NEW YORK -- Construction of New York City's third water tunnel -- the largest public works project in its history -- has entered a new phase with the start of work on an 8.5-mile, $750 million stretch linking lower Manhattan with sections in midtown and northern Manhattan, the Bronx and Queens.
Mayor Michael R. Bloomberg said the overall project, begun in 1970 and scheduled for completion in 2020, would cost $5.5 billion to $6 billion to bring the city more than 1.3 billion gallons of water a day from reservoirs in Westchester County and Connecticut, 60 miles to the north.
Bloomberg and Department of Environmental Protection Commissioner Christopher Ward said the city needed the third tunnel to protect the city's access to water in the event one of the two others, built in 1917 and 1936, malfunctioned or required maintenance.
"If something should happen to one of these tunnels, this city would be without water," Bloomberg said at a news conference at the entrance of the construction site on West 30th Street and 11th Avenue. "It could take up to a year to dig down and repair it, and put it back in service. Having Tunnel No. 3 would give us a level of security that really we absolutely have to have."
With the completion of the third tunnel, each of the other two could be shut down and drained at different times so that they can be inspected, Bloomberg said. Valves in the older tunnels, which have never been used, are so old that engineers can't say with certainty whether they could be turned back on after they were shut off, he said.
The Manhattan sections -- Stage Two of the four-phase construction process -- will be dug at an average depth of 550 feet, with one leg running south to Lower Manhattan from 30th Street on the far West Side, while a second section runs north to Central Park, Ward said. Engineers expect it to be activated in 2011, he said.
Another part of Stage Two that extends through Brooklyn and Queens at a cost of $1 billion is complete. Engineers expect it to be operational by 2007.
Stage One, which went into service in 1998, cost $1 billion. It's a 13-mile segment running from Hillview Reservoir in Yonkers, through the Bronx into Manhattan, across Central Park and under the East River to Queens.
Stage Three extends 16 miles from the Kensico Reservoir in Westchester County to a valve chamber in the Bronx designed to route the water through any of the three tunnels. Stage Four will allow water to flow 14 miles to eastern parts of the Bronx and Queens from the valve chamber in the Bronx, scheduled for completion in 2020.
The tunnel's depth will range from 400 to 800 feet. Its diameter will vary from 10 to 24 feet.
Stage One was dug using "drill and blast" methods, while Stage Two has used a tunnel-boring machine, a drill-like device lowered in sections and assembled at the bottom that excavates twice as fast and more safely than explosives, Ward said.