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Tunnel Team Tackles Challenges

North Shore Connector Project Allows for Future Redevelopment Opportunities
As reported to Jamie Ives

Riders of Pittsburgh’s North Shore Connector, a 1.2-mile extension of the city’s light rail system (commonly known as the “T”) may not stop and marvel at this engineering feat. But, if they did, they would realize how fascinating a story it is.

It begins in 2006, shortly after Trumbull Corporation of Pittsburgh, a member of Constructors Association of Western Pennsylvania, along with Obayashi Corporation of Japan, an AGC of California member, won the contract to bore twin-tube tunnels under the Allegheny River.

The North Shore Connector project, which links downtown Pittsburgh with its north shore, cost over $562 million and was completed in September 2011. The following year, it won the 2012 Alliant Build America award in the category of Highway & Transportation New.

Contractor managers from Trumbull and Obayashi inspect the work done by the Herrenknecht 22-ft, 8-in slurry pressure balance tunnel boring machine. The cutterhead is equipped with 24 twin disk 17-in cutters, 20 scrapers, and bucket lips.

A state-of-the-art tunnel boring machine (TBM) played a key role in the success of the project. It was assembled in the North Shore launch pit. From there, crews bored the first tunnel under the river to the central business district on the south side. The TBM was then rotated 180 degrees to bore the second tunnel.

John Murray of Trumbull, involved with the project from the start, believes that this project was one of the first in the United States to do tunnel work with slurry.

“Trumbull is very good at building bridges and roads,” says Murray. “Most of these projects are relatively short-term, traditional, and straight-forward. The tunneling aspect of this project made it completely different: plenty of surprises and technically challenging. Overall, it was a great experience … a privilege to work on this project.”

Elsewhere, crews on both sides of the river, proceeded with cut-and-cover tunneling work. This included station shells for two underground stations and the construction of launch and receiving pits for the TBM.

Among the key aspects of the job were shoring of the excavated tunneling pits, soil stabilization, monitoring building movement, and placement of the precast tunnel lining.

A Herrenknecht 22-ft, 8-in Slurry Pressure Balance Tunnel Boring Machine was selected to bore the twin tunnels. This proved to be the right choice for handling the varied soil and rock conditions – soft soil and mixed-face bedrock — under the river.

The cutterhead is equipped with 24 twin disk 17-in cutters, 20 scrapers, and bucket lips.

The TBM is designed to navigate a 574-ft radius curve, with the ability to correct alignment at a 458-ft radius. The articulated shield is 27.1 feet in length and is equipped with provisions to drill probe and grout holes.

The overall length of the TBM and trailing gear is 151 feet, with a total weight of 490 tons. Twenty-four, 10.2 in-thrust cylinders generate 4,200 tons of propulsion power. Direction of rotation can be left or right with a rotational speed of 0-2.7 rpm. The main bearing is 8.5 feet in diameter.

The tunnels are lined immediately behind the TBM with a 4-ft long tapered precast segment ring. The inside diameter of each erected 4-ft ring is 20 feet with an outside diameter of 21 feet, 10 inches.

At the completion of the tunneling, crews sealed segment joints and installed a cast-in-place concrete invert. The TBM was equipped with a double chamber man lock that held four persons and a single material lock. Its maximum operating pressure was 58 psi.

“Meetings were held every day at 6:30 a.m. to discuss progress from the previous day,” recalls Murray. Project staff had the authority to deal with the owner, the Port Authority of Allegheny County, and make contract decisions. There were also two longer meetings held each week. Tunnel crews worked two shifts: 7 a.m. to 5 p.m. and 5 p.m. to 2 a.m.

The project had a multi-cultural management team, with literally, a world of experience. Key subcontractors were Nicholson Construction, Cuddy, PA, owned by Soletanche of France, and Herrenknecht of Germany. “Members of the management staff worked hard to ensure that all parties understood fully all aspects of the discussion,” says Murray. “At first it took some time; translation was required; however as the months passed we became a closer knit group and communication and efficiency also improved.”

