company: "We've been using Atlas Copco drill rigs almost exclusively for the past10 years."

For them, the rapid drilling work was a result of favourable rock conditions and two new Atlas Copco Rocket Boomer drill rigs, a three-boom 353S model and a two-boom 352S unit, both equipped with COP 1838 rock drills. The two machines were rented with an option to buy.

For the book-keepers, the relatively uncomplicated and precise drilling meant little or no cost overruns. And for Atlas Copco the successful project was solid confirmation that the equipment more than met the challenge.

Reasonable rock

The rock, although poor by international standards, is considered reasonable for Alpine conditions, consisting of various forms of highly-fractured phyllites, mixed with green shale.

Says Site Manager Helmut Lutz of Stuag: "We were able to drill an average of 6.6 rounds a day with one round equal to a length of between 1.6 and 2.2 metres, depending on the conditions. We had set ourselves a goal of four to five rounds a day."

The area through which the tunnel passes is familiar ground for Mr Lutz. He worked on a railway tunnel project not far from the latest site and he knew what to expect in terms of rock type and formation.

"We were also familiar with Atlas Copco machines," he continues. "We've been using them almost exclusively for the past 10 years. If we'd changed to different products, it would have taken time for our mechanics and operators to get acquainted with them.

"We're used to not having problems - and that was one of the major reasons for staying with Atlas Copco, even though there were some attractive competitive bids. We trusted Atlas Copco's ability to fulfil the contract."

And the combination of equipment quality, reliability, high capacity and good service enabled the drillers to finish two months ahead of schedule, working three eight-hour shifts per day.

Keeping costs down

Keeping costs down was an important part of the project and Mr Lutz continues: "We managed to keep within the budget by hiring experienced crews, by good preparation and by using new equipment which meant fewer mechanical problems and maximum availability."

The cost factor was one reason the contractor decided to rent the best rigs available and both rigs have now been bought by the companies. Universale Bau has the three-boom rig and the two-boom rig is to be bought by Stuag.

Star performer leaves the tunnel: Lights ablaze, one of the Boomer rigs comes out through the east portal.

For Mechanical Foreman Johann Lerner of Universale, the most important aspect for his servicing crew was the fact that his mechanics had all worked with Atlas Copco rigs before. "The main advantage was the uncomplicated operation and the quick provision of spare parts," he says.

Praising the co-operation with Atlas Copco service representatives, he pointed out: "Someone was on call 24 hours a day, seven days a week, and there were monthly equipment checks."

Easy servicing

Mechanic Franz Seidl says he liked the Boomer rigs for their relatively easy servicing intervals. "Normally we brought them in for servicing after 400 operating hours. That's about 80,000 drill metres. This tunnel involved about 500,000 drill metres."

Excavation Foreman Joseph Lederer, of Universale Bau, said that one of the most important features on the three-boom rig was the raiseable cab.

"This gives the operator a better view of the drilling direction in the roof section. A better view means less overbreak and lower costs when applying the concrete lining."

The two Rocket Boomer rigs were also the first generation of electronically-controlled machines. The operator has to use only three joysticks instead of the standard forest of levers. Two joysticks position the booms and a further controls the drilling.

The rigs were operated entirely manually with a drilling pattern consisting of 80 to 100 holes 45 mm in diameter, depending on the properties of the rock. The contour holes were placed at a maximum spacing of 30 cm.

Co-operation and teamwork during the project: From left, Mechanical Foreman Johann Lerner, Mechanic Franz Seidl and Atlas Copco's Peter Aigner with the Rocket Boomer 353S drill rig.

Another important contribution to the low cost and rapid completion of the drilling operation was the decision to use Atlas Copco's four-metre-long Super Swellex bolts to reinforce the roof. Helmut Lutz says these were quick to install and extremely reliable.

All the rock bolts used in this operation were Super Swellex from Atlas Copco, as were 60% of those used in the side walls. Other reinforcement methods included the installation of wire mesh, steel arches and shotcreting. The whole tunnel will be concrete lined.

First choice

Swellex is now rapidly becoming the first choice for designers and contractors using the NATM technique, replacing the more traditional grouted rebars.

Rock bolts are fundamental for stabilizing the rock mass between the excavation phase and final lining. Therefore, NATM requires bolts that are not only reliable, easy and fast to install, but which also give immediate support along their whole length and in a wide range of rock.

Atlas Copco representative Peter Aigner says the Schönberg project is also a good example of how contacts between the supplier and the contractor can be strengthened, both professionally and personally.

"Both sides knew that they were confronted with tough market conditions but we tried to treat each other in the fairest way possible," he says.