Twin Robbins TBMs Play Key Role in Austrian Hydro Project

Robbins_Pfunds_0206 Power Play

While the most typical applications for tunnel boring machines today are related to water, sewer or transportation projects, the use of TBMs for hydropower projects dates back to the development of the technology itself. In 1952, James Robbins, founder of The Robbins Company, developed the first practicable TBM, which was used on the Oahe Dam diversion project in South Dakota, ushering in a new era in the tunneling state-of-the-art.

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While earlier TBMs had been used with limited success on other projects – including the Hoosac Tunnel in Massachusetts and an 1800s attempt on the Channel Tunnel – it was at the Oahe Dam where the TBM as we know it emerged.

It is fitting then that when German contractor Hochtief needed to carve a 20-plus km tunnel for a hydropower project in Austria, it turned to The Robbins Company. The $500 million project for Gemeinschaftskraftwerk Inn (GKI) is being constructed in the Tyrol area near the border of Austria and Switzerland. It consists of the construction of a weir and intake facility along the River Inn in Orvella, Switzerland, and a powerhouse near Prutz, Austria, and, of course, the hard-rock tunnel connecting the two.

The project is being constructed through a joint venture of Austrian power companies TIWAG and VERBUND, and Swiss power company Engadiner Kraftwerke. Work began in 2014 with a projected completion date of 2018. Upon completion it will have an installed capacity of 89 MW and an annual generation of about 400 gigawatt hours. GKI officials say the project represents a major step in achieving Tyrol’s power autonomy as well as achieving the goal of the European Energy Strategy.

Robbins_Pfunds_0209 Tyrolean Tunneling

One of the three construction lots of the GKI project, the headrace tunnel consists of two drives – a 12.1-km drive southward to Orvella, and a 9.4-km drive northward to Prutz. The headrace tunnel is being built in the Alps with depths ranging from 300 to 1,200 m. The ground conditions consist primarily of schist averaging 70 MPa (10,000 psi) UCS, and reaching upward of 160 MPa (23,000 psi). The ground also presented the potential for squeezing conditions.

Hochtief ordered two identical Robbins TBMs – Nos. 400 and 401 – for the project. The TBMs were built at Robbins’ facility in Narni, Italy, and trucked to the jobsite at Maria Stein, Austria – roughly the midpoint of the tunnel.
The TBMs are double-shields, which are capable of simultaneously advancing and erecting segments or operating in single shield mode in bad ground. The TBMs are 6.53 m OD, and have specially designed features to accommodate difficult and squeezing ground.

One of the unique abilities of the machines is that the diameter can be extended in bad ground, according to Andy Birch, Field Service Manager for Robbins. By shimming the gage cutters on the periphery of the cutting head, the diameter can be extended 100 mm. In extreme conditions, the diameter can be extended an additional 100 mm by unbolting and raising the main bearing. Additional measures against squeezing ground include multiple bentonite injection ports to maximize lubrication and high-pressure breakout force (57,000 kN). The TBMs have the ability to probe ahead of the machine and inject ground improvement grout as needed.

The twin 600-ton TBMs will install precast concrete segment, four to a ring (one invert, one crown and two universals). The segments are 1.66 m in length and 27 cm thick and are set on the TBM with a mechanical erector.
Segments, reinforced with wire cage rebar, are being made on site by Hochtief using Herrenknecht formworks with concrete from Hilte & Jehle, which has a plant adjacent to the site. Of note, muck from the tunnel is being sent to Hilte & Jehle, which is then using the rock for concrete aggregate. The segment carousel comprises 12 complete sets of molds and is capable of producing 40 rings per day.

Assembly of the first TBM began in Narni in April 2015, delivered to the jobsite in August, and began mining in October. The second TBM began assembly in June and was delivered to the site in October. It will begin mining when the first TBM has advanced to the point where the starter tunnel is clear, which is expected to occur in January 2016.

Robbins_Pfunds_0081 Through the Mountain

In order to launch the TBMs, Hochtief constructed a 650-m long access tunnel through which both the southbound and northbound drives would commence. Construction of the access tunnel began in October 2014 by drill-and- blast, using a Sandvik three-boom drill and Meyco Potenza equipment to place the synthetic fiber reinforced shotcrete. Excavation of the access tunnel was completed in June with concreting finished in July.

The first TBM was placed on specially designed cradles and pushed through the access tunnel through a tight 60-m radius curve by six Schoma locomotives. After mining advances far enough southbound, the second TBM will be pulled into the started tunnel to begin final assembly, testing and commissioning. In the cavern area at the end of the access tunnel, a California switch will be installed so that trains can service both the southbound and northbound TBMs.

The trains will carry in segments and pea gravel to offload, and will be loaded with muck from the TBM’s shuttle conveyor. The shuttle conveyor improves safety and efficiency and reduces wear on the rolling stock by minimizing the movement of the trains.

When the TBMs reach their destination, they will be pulled back through the tunnels and removed via the access tunnel at Maria Stein.

The TBMs being used on the GKI project demonstrate the latest in mechanical excavation that has changed the landscape of the tunneling market. “It used to be that in bad ground the conventional thinking was to use drill-and-blast,” said Detlef Jordan, Robbins’ European Sales Manager. “But today, we have developed means with our experience working with contractors to overcome difficult ground. Together we have shown that you can go fast, and reliably, using TBMs.”

Jim Rush is editor/publisher of TBM.

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