Throughout the project, the owner and team members conducted an ongoing outreach program that supplied progress updates to businesses on both sides of the river.

Care was taken to minimize the effect of the project on the people of Pittsburgh. Throughout construction, pedestrian and vehicle accesses were always a priority.

Crews closely checked all excavated materials according to environmental requirements. Discharge water was closely monitored to ensure compliance with the clean water act.

At the old Gateway Station downtown, a giant 780-sq-ft mural was lovingly preserved and restored. Entitled Pittsburgh Recollections and created by famed artist Romare Bearden, this $15 million mural was removed tile by tile and then reinstalled at the new station.

The team’s primary concern throughout the project was multiple ground conditions. In many instances, there was little cover between the tunnel and river. The close proximity of structures to the tunnels was also a test for the project team.

On the south side of the river, the tunnels pass beneath two major roadways and are adjacent to the 150-year-old Joseph Horne Building, a historic structure constructed with shallow spread footings above the tunnel spring line.

Adding to the challenge was the congested nature of the central business district of downtown. This left limited space for a lay-down area for equipment and material. On the north side, the tunnels passed beneath the newly constructed Equitable Resources Building.

Four locations were selected for ground improvement in the form of jet grouting: Launch Pit exit block, Receiving Pit entrance block, 10th Street Bypass, and a section of Stanwix Street in front of the Joseph Horne Building.

The Launch Pit exit block and Receiving Pit entrance blocks were designed as gravity structures to be incorporated into the support of excavation. These areas were constructed with 100 percent coverage at a minimum 150 psi.

On Stanwix Street in the central business district, a 468-ft jet grout zone was constructed from the Receiving Pit north along the tunnel adjacent to the Joseph Horne Building. Jet Grout ground improvement was specified in lieu of structural underpinning to control the movements on the Joseph Horne Building.

During underpinning installation, management anticipated a disruption in building operations; however, the results were better than expected. Building settlements during underpinning were expected to be in the .4 to .6 inch range.

The goal was to keep angular distortions in the range of 1/600 or less. With these guidelines, the project team believed that if damage developed, it would be substantially limited. Maximum settlement on the first drive through an unimproved zone was .2 in.

In addition to protecting the above streets and the Horne building foundations, the bored tunnels, due to their close proximity to each other, required ground stabilization for the pillar between them.

Crews also put in place temporary steel ribs to protect the first tunnel from possible damage during the boring of the second tunnel.

The newly constructed parking garage at the Equitable Resources Building on the north side was another challenge. To limit potential damage to the parking garage from tunneling, crews applied compensation grouting under the basement slab. “It was,” says Murray, “an effective approach, as the final settlement was less than 3/8 inch.”

The north side structural shell station platform is 49 feet below ground level. To construct it, crews chose a top-down cut-and-cover method using slurry walls. “It just made good sense,” explains Murray. “It was cost-efficient and it allowed us to work in close proximity with the new parking garage without closing it down.”

Cement deep soil mixing walls (CDSM) was the primary perimeter element used to support the Launch Pit, Receiving Pit, and cut-and-cover tunnel excavations.

In the CDSM method, subcontractor Nicholson Construction Company used a base machine with 70 ft Kelly bar and twin hydraulically driven mixing heads. The walls were constructed using rectangular elements of cement grout mixed with in-situ soils.

The rectangular elements are formed vertically by first mixing bentonite slurry with the in-situ soils from grade to the top of rock. Mixing is from the top down. Upon reaching the top of rock, a 3-ft deep socket was cut into the foundation rock to provide ground water cut off. Slag/bentonite grout was then injected into mixing heads so that the grout was blended with the in-situ soils as the mixing head was withdrawn.

Looking ahead, the new extension enables the Port Authority to construct further extensions of its light rail transit destinations within Allegheny County. This will likely act as a catalyst for future development opportunities throughout the region.