The Port Authority plans to test-drive early next year the 500-ton cylindrical boring machine that will dig two light-rail tunnels beneath the Allegheny River.

The giant drill is sitting beneath West General Robinson Street on the North Side. Workers now plan to move it through about 50 feet of earth in the first week of January to make sure it is functioning properly, said Kirk Thompson, project manager for the Port Authority of Allegheny County.

Once the test drilling is completed, contractors will begin continuous boring, probably later in January.

The first 2,400-foot-long tunnel is due to reach the "receiving pit" at the site of the new Stanwix Street station Downtown in April or May. The boring machine will then make a u-turn and dig a second 23-foot-diameter tunnel back to the North Shore.

At no point will the tunnel pass through water. It will be at least 20 feet beneath the riverbed, and even deeper at the midpoint of the Allegheny.

As the boring machine churns forward at about 1 foot per hour, a milkshake-thick slurry of water and bentonite clay will be piped to the space in front of the cutting heads, making it easier for the machine to chew through the soil. Then, other pipes will take the mixture of earth and bentonite back to a white-and-blue-striped building on the North Shore, where the two substances will be separated in a settling pond.

The bentonite will be reused, while the tailings from the tunnel will be piled outside the building and hauled away to special landfills in Beaver and Washington counties, Mr. Thompson said.

The tunneling will remove more than 65,000 cubic yards of earth and stone -- enough to fill 440 tractor-trailers.

The Port Authority is making progress on other parts of the project as well.

The walls for the first North Side station, which will sit beneath the West General Robinson Street parking garage near PNC Park, are nearly complete.

They are being built with slurry-wall construction methods, in which the spaces for the walls are excavated and filled with a bentonite slurry to maintain their shape. Workers then lower supportive rebar into the slurry. Finally, concrete is pumped into the opening and slowly displaces the slurry.

After the walls are finished, contractors will build the station's roof below ground level, and then the space beneath the roof and inside the walls will be dug out. "It's known as top-down construction," Mr. Thompson said.

At the site of the new Downtown station beneath Stanwix Street, workers are relocating a 42-inch storm sewer line in the final phase of utility line relocation there.

They are also using the slurry-wall method to build temporary walls for the underground receiving pit, which will stretch 50 feet wide, 40 feet long and 40 feet deep from Stanwix and Penn Avenue back toward Liberty Avenue.

Stanwix Street, which has been closed between Penn Avenue and Fort Duquesne Boulevard, is scheduled to reopen in mid-February, but workers will probably still be excavating the receiving pit beneath temporary decking, Mr. Thompson said.

The $435-million project is expected to be finished by 2011, and is designed to extend the Port Authority's light rail system from Downtown to the fast-developing North Shore. Besides new stations Downtown and near PNC Park, a third new station will be built above ground at the end of the line near Heinz Field.