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Rare Procedure Aids Twin Tunnel Job in Pittsburgh

Wed July 25, 2007 - Northeast Edition
Mary Reed

There won’t be the usual crowd of interested onlookers watching construction work when Pittsburgh’s twin tunnel project gets fully under way.

That’s because there will be little to see on the surface once the boring of two 2,400 ft. (732 km) long tunnels 40 to 60 ft. (12 to 18.2 m) below the Allegheny River moves full steam ahead.

The tunnels are part of the first phase of the Port Authority of Allegheny County’s North Shore Connector project, a $435 million 1.2 mi. (1.9 km) expansion of the city’s light rail transit system. This new section of the 25-mi. (40 km) line will eventually link the central business district in downtown Pittsburgh to the rapidly developing North Shore and preserve the ability for future extensions to the Pittsburgh International Airport.

The project also includes reconstruction of retaining walls on the 10th Street Bypass as well as building a trio of LRT stations. In addition, pilings supporting the Route 65 overpass on the North Shore will be replaced by drilled caissons spaced in such a fashion so as to allow room for a cut and cover tunnel to pass between them.

It is anticipated the entire light rail expansion project will take 4.5 years to complete and once open will carry more than 14,000 riders on weekdays by 2025. This estimated average ridership includes thousands who attend concerts and sporting events held at the Pittsburgh Steelers’ Heinz Field and PNC Park, home of the Pittsburgh Pirates.

Funding for the project is being provided by the Federal Transit Administration, which is contributing $348 million. The remaining $87 million will come from state and local sources.

The $156.5 million tunnel will be constructed by North Shore Constructors Obayashi Trumbull JV (NSCOT JV), a joint venture formed by Obayashi Corporation of San Francisco, Calif. (headquartered in Tokyo, Japan) and the Trumbull Corporation, based in West Mifflin, Pa. The consortium was awarded the contract by the Port Authority of Allegheny County in July 2006.

This phase of the overall project includes the building of 1,500 ft. (457 m) of cut and cover section and 2,400 ft. (731.5 m) of bored tunnel, extending the existing light rail line by 3,700 ft. (1,128 m). In early 2007, open excavation began on a launch pit from which a huge tunnel boring machine will begin chewing out the first of the two tunnels.

Giant Tunnel Boring Machine

The gigantic Shield Machine, which weighs more than 450 tons (408 t), has a diameter of approximately 23 ft. (7 m), and is approximately 30 ft. (9 m) in length. It was manufactured by the German company Herrenknecht AG, based in Schwanau, Germany, and shipped in early July. It is expected to arrive in Pittsburgh in August.

Shield Machines are frequently used to bore through soft ground, a spokesman of North Shore Constructors Obayashi Trumbull JV said.

“The Shield, a thick cylindrical steel plate, is the ground support that prevents the bored tunnel from caving in on the machine and personnel inside the tunnel.

“Although the Shield completely supports the side wall of the bored tunnel around the TBM, the front face of the TBM cannot be fully supported, since excavated material needs to be transported through an opening just behind the cutting face while enough pressure to withstand the ground pressure ahead, and most of the time hydrostatic water pressure as well, is maintained,” he said.

“The critical thing is to take out the same volume of ground as the volume just displaced by the TBM advancement,” he continued.

“There are several ways to excavate a controlled amount of soil from the face while maintaining the in-situ face pressure at same time.”

A similar type of TBM has been used on Seattle’s ongoing Sound Transit Light Rail Project, which remains under construction by Obayashi Corporation. However, the borer to be used for the Pittsburgh job has a different approach to maintaining pressure at the cutting face than the TBM operating in Seattle.

“While the TBM in Seattle uses excavated material to balance the earth pressure in a process called Earth Pressure Balance [EPB] Shield Tunneling, the TBM for this project will use slurry consisting of bentonite and water, along with controlled compressed air.

“Both type of Shield Machines have pros and cons, and we had chosen to go for the Slurry Shield Machine based on our preliminary study for the project conditions,” the spokesman added.

How the Shield Machine Operates

During its excavations the Shield Machine will make an S-shaped bend beneath the middle of the Allegheny River to avoid passing under PNC Park, whose home plate sits approximately 443 ft. (135 m) from the shoreline.

The first tunnel will take approximately six months to reach the opposite shore, after which the boring machine will execute a U-turn and excavate a tunnel back under the river in the other direction.

