At 7:46 p.m. yesterday the concrete wall below Stanwix Street began to crumble, then quickly burst, black earth and slurry streaming down into the pit.

Several dozen construction workers perched above the hole let out a cheer as the metal teeth of a 500-ton tunnel boring machine saw daylight for the first time since January, when it began its journey from a hole next to PNC Park.

And with that, the German-made machine completed one half of a tunnel under the Allegheny River, a $435 million project that eventually will bring light rail service to the North Shore.

Yesterday's final push was beset with delays.

The machine was supposed to pierce the 4-foot concrete wall in the receiving pit near Stanwix Street and Penn Avenue by mid-morning. But a half-hour after the machine started digging at about 6:30 a.m., a bearing broke on a conveyor system that carries slurry -- a mix of dirt, crushed stone and water -- to a separation plant, where waste is trucked away for disposal. As a result of the glitch, a replacement part had to come from Ohio.

Port Authority spokeswoman Judi McNeil likened the delay to the arrival of a baby.

"Just when you think the baby is going to be born, the baby has a different idea," she said. "Obviously, this is very bad timing."

After the mechanical delay, the machine made slow progress on the final 6 feet of earth, as workers were careful to avoid disrupting the water table or crashing recklessly through the wall.

The late-afternoon target time stretched into evening as the authority's mascot for the project -- Boris the badger -- handed out brochures near the pit. When the moment finally came, one of the workers sounded an air horn.

Workers now will spend several weeks turning the machine around, first tearing down the steel beams that reinforced the wall, then guiding the $10 million machine onto metal plates to turn it.

They will set up the mechanisms that will enable the cylindrical giant to start digging a parallel tunnel to carry trolleys in the northbound direction from Gateway Center once the 1.2-mile-long light-rail extension becomes operational sometime in 2011. The boring of the northbound tunnel is to be completed late this year.

The 2,240-foot tunnel is the first built under one of the region's three rivers; the first tunnel built in the city since the Fort Pitt and Squirrel Hill tunnels in the 1950s; and the first built in Allegheny County since the 1980s, when the Port Authority excavated a tunnel through part of Mt. Lebanon as part of reconstructing the South Hills light-rail line.