Tunnel interior (Port Authority)
Profile drawing looking downstream
System Map (Port Authority) Hi-Res Map;|
Tunnel interior (Port Authority);
Northern tunnel portal (Port Authority)
Tunnel Boring Machine (Herrenknecht AG);|
Rotating cutter face (Civil Engineering magazine)
Tunnel interior (Civil Engineering magazine)
North Shore Connector tunnel
Downstream tunnel referred to as the "left tunnel"
USGS 7.5" Topo Quad - UTM Coordinates:
Pittsburgh West - Zone 17; 0584 4477
Port Authority "T" Subway / light rail
Allegheny "T" station near Heinz Field;
(north portal near Reedsale St at Art Rooney Drive)
North Shore "T" station (Reedsale St at Tony Dorsett Drive);
Gateway "T" station (Stanwix St between Liberty and Penn Avenues)
(south to north) Stanwix St; Ft. Duquesne Blvd; Tenth St Bypass; Allegheny River; Mazeroski Way; West General Robinson St; Reedsdale St
TYPE OF CONSTRUCTION / DESIGN:
Cut and cover:
Gateway station to Stanwix receiving pit;
TBM receiving pit:
At Stanwix St, north of Penn Av;
TBM - Twin bored tunnels:
Under Allegheny River (Herrenknecht AG tunnel boring machine);
TBM launch pit:
West General Robinson St at Mazeroski Way, diagonally opposite PNC Park;
Cut and cover:
TBM entry pit to Reedsdale St near Art Rooney Drive
North Shore Connector project typically quoted as 1.2 miles:
3,850 ft est (tunnel extension beyond original downtown T system);
2,200 ft est (elevated structures connecting to Allegheny station);
Former Gateway Center station loop to Stanwix receiving pit (cut and cover): 400 ft est;
Stanwix receiving pit to West General Robinson launch pit (TBM): 2,240 ft [875 ft river width];
Gen Robinson launch pit to north portal (cut and cover): 1,200 ft est;
North portal to western end of Allegheny station (elevated): 2,000 ft est;
Bridge structure beyond station for future extension (elevated): 200 ft est
Twin bored tunnels: each 22 ft diameter extrados;
River bed to top of bore: ~20-25 ft;
Precast concrete ring segments: 11 in thick x 4 ft wide;
Seven segments complete each 20 ton ring;
7 inches of insulation inside segments
Horizontal curvature of tunnel: 600 ft min radius
Typical distance between tunnels: 10m center-to-center
YEAR ERECTED / ENGINEER:
Studies to extend T Light Rail to North Side and Oakland, August 1994
Proposals to reach North Side include tunneling, February 1998
First bids received, August 2005
TBM assembly begins, November 2007
TBM tunneling begins, January 2008
TBM tunneling completed, January 2009
North Shore "T" service begins, March 25, 2012
General Contractors: North Shore Constructors,
joint venture of Obayashi Corp. and Trumbull Corp.
Engineer (draft environmental impact statement and preliminary engineering): DMJM Harris (AECOM-USA)
Jet grouting specialty contractor: Nicholson Construction Company; tunnel-boring machine manufacturer: Herrenknecht AG; precast-concrete ring segment manufacturer: A.C. Miller Concrete Products; geotechnical subconsultants: Boscardin Consulting Engineers, Inc., GEI Consultants, Inc., and GeoMechanics, Inc.
Owner: Port Authority of Allegheny County
The idea of building a subway connection to the former city of Allegheny dates at least to 1906. In fact, the piers of the Roberto Clemente Bridge at Sixth Street were designed with a removable diaphram to allow for future tunneling.
The "T" light rail vehicles operate above ground in the South Hills, south of the Monongahela River. After crossing the river and passing the First Avenue Station, the tracks enter a tunnel which is, in part, a rebuilt railroad tunnel. Diverging from the old tunnel at Steel Plaza Station, the subway travels about 35 feet below street level in a cut-and-cover tunnel -- below Sixth Av to the Wood Street Station and then under Liberty Av to Gateway Center.
The North Shore Connector project begins at Stanwix St and Liberty Av where the tunnel was extended about 350 feet northwest of the previous Gateway Center station, through the newly-constructed Gateway station and passing under the Allegheny River. The tunnel continues through the North Side station, turning west to emerge and rise to the elevated Allegheny station.
Studies began in August 1994 to extend the "T" light rail system to the North Side and Oakland at an estimated cost of $1 billion. Allegheny County commissioners halted the study, then allowed it to continue with the restriction that the extension would only serve the North Side.
In February 1998, reaching the North Side by tunneling was included in the options being studied. After taking over the project planning in 1999, the Port Authority narrowed the alignment options to two: a tunnel from Gateway Center under the Allegheny River, or an extension from Steel Plaza past the convention center and over the Fort Wayne railroad bridge. Environmental impact statements favored the Gateway alignment and in conjunction with additional studies, the Port Authority concurred. Estimates in 2000 were $390 million with completion in 2007.
