(Editor's Note: This article is part of an occasional series that takes a look back at iconic United States construction projects. It originally appeared in the pages of CEG in September of 2006.)
One of the world's largest and most critical construction projects is moving along foot by foot in New York City, unseen and as deep as 800 ft. (243.8 m) beneath the streets. It's taking half a century to complete.
Sandhogs using a huge "mole" mechanical rock excavator are boring 50 to 100 ft. (15.2 to 30.5 m) a day through bedrock to painstakingly construct a 60-mi. (96.6 km) long third tunnel for the city's water in the dim netherworld under the city. When complete in 2020, the third tunnel network will be able to carry 1.3 billion gal. (4.9 billion L) of water per day for the nine million residents of the metropolitan area.
Financed through water bonds and the collection of water and sewer fees, the $6-billion tunnel is the largest and most expensive capital construction project in the city's history. It will be able to supply all the city's water, allowing two older tunnels, completed in 1917 and 1936, to be inspected and repaired.
Serious Situation
The two older tunnels are in questionable condition. Neither has ever been taken out of service since being activated in the first half of the last century.
Since the two serve different areas of the city, if one fails, the other can't take its place.
"Without the third tunnel, there really is no way of replacing a failed section," said Lou Huang, chief of waterworks construction of New York City's Department of Environmental Protection (DEP). "Completing the new tunnel is very important in terms of being able to keep the city functioning. Besides having drinking and bathing water available, we also need water at all times for firefighting. If the present Tunnel Number 1 under Manhattan failed, how long could the city as we know it continue to function?
"With no way of fighting fires, some of our industry would have to leave. Inevitably the population would decrease, and a lot of services would go by the wayside."
City officials have said that if City Tunnel No. 1, which is considered the most vulnerable, caved in, all of lower Manhattan and downtown Brooklyn, as well as parts of the Bronx, could lose its water supply. Fears are compounded by the always-lurking threat of a possible terrorist bomb rupturing one or both of the older tunnels.
(Once the third tunnel handling the water, Tunnel 1 would probably be the first to be inspected and improved. The city would carefully assess its structural, mechanical and electrical systems and proceed with repairs depending on condition.)
New York City realized as early as 1954 that it needed to do something about its aged water supply system. It began planning the third water tunnel, called "City Tunnel No. 3", in the early 1960s and started digging it in 1970, making the project 36 years old.
"The basic design of the new third tunnel was done in the Cold War and had the threat from the Cold War in mind," Huang said. "The tunnel is hardened a lot more than the old tunnels and is designed to survive a hit from a nuclear blast not too far away."
Maze of Obstacles
The obstacles to designing and building the new tunnel under the city are horrendous.
Water flows by gravity from reservoirs upstate and descends through aqueducts from as high as 1,400 ft. (426. 7 m) above sea level to tunnels as deep as 1,000 ft. (304.8 m).
Much of the third tunnel is being constructed 600 to 800 ft. (182.8 to 243.8 m) below the surface to avoid the maze of electrical and other lines under the city's buildings and streets. This means boring through the hard bedrock Manhattan Schist — once alpine-height mountain rock — which was formed more than 400 million years ago.
Also beneath the streets are more than 32 million mi. (51.5 million km) of gas mains and other utility lines, 443 mi. (712.9 km) of subway tracks, 6,000 mi. (9,656 km) of sewers, 22 tunnels, and, of course, even some people who live here out of sight.
The city's gas mains and steam pipes would reach across the United States and back three times. The streets are dotted with 750,000 manholes, usually unnoticed access points to the hidden world below. And people travel 18.5-billion mi. a year on public transit through this labyrinthine world.
Building a tunnel 10 to 24 ft. (3 to 7.3 m) in diameter through all this is one of the world's most challenging construction projects.
Past Tragedies
New York City has experienced enormous tragedies, so there's real reason for worry.
