North Shore Connector
Twin-tube Tunnel Key to Pittsburgh's Light Rail Expansion
By Greg Thompson

In the late 1990s, when Pittsburgh city planners turned their attention to revitalizing the downtown area, a rapid transit component seemed a natural fit. Indeed, Pittsburgh has experienced a renaissance of sorts – bouncing back from the troubled economy caused by a mid-1970s slowdown in the steel industry – with reimagined and reinvigorated industrial sites and an economic strength based on cutting-edge technology and investments.

Pittsburgh’s “T,” as the light-rail mass transit system is called, is comprised of 25-miles of at-grade and subway lines that connect downtown Pittsburgh to surrounding suburbs. The first portions of the T opened in 1984 and the system serves thousands of riders each day. As the downtown revitalization took root, expanding the rapid transit system took on increased importance as a way to serve the burgeoning “Golden Triangle” (the downtown region where the three rivers – Allegheny, Ohio and Monongahela – meet) and to reduce traffic on the roadways and bridges on game days – both Heinz Field (home of the Steelers) and PNC Park (home of the Pirates) are in the North Shore area served by the new line.

Startup

A major component of the project is a twin-tube tunnel to bring the transit line from downtown, under the Allegheny River and onto the North Shore near the ballparks. The Port Authority of Allegheny County assumed control of the North Shore Connector in 1999 and furthered the environmental study, design and engineering of the new line. In the years following, some 300 meetings were held with the public, the professional sports teams and others as plans were finalized as to how best serve the community and businesses along the proposed alignment.

The bidding phase of the project got off to a rocky start. Initial bids came in nearly 25 percent higher than estimates and all were rejected. Following an extended bidding phase, the budget increased to $435 million and the job was awarded to a joint venture of Obayashi and Trumbull (collectively North Shore Contractors).

The North Shore Connector is said to be among the last to be eligible for 80 percent Federal funding – similar projects more typically receive 50 percent funding. According to the Port Authority of Allegheny County, the funding breaks down as follows: $348 million (80 percent) from the Federal government, $72.5 million (17 percent) from the State of Pennsylvania and $14.5 million (3 percent) from Allegheny County.

Notice to proceed was issued in November 2006.

The Project

All told, the North Shore Connector project is comprised of twin 2,240-lf tunnels 21-ft, 10-in. diameter in addition to a cut-and-cover station and supporting structures. The under-river portions of the tunnels are approximately 867 ft each way with the remainder of the alignment passing below the business district. The tunnel passes roughly 21-ft below the river bed and through fluvioglacial and alluvial deposits.

Prior to and during construction, ground improvements in the form of jet grouting were performed to prevent settlement along the alignment, which, in addition to the river, passes beneath three highways and several office buildings.

To bore the tunnels, a Herrenknecht slurry pressure balance mixshield TBM was selected. The TBM launched in January 2008 from a pit near PNC Park. The 22-ft diameter TBM with 17-in. cutters was designed to mine through both soft ground and rock, leaving behind a tunnel lined with precast, segmented rings.
Industrial communications provider Wholesale Mine Supply and its HC Global division through a joint venture with Gomez Tunneling provided two-way radio communications to the project. According to Wholesale Mine Supply, the VARIS leaky feeder network system was the first in the world to communicate to a tunnel boring machine located under ground and provide both leaky feeder communications and Ethernet capabilities over the same cable. The system also provided two-way radio communications to both the underground and surface areas of the jobsite.

The VARIS network enabled communication between the TBM and engineers in Germany, allowing the engineers to monitor the TBM’s navigation system even control the machine.

Sticky Clay About 700 ft in, the trouble began. The TBM had been making smooth progress, bettering the estimated daily advance rate of 25 ft per day by 10 to 15 ft. The TBM ran into what was described by Obayashi project manager Asao Nomura as “sticky clay.” Not only that, but buried tree roots became stuck between the crusher and the gate and the machine stopped. A change order was issued to address the trouble and work stopped for a solid month as the clay and tree roots were tackled. Urethane foam was injected at the face to stop leakage and pressurize the face in order to work. Once the crew cleared the debris and the slurry lines, the TBM resumed mining. Turning Around and Heading Home The first tunnel holed through on July 11, 2008. For several weeks, crews turned the machine around in the receiving pit to set up for the second bore. Tunneling resumed Sept. 3 and the lessons learned on the first bore proved invaluable on the second pass. The second tunnel reached its mark on Jan. 15, 2009, meaning the second trip of 4.5 months was a full month faster than the first.

The TBM has returned to its launch point now and disassembly is under way and is expected to be complete by early March. Tunnel invert concrete is under way as well and will be followed by the tunnel cleanup process. By the middle of March, crews are expected to begin launch pit structural concrete work. The tunnel and connecting structures are expected to be complete in spring 2010.
The 31,000 cu yd cast-in-place concrete for the cut-and-cover structures, including the North Side Station shell, is 95 percent complete and on schedule to finish in March 2009. Backfilling of the box sections boxes is complete and surface restoration is in progress.

Connected

The North Shore Connector is scheduled to begin operation in 2011. It will add 1.2 miles to the 25-mile T and is expected to support the estimated $1 billion in developments in the North Shore area. Not only that, but officials hope that the new line will spur future light rail extensions, such as a run to the airport.

In addition to the dramatic reduction in game-day traffic to and from Heinz Field and PNC Park, the line will serve other attractions such as the Carnegie Science Center and the Warhol Museum. With fuel costs always in flux and time an increasingly precious commodity, Pittsburgh’s commitment to rapid transit will serve the city and its people well into the future.

Greg Thompson is assistant editor of Tunnel Business Magazine.