In 2005, the first construction bids to bore beneath the river came in 25% over budget; the Port Authority scaled back the project by eliminating the proposed convention center spur and station. By the following year, the cost estimate had increased to $435 million with $50 million having been spent to date on planning, engineering and regulatory clearances; though property acquisitions had been cleared, no construction had yet occurred.
The Port Authority re-bid the project in 2006 and awarded a $156.5 million contract to North Shore Constructors, a joint venture between West Mifflin-based Trumbull Corporation and Obayashi Corporation of San Francisco. It was the first of 16 contracts to be awarded for the $435 million project. In addition to the twin bored tunnels, that first contract included construction of 1,200 feet of right-of-way on the North Shore and the shell for the North Side Station.
With the Federal Transit Administration committed to funding 80% (amounting then to $348 million) of the project, construction began.
Arriving from Herrenknecht AG in Schwanau, Germany, tunnel boring machine (TBM) assembly began in November 2007. The TBM, measuring 200 feet long and weighing 500 tons, was lowered into a 55-foot-deep launch pit excavated near the intersection of West General Robinson Street and Mazeroski Way near PNC Park. Tunneling began in January 2008.
July 10, 2008, the TBM holed through into the receiving pit in Stanwix Street near Penn Avenue downtown. The machine was hoisted by crane, turned around and began digging the second parallel tunnel September 3, arriving back at the North Side launch pit January 15, 2009. Completing the second tunnel in 4.5 months showed the benefit from experiences gained; the second tunnel was a full month faster than the first.
The laser-guided Herrenknecht slurry pressure balance mixshield TBM uses a 22-foot diameter rotating head (typically 1 RPM), featuring 17-inch cutters, driven by electric motors. Digging through glacial/fluvial gravel and rock, the working face is stabilized with a pressurized bentonite-water slurry; the excavated material is transported by the clay slurry through pipes back to a separation plant above ground. Excavated sand and gravel are separated from the slurry, allowing the slurry to be reused and the other materials to be reserved for future use elsewhere. The Pittsburgh TBM was generally operated in two 12-hour shifts, five days a week, averaging 34 feet per day. As the front of the TBM cut, a steel shield in the trailing section holds the cavity open and 4-foot-wide, precast concrete segments are bolted together to form the tunnel liner (seven modules complete the circumference of a given ring). The TBM then uses hydraulic legs to push off the placed concrete rings as it moves forward. The main body of the TBM is one inch smaller in diamter than the rotating cutting head, resulting in reduced friction and allowing for easier movement and steering. The complete mining assembly measured approximately 150 ft from the cutter head to the end of the trailing gantry system.
The tunnel lining is designed something like the adjustable duct elbows used in HVAC. According to Civil Engineering magazine: "To create curves using the rigid tunnel lining sections, the concrete rings were designed with a universal taper, meaning that the actual length of each ring varied along its perimeter in a uniform taper. If the point of maximum ring length was placed in contact with the point of minimum length of the preceding ring, the result was a straight, level path. However, rotating the new ring with respect to the preceding one would allow the lining to follow the change of alignment. Because of the preset bolt patterns, each ring could be rotated to 1 of 24 different positions with respect to the preceding ring. The degree of rotation determined the amount and direction of the change."
Paralleling the western side of Mazeroski Way, the 2,240 ft TBM section of the tunnel passes below the Equitable Resources building. The tunnel descends on a 6.6% grade from the North Shore to a depth of 69 feet (25 feet river depth, 22 feet further to top of 22-foot diameter tunnel bore). Below the Allegheny River, the path turns left then right, about 45 degrees each time, to align with Stanwix Street. The tunnel ascends a 7.6% grade to arrive at the Gateway station.
Final cost of the project is stated at $553 million.
With the opening of service on March 25, 2012, the North Shore Connector projects to a northward future extension of the light rail network now that the most difficult part that vision being achieved in crossing the Allegheny River. As part of the effort to promote North Shore development (and use of parking lots there), the Pittsburgh Stadium Authority and Alco Parking provided underwriting funds to extend the Downtown Free Fare Zone. After April 1, 2012, rides from Downtown to the North Side station are to remain free but trips to Allegheny station will require a full $2.25 Zone 1 fare.
Port Authority of Allegheny County, Pittsburgh Tribune-Review, Pittsburgh Post-Gazette, Herrenknecht AG website, TBM: Tunnel Business magazine (Feb 2009), Civil Engineering magazine (Mar 2009)
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Gateway "T" station
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North Side "T" station
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Northern tunnel portal
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Allegheny "T" station
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