An Engineering Feat

Digging Begins on Manhattan Section of City Water Tunnel No. 3

by Natalie Keith

Though few will ever see it, deep beneath Manhattan's streets is a $750 million "engineering feat of gigantic proportion."

Since last fall, workers have been digging the Manhattan section of City Water Tunnel No. 3, a project that is being described as the largest public works project the city has ever undertaken. The size and length of the tunnel, its sophisticated control system, the placement of its valves in special chambers, and the depth of excavation all make this a challenging job to complete.

"Though rarely seen by the public, this project will increase our water systems reliability and will instill a sense of security in our city's ability to provide essential services for future generations," said Mayor Michael Bloomberg.

Located 550 ft. under the streets of Manhattan, the tunnel will deliver safe drinking water to city residents. The section of the tunnel currently under construction originates from 30th Street on the west side of Manhattan and will run downtown to the Holland Tunnel. A second section will loop north up the west side to the Lincoln Center area. This 8.5-mile section of the tunnel will cost $750 million and will be activated by 2011.

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Planning for City Tunnel No. 3 began in the early 1960s with construction starting in 1970. The 60-mile-long Tunnel No. 3 is being constructed in four stages and is expected to be completed by 2020 at a total cost of $5.5 billion to $6 billion. The project is being financed through the sale of water bonds and the collection of water and sewer charges, according to the city's Department of Environmental Protection, which is overseeing the project.

Stage 1, completed in 1998, runs 13 miles from the Hillview Reservoir in Westchester, through the Bronx, Northern Manhattan and Queens. It cost approximately $1 billion and is already delivering drinking water to parts of these boroughs. Eventually, the Manhattan leg will be connected into the Stage 1 portion of the tunnel.

Stage 1 is a 24-ft. diameter concrete lined pressure tunnel that steps down in diameter to 20 ft. As water travels along this route, it will rise from the tunnel via 14 supply shafts and feed into the distribution system. Three of the four unique subsurface valve chambers have already been built to allow the connection of future stages of the tunnel without removing the water or taking any other stage of the tunnel out of service, according to the DEP.

The three valve chambers are located in the Bronx at Van Cortlandt Park and in Manhattan at Central Park and Roosevelt Island. Each valve chamber contains a series of 96-in. diameter conduits with valves and flowmeters to direct, control and measure the flow of water in sections of the tunnel, according to the DEP.

The first section of Stage 2, the Brooklyn-Queens portion, is completed and will be activated by 2007. This 10.5-mile leg of the tunnel runs from Red Hook, Brooklyn to Woodside and Astoria, Queens.

"The activation of Stages 1 and 2 of Tunnel No. 3 will provide the system with the ability to bypass one or both of City Tunnels No. 1 and 2," said DEP commissioner Christopher Ward. "Operation of the new tunnel will allow for inspection of the older tunnels for the first time since they were put into operation in 1917 and 1936 respectively."

The tunnel will require 79,000 tons of concrete to complete and the amount of rock displaced during excavation will be approximately the size of a football field piled 250 ft. high.

So how are workers able to remove that large amount of rock? They're using a tunnel boring machine, a mechanical rock excavation technology never before used in City Tunnel No. 3. The TBM, also called the "mole," was lowered in sections and assembled at the bottom of the shaft in a short section of tunnel called the "bell out." The TBM is about 70 ft. long and is supported by 700 ft. of trailing gear, according to the DEP.

It is a Robbins 1215-257 hard rock-main beam type tunnel boring machine and uses a rotating cutter head containing 27 17-in. diameter disc cutters to crush the rock. The disc cutters push into the rock under a thrust load of 1.9 million pounds and crush the rock into small fragments. Four 450 horsepower electric motors rotate the cutter head at approximately 12 revolutions per minute, according to the DEP.

The crushed rock, or muck, is removed from the face of the tunnel through the TBM and over the trailing gear by conveyor. The conveyor discharges the muck into muck cars. Trains of four muck cars pulled by locomotives transport the muck through the bored tunnel to the access shaft, where the muck is transferred and raised to the surface by a vertical conveyor belt, according to the DEP.

Initial excavation will be south of the shaft. After the south portion is completed, the TBM will be backed out to the shaft and turned around to do the north portion of the tunnel. It is anticipated that the TBM will excavate bedrock 24 hours a day, five days a week, at a rate of 75 ft. per day, according to DEP.