Port Authority of Allegheny County TBM is stuck in its tracks.

For the second time in two weeks, the 500-ton boring machine digging Pittsburgh's North Shore connector light-rail project is jammed.

In early April, a large tree trunk lodged at the front of the machine and clogged a 10-inch “slurry” outtake pipe. The pipe mixes water with excavated dirt and crushed rock and pumps it to a surface pit for disposal. The 20-inch stump was removed and boring work resumed after a one-day delay, says David Whipkey, spokesman for Port Authority of Allegheny County, owner of the project.

Shortly after the first jam, the German-made machine's outtake again became clogged with wood. The machine will not be functioning again for about two weeks, Whipkey says. The blockage has stopped tunneling work on the North Shore Connector, which when completed will include twin tunnels beneath the Allegheny River that will connect the city's light rail system from downtown Pittsburgh to stops on the city's North Side.

The boring machine had dug 720 ft of tunnel from its North Shore launch pit, and is about 10 ft from the river. Drilling began on Jan. 22 and had been progressing at a rate of about 30 ft per day—exceeding the Port Authority's goal of 25 ft per day.

To repair the machine, soil and rock at the front of the machine's 21-ft-diameter cutting head will be grouted with concrete before a repair team enters a pressurized chamber to remove the blockage, which consists of pieces of wood and clay. The repair crew will inspect and replace worn teeth on the cutting head and perform any other needed maintenance.

German firm Herrenknecht manufactured the boring machine. North Shore Constructors, a partnership of Pittsburgh-based Trumbull Corp. and Japanese firm Obayashi Corp., is the North Shore Connector project contractor.