About a month after it reopened, Stanwix Street will be closed again between Fort Duquesne Boulevard and Penn Avenue starting Monday to accommodate the Port Authority's North Shore Connector project.

This time, the block-long Downtown street closing will last less than three weeks as the 500-ton tunnel boring machine is turned 180 degrees while sitting in a receiving pit excavated in the middle of the street.

Several weeks later, the machine will start drilling the northbound tunnel under the Allegheny River, another link in the 1.2-mile light-rail extension between Gateway Center and the North Shore.

As of yesterday morning, the contractor had finished 1,908 feet of the southbound tunnel, including the installation of 472 prefabricated, interlocking concrete rings that make up the shell of the 21-foot diameter hole between Downtown and the launch pit near PNC Park.

During the previous 24 hours, the huge wormlike machine had advanced 44 feet, leaving 332 feet until it punches through the earthen wall of the receiving pit, whose bottom is 55 feet below Stanwix Street.

Authority officials had been planning a low-key public reception for the milestone. They have since nixed the idea because deliberately slowing or shutting down the complex tunnel boring machine could cause complications and add to already tight costs.

Instead, the authority plans to install a camera in the pit to provide a live video feed to a large-screen television that will be set up somewhere around the Stanwix Street-Penn Avenue intersection.

"They could 'hole through' in the middle of the night," authority spokeswoman Judi McNeil said. "The time it happens is something we won't know until the last minute."

The Port Authority reopened the section of Stanwix Street May 28, but bus service didn't resume there for several reasons, including the upcoming need to close it again for the tunnel boring machine's arrival and events like the Tour of Pennsylvania bicycle race this weekend.

The $435 million North Shore Connector, which is facing overruns because of escalating construction costs, is to open in 2011. It will include a new Gateway Center station with dual platforms and two stations on the North Shore.