The huge tunnel-boring machine digging the first of two twin subway tunnels beneath the Allegheny River is expected to "hole through" sometime between 8:30 and 10 a.m. tomorrow.

The rotating teeth on the front of the 500-ton machine will grind through dirt and stone and break into the light at the end of the tunnel in a "receiving pit" 55 feet below Stanwix Street near Penn Avenue.

The Port Authority has set up a large-screen television so bystanders can watch the milestone event.

The 2,240-foot-long 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.

The Allegheny River tunnels are part of more light-rail work, this time a 1.2-mile extension from Gateway Center, Downtown, to the North Shore.

When the $10 million, 30-foot-long tunnel-boring machine breaks into the receiving pit, it will be slid onto steel plates and turned around with hydraulic jacks so it can start drilling in the opposite direction.

The twin tunnels -- one for northbound trolleys, one for southbound trolleys -- are the key component of the project whose price tag remains $435 million for now but is certain to require extra money because of soaring energy, steel and concrete costs.

The project includes building a new Gateway Center Station, with dual platforms, and two stations on the North Shore. The line is to be in operation sometime in 2011.