Starting this week, contractors for the Port Authority's $435 million light-rail extension project will take two major steps on the North Shore.

First, the mammoth tunnel-boring machine will begin its descent toward the Allegheny River by grinding its way under part of the six-story Equitable Resources headquarters west of PNC Park.

The second event involves moving piers that support several elevated ramps connecting Route 65 and the Fort Duquesne Bridge onto a new substructure, a move that will enable workers to sever and remove underground concrete pilings blocking future tunnel construction along Reedsdale Street.

Neither activity is expected to cause major disruption.

Equitable Resources is looking to conduct business as usual for the estimated three days that the 500-ton, worm-like tunnel boring machine will be in operation below its foundation.

"It's no big deal," said Winston Simmonds, the Port Authority administrator overseeing construction. "The foundation was designed so this work would not impact the building. We don't anticipate damage."

Contractors are injecting a special grout to fill any voids that develop between the earth and 21-foot-diameter tunnel rings installed as the boring machine advances.

As of Friday, the machine was about 170 feet from the Equitable Resources building. Two months from now, it should be mining in bedrock below the river en route to Downtown and a new Gateway Center T station.

The work involving Route 65/Ohio River Boulevard will require closing three ramps for two to three weekends, when traffic is light -- Fort Duquesne Bridge ramps leading to and from Route 65, and the North Shore ramp between the bridge and the PNC Park-Heinz Field area.

Ramp traffic will be detoured via the West End Bridge, probably starting March 21-24, from Friday night through early Monday morning.

Interstate 279 will remain open in both directions, as will the Route 65 mainline between the West End Bridge and the North Side, Route 28 and I-279 north.

"We've been coordinating with the Port Authority from the start," said Jim Struzzi, spokesman for the Pennsylvania Department of Transportation. "We're OK with everything."

The work involving the piers is called "underpinning." The piers, and the bridge superstructure they carry, currently sit on a series of underground pilings that extend down to bedrock. But the pilings stand in the way of cut-and-cover tunnels west of PNC Park, where the light-rail line starts making a transition to elevated track.

In order to remove the pilings, the authority built thick slabs of concrete slightly below ground level and surrounding the base of the existing piers. The slabs, in turn, sit on concrete caissons much larger than the ramp pilings but also extending down to bedrock.

Special monitoring devices will be used to measure vibration and detect the slightest movement of the bridge ramps while the weight of piers is shifted from pilings to the caisson-supported concrete slabs.

The overall project will extend the authority's Light Rail Transit system a total of 1.2 miles from a new Gateway Center Station to the North Shore via twin tunnels under the Allegheny River. There will be two North Shore stations, the North Shore Station at the northwest corner of PNC Park and the Allegheny Station at the northwest corner of Heinz Field.

The federal government is paying 80 percent of the cost, PennDOT 16.67 percent, and Allegheny County $12 million, with the balance from the special capital funds that come to the Port Authority.