The high-tech, computer-driven, 500-ton machine boring the first of two twin light-rail transit tunnels under the Allegheny River is stumped.

Port Authority officials revealed yesterday that a large tree trunk, complete with roots, somehow became lodged at the front of the machine and clogged a 10-inch "slurry pipe" that mixes water with excavated dirt and crushed rock and then pumps it back to a "launch pit" for disposal.

Because of the blockage, the machine has been shut down for up to three weeks and tunnel digging has stopped beneath the North Shore.

Soil and rock at the front of the machine's 21-foot-diameter cutting head will be grouted with concrete before a repair team enters a pressurized chamber to remove the stump, which apparently had been buried some time ago.

While there, the repair team will inspect and replace worn teeth on the cutting head and perform any other necessary maintenance.

Boring the twin tunnels is part of a $156.5 million contract for the 1.2-mile light-rail extension from the Gateway Center T Station to the North Shore, where stations near PNC Park and Heinz Field are part of the federally funded $435 million project.