Pittsburgh’s North Shore Connector uses precast panels for an underwater rail tunnel.
By Jonathan Barnes – Photos courtesy of Technopref Industries
Pittsburgh is known as the City of Bridges, because of the many spans that cross its rivers and valleys. But in a twist of engineering fate, the largest, newest structure to traverse a river in the city is being built underground. The subterranean North Shore Connector, a 1.2-mile extension of Pittsburgh’s light rail system, part of which runs beneath the Allegheny River, relies heavily upon precast concrete segment rings for its structure. The segments are the exoskeleton of the dual-tunnel, underwater portion of the light rail structure and line the interior of the tunnels.
Delivery of all 1,120 ring segments was to be completed by the end of 2008. Each ring is made up of seven precast segments weighing between 1 and 3 tons each. Once complete, the individual rings each weigh about 40,000 pounds.
The North Shore Connector is extending Pittsburgh’s light rail system from the city’s downtown, under the Allegheny River, to stops on the city’s north side. The complex job brought together two diverse companies – one a family-owned precast manufacturer based in western Pennsylvania and the other a global construction firm based in France. A.C. Miller Concrete Products Inc. of Blairsville, Pa., worked with Technopref Industries Inc., a U.S. subsidiary of Demathieu & Bard, S.A., which oversaw production of the concrete segments for the tunnel. Technopref provided the formwork and its expertise in working on segmented precast concrete tunnels. Technopref also provided equipment such as a vacuum lift crane to move the segments and a semiautomated finishing line used to prepare the segments with gaskets and packing material.
A.C. Miller, in addition to its understanding of the local market, provided the partnership with its expertise in precast concrete production, experienced employees and its nearby production facility. The plant is about an hour’s drive from Pittsburgh, and the close proximity to the work site was crucial to completing the job on time.
Underwater/underground tunneling work has a long and varied history in Europe, and many of the experts in this area of construction are from Europe. Technopref and its parent company have been involved in underground tunneling work for more than 20 years and have been working in the United States since 2000.
But even with its depth of experience, Technopref couldn’t have predicted the great success its partnership with A.C. Miller. Working together, the two companies have consistently beat construction timelines for the $435 million Connector project. Due to the efficiency of the Blairsville facility and because of the uniquely slow nature of the tunneling process, the production of the concrete segments has outpaced mining work on the tunnel job.
“The Connector job has a uniquely complicated tunnel alignment snaking into complex curves,” says Carlo Cattelan, Technopref’s project manager on the Connector. Cattelan oversaw production of the concrete segments along with Fred Esch, Technopref’s general superintendent. “It’s a short tunnel with a complicated alignment.”
Just one of the two concrete tunnels to be built under the river for the Connector has been completed, but all of the 1,120 ring segments already have been produced. Since there is little room on the work site for storage of the segment rings, delivery occurs in conjunction with the mining in a “just-in-time” fashion. Depending upon the need, A.C. Miller ships by tractor-trailer an average of eight to 10 rings per day to the site when the tunnel-boring machine is mining. At its peak, the company delivered 16 rings to the site in one day.
Initial meetings with three local concrete manufacturers quickly led Technopref Industries to zero in on A.C. Miller. But before the company could meet the expected demand of producing and delivering many concrete segments daily, it had to upgrade its Blairsville facility and hire more workers. The company also made $800,000 in improvements to the plant and, to bolster its staff of 60 at the facility, it hired 40 more workers specifically for the Connector project.
The A.C. Miller/Technopref partnership has been very successful, says Paul Zick, project director for the North Shore Connector. The two companies achieve a high level of precision in manufacturing the 11-inch-thick segments for the 20-foot-diameter, 4-foot-long rings. The structural and dimensional integrity of the rings is important, since the mining process and the life of the product depend on it.
“The attention to detail was tremendous,” Zick says. “On the segments, the important part is the tolerances. The pieces have to be interchangeable. If, during installation, you damage a piece from one ring, you should be able to replace it with another piece from a similar ring – even if the dimensional tolerances on the segments is less than one-sixty-fourth of an inch.”
Cattelan says Technopref’s history of working with tunnels gives it an advantage in the market. “This job is about precasting, precision, consistency, high-quality manufacturing standards, and having knowledge of what goes on with the tunnel boring machine (TBM). You need to be mindful of how the TBM works and how the TBM staff wishes to operate. You always have to give the highest priority to product tolerances,” he says.
The rings are made of a much more sophisticated mix of concrete than the type used for roads, says Mike Buchan, vice president of A.C. Miller Concrete Products. “Its compression strength is very high, attaining up to 12,000 psi in compressive resistance with very low permeability to water,” he adds.
The concrete used for the rings is comprised of portland cement and silica fume. When mixed with cement, the silica fume powder gives the mix added strength and density. The rings are cast in forms that were manufactured in France specifically for the Pittsburgh job.
After delivery to the work site, the ring segments must then be moved into position for installation. In some tunneling operations, a rail line would carry them into the tunnel. Such a construction transport method was impossible for the Connector, however, because of the grade of the tunnels. Custom-built flatbed wheeled vehicles (nicknamed “skateboards” by the mining crews) are being used instead. At the site access shaft (known by workers as the Launching Pit), the installation process begins with lowering the ring stacks onto the skateboards that cart the ring stacks into the tunnel, to the docking station and to the TBM. At the docking station, hydraulic equipment lifts the segments onto a “sled,” which carries them toward the TBM erector. TBM crews then operate the ring erector machine’s hydraulic arms to assemble the segments into a ring. Workers then use pneumatic impact wrenches to bolt the ring into place.
The TBM drives itself forward by pushing off the rings that are installed. “The TBM puts 8 million pounds of pressure on the rings while pushing itself,” Buchan says. Once assembled by the TBM erector, the segment rings, along with compressed gaskets, are held together with bolts and dowels.
The partnership between the two companies has gone so well that they are working together on a second project, a subway line in New York City. The partnership has also provided A.C. Miller with expertise in a niche area of construction. That newly gained expertise has made the company more competitive, Buchan says. “It’s our goal to continue producing tunnel liner rings and to continue our relationship with Technopref,” he says.
A.C. Miller’s involvement in the Connector project has helped to ensure that some of the knowledge required for the project stays in the Pittsburgh region. Dave Whipkey, spokesman for North Shore Connector owner Port Authority of Allegheny County, says the Authority is pleased with the work done by A.C. Miller and Technopref. Their work is helping with a project that will continue the transformation of Pittsburgh’s North Side, where both the Pirates and the Steelers have recently built a new stadium. “A.C. Miller’s reputation and attention to detail in producing precast rings is second to none,” Whipkey says. “When completed, the North Shore Connector will create a cohesive business district that will link Pittsburgh’s South Hills, downtown and the booming North Shore.”
Jonathan Barnes is an award-winning freelance writer based in Pittsburgh.