PITTSBURGH — Pittsburgh's light-rail tunnel project is stalled for about three weeks because a boring machine unexpectedly hit a layer of clay near the Allegheny River. That happened after a piece of wood jammed the machine's drill last week.

The 1.2 mile, $435 million tunnel will connect downtown Pittsburgh to its northern shore where PNC Park and Heinz Field are located.

The problems will delay digging about three weeks. They became known Friday at a meeting of the Port Authority of Allegheny County board, the agency that oversees county transit.

The project has already been delayed by other problems.