Barge-mounted rigs began drilling for deep soil samples in the waters beneath the Tappan Zee Bridge Monday as work began on the five-year, $3.9-billion project to replace the Rockland-to-Westchester span.

During the next eight weeks, teams from Tappan Zee Constructors -- the consortium of companies chosen to build the bridge -- will assess soil conditions on the Hudson River bottom and use the information to gauge the size of piers and supporting foundations needed to support the new bridge, state officials said.

The barges, recently relocated from the coast of Louisiana, are the first of as many as 100 floating work stations that will be deployed when construction activity reaches its peak, according to Darrell Waters, the president of Tappan Zee Constructors and a veteran of bridge projects on five continents.

By early summer, some 50 to 60 barges will be on the river, building the foundation for the new bridge.

Monday, one rig was situated some 100 yards north of the bridge towers as motorists streamed by overhead. Others were on their way. State officials said the rigs will continue working around the clock this week, making their way across the river.

There is expected to be little or no noise that can be heard from the shoreline.

There will also be limited test-boring operations on land owned by the New York State Thruway Authority at either end of the existing bridge. Neither operation is expected to slow traffic.

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The current bridge will be demolished and most of the steel in it will be recycled once traffic is up and running on the new span, Waters said.