Wayss & Freytag Ingenieurbau is currently at work constructing the 7 km long Finne Tunnel through Germany's Schnecktal valley. Having completed the bore, the contractor is now using a Gomaco Commander III to finish the twin-tube rail tunnel's track bed and walkway.

The Finne Tunnel is part of a new 123 km high-speed rail link between Leipzig and Erfurt, Germany, which will see trains reach speeds as high as 300 km/h. Initial construction began in April 2008 using two 10.8 m diameter tunnel boring machines (TBMs). Now they have been excavated and lined, Wayss & Freytag has moved on to the slipforming phase, for which it has selected a four-track Commander III paver and Leica Geosystems 3D control system, rather than a traditional string line.

The concrete is a dry, low slump mix with a low percentage of cement as Wayss & Freytag project manager Christian Korndörfer explained, "We had concrete with less cement because of the size and depth of the applications. The floor is over 1 m thick and we didn't want the concrete curing process to generate too much heat inside the tunnel or result in any cracking within the concrete."

Rather than delivering the concrete to the paver using traditional mixer trucks, which would have to have backed-up all the way to the work area, Wayss & Freytag developed a faster two-part solution. The floor of the tunnel was paved in a special sequence.

A weekly paving production goal of 1000 m was established, with an average paving goal of 250 m per day. At the beginning of each week, the four-track Commander III was set up to pave 1000 m beyond the section m completed the week before. The concrete trucks drove in forward gear on the completed tunnel floor to a turntable at the end of the section. The turntable then rotated the three-axle trucks 180 degrees so they could drive in reverse to the paver, dump their load of concrete in front of the Commander III, and then drive out of the tunnel in forward gear.

Operating the paver itself required some interesting adaptations. "The tunnel floor is 6 m wide. In the circular tunnel, at its deepest point in the centre, the floor was 1050 mm (41.3 in). We turned all four tracks on the Commander III to 35° angles so the paver could drive on the round walls," said Mr Korndörfer.

The slipform mold was designed for a drainage channel in the tunnel floor. The channel measured 180 mm deep and 720 mm wide at the top tapering down to 540 mm wide at the bottom. A height tolerance of +/- 10 mm had to be met on the new tunnel floor to ensure the accurate installation of the future track rail. In the event, a tolerance of +/- 3 mm was achieved.

The second phase involved Wayss and Freytag converting their Commander III to a three-track paver to slipform a walkway against one wall of each tunnel. Gomaco built a variable height, variable width walkway mold and hopper to accommodate changes in the tunnel wall face.

The top width of the walkway varied between 1.05 m and 1.75 m, and the height changed between 0.6 m and 0. 95 m.

"The walkway was a much more challenging profile to slipform than the tunnel floor," Mr Korndörfer explained. "It had to be placed with 100 % accuracy and the Commander III slipformed the walkway very well. We had no problems and were able to achieve production rates from 170 to 200 m per day.

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