When boring begins this fall, 20 to 30 field employees will be working each shift of the 24-hour operation with an additional 30 to 50 carrying out structural work in other areas.

The Shield Machine performs a number of operations.

The first major task is to excavate the ground at the face and pump out the excavated material to a separation plant located at the ground level.

The tunnel borer moves forward by means of a thrusting force produced by numerous hydraulic jacks connected to the shield and cutter head. Electric motors drive the cutter head. The crushed/shredded/grinded soil and rock from the cutting face will be mixed with the slurry mentioned above, and pumped through a steel pipe for disposal.

The second major task is to erect circular concrete rings, which consist of several precast concrete segments per ring that are assembled within the rear section of the TBM shield behind the TBM. As the completed rings leave the protection of the shield and are exposed to the ground, other TBM equipment will inject cement grout behind the rings to fill the annular void space.

The completed rings also serve as a reaction structure for the hydraulic jacks that thrust the TBM forward.

These segments will be shipped to the job site on a daily basis as needed. One ring will typically consist of seven pieces, and the gaskets glued at its joints will be compressed when assembled in place to form a watertight joint, the NSCOT JV spokesman stated.

The TBM’s digital guidance system indicates a variety of information on a monitor, including the position of the machine and any deviation from its designed alignment, allowing its operator to control and steer the machine based on continually presented information.

The TBM will begin by excavating the southbound tunnel. It can advance 30 to 60 ft. (9 to 18 m) a day, depending on ground condition and will eventually pass through more than 70,000 cu. yds. (53,519 cu m) of various types of soils including sand, gravel, clay, and claystone as it burrows under the river. The overall excavation volume of 200,000 cu. yds. (152,911 cu m) includes both bored tunnels and the cut and cover tunnel.

Challenges to Be Overcome

“There are many challenges in the project,” the North Shore Constructors Obayashi Trumbull JV company spokesman stated.

“For the TBM operation, they include boring underneath the Allegheny River as well as under a historical downtown area where a lot of old utilities exist, U-turning the TBM, and a sharp curve radius and inclined/declined grade more than 7.58 percent for the TBM alignment.

“Each of us on the project team has their own expertise and a variety of experience in their field. Gearing these multiple forces into one direction to reach our goal is the most challenging part. Teamwork and partnership will be the key to success in this type of complicated infrastructure project.”

Shield Machines do not represent a new technology, their use having been pioneered on the Thames Tunnel project in London in 1825 by the French engineer Sir Marc Isambard Brunel, assisted by his son Isambard Brunel, assisted by his son Isambard Kingdom Brunel.

However, the Slurry Shield Machine is relatively new and was introduced in Japan in the 1960s. “A number of Slurry Shield Machines have been used, mostly in Europe and Asia where soft ground tunneling is inevitable due to the geological nature of the motherland,” the NSCOT JV spokesman said.

“As for Slurry Shield Machines in the U.S., this project will be a rare case, being only the second — or possibly the third, depending on the time of completion — on which a Slurry Shield Machine with this large a diameter will have been used.

“In addition, several specialized types of equipment rarely seen in the American market will be utilized, such as the slurry mixing and separation plant, the cutter soil mixing wall rig, and the slurry diaphragm wall rig,” he concluded.

About the Companies

General contractor Obayashi Corporation is based in Japan. Founded in 1892, it has completed major heavy civil projects and architectural buildings nationally and worldwide, including the Trans-Tokyo Bay Highway Tunnel in 1997 and the Akashi Kaikyo Bridge in 1998.

Its first public project in the United States was the North Shore Outfalls Consolidation Contract in San Francisco in 1979. The corporation’s heavy civil division has completed several major projects in the past 25 years, such as the Light Rail Subway Tunnel in Minneapolis, Minn., Nancy Creek Sewer Tunnel in Atlanta, Ga., and the Upper Diamond Fork Tunnel Project in Utah.

Trumbull Corporation, general contractor based in West Mifflin, Pa., was founded in 1955. The company has completed major heavy-civil projects in Pennsylvania and bordering states.

Specializing in highway and bridge projects, its recent jobs include reconstruction of the Fort Pitt Bridge in Pittsburgh, Pa. and construction of the Woodrow Wilson Bridge in Washington, D.C.

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