The Port Authority's tunnel-boring machine is ready to start digging its way from Downtown to the North Shore this week, probably today, as work advances on the 1.2-mile light-rail extension.

The 500-ton behemoth has slowly and gingerly been turned around in the Stanwix Street "receiving pit," which it reached July 10 after a six-month, 2,240-foot southbound trip under the Allegheny River.

Workers and engineers have pushed the machine up against the wall where the second tunnel will begin and positioned laser equipment for guidance for the northbound trip.

Meanwhile, they've installed a "seal ring," akin to a rubber washer inside a garden hose, and are connecting electrical, hydraulic and other support lines and systems. The work is taking place in a fenced-off area near the Stanwix Street-Penn Avenue intersection.

Once the drilling starts, the machine will make an initial bore of about 150 feet. Then it will be stopped until workers attach "trailing gear" that includes a pipe that will carry mined earth and stone mixed with water as a slurry through the completed tunnel back to a separation plant on the North Shore, near PNC Park.

Project officials said boring the second tunnel should take less time than the first one because of lessons learned, including dealing with a tree trunk that jammed the machine. The second 21-foot-diameter tunnel is expected to be finished before the end of the year.

Once the machine passes Fort Duquesne Boulevard, the authority expects to reopen the block-long section of Stanwix back to Penn Avenue. Before then, however, the block of Stanwix between Penn and Liberty avenues will be closed while a contractor begins building a new Gateway light-rail station, essentially in front of Fifth Avenue Place.

Meanwhile, the authority is soliciting bids for the next contract for the purchase and installation of eight elevators and eight escalators for the Gateway, North Shore and Allegheny stations. Bids are due Oct. 15.

The 1.2-mile extension is scheduled to be operational in 2011.