Because of mosquitoes breeding in foul drinking water, thousands of its residents died from yellow fever epidemics in 1798, 1805, 1819, and 1822. Thousands more died from asiatic cholera in 1832. After approximately 700 buildings, approximately one-third of the city, burned down in 1834, with little water to fight the flames, the city completed the 30-mi. (48.3 km) Croton Aqueduct in 1842, followed by the Catskill Aqueduct, completed in 1907 at a cost of 10 workers killed or injured out of each 100.
City Tunnel No 1, begun in 1911, brought the water from Yonkers to the city. Both it, and City Tunnel No 2, begun in 1929, were built using underground caissons filled with compressed air and requiring workers to enter through airlocks. Many perished from the "bends."
(A third underground aqueduct, which brings water 84 mi. (135.2 km) from the Delaware River, has leaked as many as 1 billion gal. of water a month, according to some DEP reports. The DEP has inspected and photographed much of its interior with an 800-lb. self-piloted submarine.)
Intrepid Sandhogs
Sandhogs (the name derives from men who excavated soft earth in building underwater caissons for the Brooklyn Bridge in the 1870s) are winched down hundreds of feet in cages to the eerie, dim worksite. During the first stage of the tunnel project, they blasted much of the rock with dynamite, "mucking" it out in huge buckets each carrying 28 tons (25 t).
Now, however, their primary equipment is a monstrous "mole," a 450-ton (408 t), 19-ft. (5.8 m) diameter, 70-ft. (21.3 m) long tunnel boring machine (TBM) similar to that used to dig the channel tunnel ("chunnel") between France and England. The TBM causes less damage at the excavation point than blasting, and doesn't rock surrounding communities with dynamite explosions.
The boring machine, which is lowered into the tunnel in pieces and assembled at the bottom, can excavate 55 to 100 ft. (16.8 to 30.5 m) of rock per day at a diameter of 23 ft. (7 m) — more than twice as fast as drilling and blasting. Applying enormous pressure, it chips off the bedrock by rotating a series of steel cutting teeth.
As the TBM crushes rock with its circular rotating head, which contains 27 cutters, weighing 350 lbs. (158.8 kg) each, the rock muck is removed by conveyor through the equipment's trailing gear. Loaded into rail cars, which a locomotive then pulls to the head of the shaft, the muck is sifted, crushed and raised to the surface on a vertical conveyor belt.
Huge Concrete Job
At its deepest, the third tunnel is 800 ft. (243.8 m) below the ground. At its most shallow, it is 400 ft. deep. The excavation is reinforced and then finished with a full-circle 14-in. (35.5 cm) layer of concrete. The Manhattan Section alone will use approximately 80,000 cu. yds. (61,164 cu m) of concrete.
The concrete for lining the tunnel comes from trucks on the surface. In completed sections of the tunnel, it has simply been dropped to the worksite through a pipe, remixed at the bottom, and sent forward to concrete forms in the areas to be surfaced. Huang told Construction Equipment Guide (CEG), however, that the contractor is now proposing that concrete be pumped down rather than using a drop-pipe.
"The method remains to be finalized," he said.
The concrete lining uses 500 ft. (152.4 m) of cylindrical steel forms. After one-half of the form is placed and set, the contractor places the second half. The first half of the form is collapsed and passed inside the second half to take more concrete at the front of the work.
"It's as if you had two cans with lips of both sides cut out," Huang said. "You put the cylinders end to end, place concrete in the first, and then in the second. Once we're done on the first can in a day or two, it can be stripped and collapsed and passed through the second."
After being lined with the concrete, the tunnel will be tested, fitted with instruments, and sterilized before water will be allowed to flow.
Massive Excavation
The amount of rock displaced during the excavation of the 8.5-mi.-long (13.7 km) Manhattan leg of the tunnel is approximately the size of a football field piled 250 ft. (76.2 m) high.
Since 1970, approximately 5,000 sandhogs have worked on the third water tunnel. Twenty-four personnel — all but three of them sandhogs — have died in construction-related accidents during the project. This is a mortality rate of roughly one man per mile.
Huang said, however, there have been no fatalities on the third tunnel project for the past few years, though underground excavation is one of the most dangerous areas of construction.
