The TBM driving the twin tunnels under the Allegheny river in Pittsburgh for the North Shore Connector light rail project has holed through on the first drive.
Contractor North Shore Constructors, a JV of Obayashi and Trumbull, launched the 6.5m diameter Herrenknecht Mixshield in late January. The launch had been scheduled for last year but poor ground conditions on the construction site on the north side of the river delayed the start of boring. The TBM arrived on site in August 2007.
Geology along the alignment comprises primarily silts, sands, gravel shales and claystone with lenses of limestone. Progress rates achieved during the drive with cover of 6.5m to the riverbed were up to 12.3m per day, as planned. The hydrostatic head at the deepest point on the 680m long drive was 14.5m.
The TBM (S-374) holed through south of the river on 10 July at the reception pit that had been excavated in the road between two buildings on Stanwix Street. The breakthrough was delayed a half day by a malfunctioning pillow block bearing on the slurry plant conveyor belt.
Last year the construction schedule anticipated that both bores would be completed by now but the shield is now being prepared to be relaunch on the northbound drive from Stanwix St. Along with the stations either side of the river and the cut and cover sections, the entire underground stretch of each tunnel is most of the 1.9km extension to the rail link.
The light rail project is being funded mostly with federal money and will enable further branches to help economic redevelopment of the region, said the Port Authority of Allegheny County.
In the early planning of the project the start of construction was scheduled for late 2005. However, when bids for the tunnel package were markedly higher than estimated the procurement was rethought and rebids were called for with different package possibilities.
The JV contractor won with a new package bid of US$156.5M for the tunnels and a stretch of alignment on the North Shore and the shell of the North Side station. The client’s estimate of US$140M. The award was confirmed in early 2007.
The TBM driving the rail extension in Pittsburgh has completed the first bore
