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Pittsburgh Begins Tunneling for North Shore Connector


February 18, 2008


The Port Authority of Allegheny County began drilling a tunnel under the Allegheny River Jan. 25 as part of the construction process for the North Shore Connector light rail link. Contractors must first drill one tunnel under the river from the North Shore to downtown Pittsburgh, then turn around and drill another in the opposite direction.

The launch pit, located on the North Shore near PNC Park, is nearly 60 feet below ground. Local news media were on the scene as the tunnel boring machine began its trip.

The 500-ton tunnel boring machine has the ability to advance 25 to 30 feet per day at top speed, reachable after three attachments, called gantries, are connected behind the main body. As Passenger Transport went to press, contractors had attached two of the three gantries, which are long trailer-type structures that sit behind the TBM and provide electrical, hydraulic, and other support equipment to the machine. A single operator, with a crew of four or five, runs the machine.

"It's routine around the world--they've been used in construction for decades," Project Manager Kirk Thompson said of the tunnel boring process. "It's a big deal here because it's never been done in Pennsylvania before, let alone the Pittsburgh area."

As the machine tunnels its way toward the downtown area, a hydraulic arm lifts pre-cast concrete rings into place and bolts them. These rings, in seven segments, eventually comprise the tunnel.

Slurry, a substance made of water and clay, is pumped toward the machine's cutting head to facilitate boring. Material removed from the earth along with the slurry is then pumped to a slurry-separation plant near the launch pit where earthen material is separated and the slurry is reused.

The huge machine is itself guided by lasers. With coordinates programmed in, the TBM in one sense runs itself. Further, it operates with such precision that it is expected to arrive within millimeters of the Stanwix Street target.

When the North Shore Connector enters revenue service in 2011, light rail passengers will have direct access to such attractions as Heinz Field, Carnegie Science Center, Community College of Allegheny County, and many restaurants, clubs, and hotels located along the North Shore. People who live in the South Hills will be able to board a light rail vehicle and take a convenient ride to downtown Pittsburgh and across to the numerous North Shore destinations.

Anyone who has ever lived near a construction project will attest, however, that such projects invariably bring problems. Thompson cited several "typical acute challenges" faced in an urban area: limited trucking hours, limited staging and lay-down areas, coordination with special events, and keeping areas routinely cleaned. "Any time you excavate, dig up dirt, it's messy," he said.

Nearby residents always like the end result, but find the process difficult. Said Thompson: "They'd like to be put into a three-year sleep and wake up and have it done."

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