A massive construction project has been underway deep in the heart of the Schnecktal valley area in Germany. The majority of the work is underground, as a joint-venture team led by German contractor Wayss and Freytag Ingenieurbau AG builds the nearly 4.3-mile-long (7 km) Finne Tunnel. Finne Tunnel is just one section of a new 76-mile (123 km) high-speed rail link between the cities of Leipzig and Erfurt, Germany.
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-ft. diameter (10.8 meter) shafts. Peak TBM production rates reached up to 80 ft. (24.5 meters) per day. The internal, lined diameter is 31.5 ft. (9.6 meters) and was formed using precast concrete lining segments or rings. They were cast on site and each ring was 6.6 ft (2 meters) long, 17.7 in. (450 mm) thick and weighed 12 tons. A total of 6,822 rings were needed to line the new tunnels.
In September 2009, the first TBM "holed through." The second one followed it a few months later when it broke through in February 2010, six months ahead of schedule. Work was far from being completed, however. It was time to start the slipforming phase within the tunnels.
Paving in Circles
Representatives from Wayss and Freytag worked with Gomaco International Ltd. to determine which paver would suit their tunnel applications. They chose the Gomaco Commander III. The slipformer would first pave the tunnel floor in four-track mode, then be converted on site to a three-track paver to slipform the tunnel's walkway.
The decision was also made early in the design phase to use a Leica Geosystems 3D control system. Setting and maintaining stringline within the tight confines of the tunnel would be nearly impossible, and the stringless system would alleviate those concerns.
The concrete for the various tunnel applications was provided by an on-site mobile batch plant with a 105-cu.-yd. (80 cu. meter) per hour capacity and is located just outside of the tunnel entrances. A dry, low-slump concrete was produced with a low percentage of cement.
"We had concrete with less cement because of the size and depth of the applications," Christian Korndörfer, project manager for Wayss & Freytag, explained. "The floor is over 3.3 ft. (1 meter) 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 also a concern. Wayss & Freytag wanted to use standard concrete trucks, but having them drive in reverse through the length of the tunnel to reach the paver would be too time consuming. There was also no room inside the tunnel for trucks to turn around.
The company developed a two-part solution. The floor of the tunnel was paved in a special sequence. A weekly paving production goal of 3,281 ft. (1,000 meters) was established, with an average paving goal of 820 ft. (250 meters) per day. At the beginning of each week, the four-track Commander III was set up to pave 3,281 ft. (1000 meters) beyond the section of floor 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° so they could drive in reverse to the paver, dump their load of concrete in front of it and then drive out of the tunnel in forward gear. Between six to eight trucks carrying 10.5-cu.-yd. (8 cu. meter) loads of concrete transported the material.
"The tunnel floor is 19.7 ft. (6 meters) wide," Korndörfer said. "In the circular tunnel, at its deepest point in the center, the floor was 41.3 in. (1,050 mm). We turned all four tracks on the Commander III to 35° 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 in. (180 mm) deep and 28.3 in. (720 mm) wide at the top tapering down to 21.3 in. (540 mm) wide at the bottom.