A massive construction project has been underway deep in the heart of Germany’s Schnecktal valley. From the surface, though, you would never be able to tell. The majority of the work is being performed underground as a joint venture led by German contractor Wayss & Freytag Ingenieurbau AG, Frankfurt, to build the nearly 4.3-mile-long Finne Tunnel. After a few years of tunnel boring operations, the contractor is at work finishing the interior of the tunnel—slipforming first the tunnel’s floor and then a walkway with a Gomaco Commander III.

Finne Tunnel is just one section of a new 76-mile high-speed rail link between Leipzig and Erfurt. Initial construction on the twin-bore tunnel began in April 2008. Two tunnel boring machines (TBMs) worked at the same time boring the 35.4-foot-diameter shafts. Peak TBM production rates reached up to 80 feet per day.

The high-speed trains traveling the new route will reach speeds of 186 miles/hour. Such high speeds and the resulting pressure waves dictated the large diameter of the tunnels. The 31 1/2-foot internal, lined diameter was formed using precast concrete lining segments or rings that were cast onsite. Each ring is 6.6 feet?long, 17.7 inches thick, and weighed 12 tons. A total of 6822 rings were needed to line the new tunnels.

In September 2009, the first TBM “holed through.” The second followed a few months later when it broke through in February 2010, six months ahead of schedule. The work, however, was far from complete. The next phase was the slipforming. Representatives from Wayss & Freytag worked with Gomaco International Ltd. to determine which paver would suit its tunnel applications. The Commander III was chosen to first pave the tunnel floor in four-track mode, then to be converted onsite to a three-track paver to slipform the tunnel’s walkway.

The decision also was made early in the design phase to use the Leica Geosystems 3D control system since setting and maintaining stringline within the tight confines of the tunnel would have been nearly impossible. The stringless system alleviated those concerns.

The concrete for the tunnel was provided by an onsite mobile batch plant with an 105-cubic-yard/hour capacity located just outside the tunnel entrances. The plant made a dry, low-slump concrete with a low percentage of cement to reduce heat of hydration. “We had concrete with less cement because of the size and depth of the applications,” says Christian Korndörfer, project manager for Wayss &?Freytag. “The floor is over 3.3 feet thick. We didn’t want the concrete curing process to generate too much heat inside the tunnel or result in any cracking within the concrete.”

Delivering concrete to the paver within the circular tunnel was another challenge. Wayss & Freytag wanted to use standard concrete trucks, but with no room inside the tunnel for trucks to turn around thye would have had to drive in reverse the length of the tunnel to reach the paver. which would be too time consuming. Instead, a two-part solution was developed. With a weekly paving of 3281 feet, a turntable was set at that distance from the paving machine. The concrete trucks then drove in forward to the turntable where they were rotated 180 degrees so they could drive in reverse to the paver, dump their load of concrete in front of the paver, and then drive forward out of the tunnel. The material was transported using between six and eight trucks carrying 10-cubic-yard loads of concrete.

“The tunnel floor is 19.7 feet wide,” says Korndörfer. “In the circular tunnel, at its deepest point in the center, the floor was 41.3 inches. We turned all four tracks on the Commander III to 35 degree angles so the paver could drive on the round walls.”

The slipform mold was designed for a drainage channel in the tunnel floor. The channel measured 7.1 inches deep and 28.3 inches wide at the top tapering down to 21.3 inches wide at the bottom.

A height tolerance of ±0.4 inches had to be met on the new tunnel floor to ensure the accurate installation of the future track rail. Control measurements confirmed the new concrete tunnel floor was always within the specified height tolerances.

“The achieved vertical accuracy of ±0.1 inches by far outperformed the required accuracy of a maximum ±0.4 inches,” says Baumgartner.

In total, 36,623 cubic yards of concrete were slipformed to build the floor in each 4.3-mile-long tunnel. Both production and quality of the finished product exceeded expectations.

The second phase of slipforming within the Finne Tunnel involved converting the 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 the varying line of the tunnel.

“The line of the train track must be 100% accurate and its placement is considered sacred,” says Korndörfer. “The walkway mold had to be able to accommodate the changing alignment of the tunnel, tunnel superelevations, and other variations created when working inside a tube.”

Gomaco engineers designed the mold and hopper with telescoping abilities. As the face of the tunnel wall changed, the mold compensated by telescoping in and out or up and down to change the size of the walkway and keep the profile in alignment with the train tracks. The telescoping feature also ensured that the mold was always kept against the tunnel wall and the accuracy of the walkway placement maintained.

The top width of the walkway varied between 3.4 and 5.7 feet. The height of the walkway was variable as well, from 23.6 inches to 37.4 inches. Hydraulic pressure-compensated cylinders controlled the changes. A finishing roller, mounted to the back of the mold, helped provide the finish to the walkway’s surface, eliminating the need for hand finishing.

A service channel in the surface of the walkway was slipformed 3.3 inches wide by 2.8 inches tall. Lighting conductor strips will eventually be placed in the keyway, tested to make sure they are operational, and then covered and sealed. The keyway allows the strips to be removed and replaced as needed without damaging the profile of the walkway. A 2% cross slope across the top surface ensures proper water drainage off the walkway.

“The walkway was a much more challenging profile to slipform than the tunnel floor,” says Korndörfer. “It had to be placed with 100% accuracy. We had no problems and were able to achieve production rates from 558 to 656 feet/day.

“The problem with the tolerances in the walkway and the need for the walkway mold to change size was one of the biggest challenges we faced on the project,” says Korndörfer. “Our other challenge was the logistics of getting concrete into the tunnel. We were able to devise solutions for both problems and achieve a quality product. Our Gomaco with the Leica system has done a good job and worked well.”

Wayss & Freytag is on schedule to complete its portion of the Finne Tunnel by the end of 2012. Others will then begin placing the track and installing the electrical and other systems. The entire Erfurt to Leipzig line will be operational in 2015 as part of a high-speed connection from Munich to Berlin. Ultimately, the line will run all the way from the Scandinavian countries to Italy.

For more information about the products mentioned in this article, visit www.gomaco.com.