The sandhogs on this project belong to Local 147, the sandhogs' union. They earn as much as $120,000 per year. Also on the job are operating engineers of Locals 14 and 15, plus DEP employees.
Important Milestone
The third water tunnel, designed and to be operated by the DEP, is being planned as four-stages.
Stage 1, which cost $1 billion, is complete. It extends 13 mi. (20.9 km) from Hillview Reservoir in Yonkers, under the Bronx and Manhattan (beneath Central Park), and under the East River and Roosevelt Island into Astoria, Queens. Opened in August 1998, it is 20 to 24 ft. (6.1 to 7.3 m) in diameter and uses 15 supply shafts to raise the water to the city's distribution system.
The project reached an important milestone when excavation of Stage 2, an 8.5-mi. (13.7 km) Manhattan segment, was completed on Aug. 9, 2006. This segment resembles three spokes radiating from a central point roughly below the intersection of West 30th St. and 11th Ave. One spoke connects with the newly completed tunnel in Central Park. A second segment goes to Lower Manhattan. The third spoke, 2.5 mi. long (4 km), goes east to 2nd Ave. and then north to East 59th St. and 1st Ave.
The Manhattan section, begun in October 2003, is scheduled to be completed in 2012 at an estimated cost of $1 billion.
The construction contractor for the Manhattan leg of Stage Two is a joint venture of Schiavone Construction Co., J. F. Shea Construction and Frontier-Kemper Constructors.
Stage 2 also includes a $750 million, 10.5-mi. (16.9 km) Brooklyn/Queens section, which is to begin delivering water by 2009, supplying water to areas in all five boroughs.
Mayor Cites Importance
Mayor Michael R. Bloomberg and Emily Lloyd, commissioner of the DEP, wearing rubber boots and yellow outer gear, descended 550 ft. (167.6 m) to the tunnel, operated the boring machine, making the final cut to mark the milestone, and signed their names on the tunnel wall.
"The Third Water Tunnel is the single largest infrastructure project in the city's history," Bloomberg said. "The building of Water Tunnels One and Two were essential in New York City's evolution into a world business and cultural center and the third tunnel will help keep our city thriving through the 21st Century."
"We can live without a lot of things; water is not one of them," the mayor said. "It would be a very big problem if one of those two tunnels were to collapse in any one portion. It could take up to a year to dig down, repair it, and get it back in service … One of the great fears is, if today you turned off one of those valves, you may not be able to get it back on, or maybe the tunnel may collapse if there's no water in it."
Bloomberg said his administration had committed nearly $4 billion to the third water tunnel, doubling the investment of the previous five administrations.
Future Tunnels
Stage 3 of the third tunnel, in the final planning stages, is a 16-mi. (25.7 km) long tunnel from the Kensico Reservoir to the tunnel's valve chamber in the Bronx. It would deliver water to the tunnel from Kensico at greater pressure than from Hillview since Kensico is at a higher elevation.
Stage 4, if required, would be a 14-mi. (22.5 km) tunnel delivering water from the valve chamber under the East River into the eastern parts of the Bronx and Queens.
New Valve System
Malcolm Pirnie Inc., White Plains, NY, design engineers for the third tunnel, designed an expandable valve system, which allows future stages of the tunnel to be connected without removing the water or taking any other stage of the tunnel out of service. This removes the worry about not being able to open valves once they are closed, and is a major new advanced feature of the third tunnel.
The system includes four huge subsurface flow-control valve chambers.
Three of the huge chambers have already been built — at Van Cortlandt Park in the Bronx, at Central Park in Manhattan, and at Roosevelt Island.
The largest chamber, at Van Cortlandt, is 620 ft. (188.9 m) long, 42.5 ft. (12.9 m) wide, and 41 ft. (12.5 m) high. It includes 34 large stainless steel valves, which can be quickly operated electronically.
Each valve chamber contains a series of 96-in. (243.8 cm) diameter conduits with valves and flowmeters to direct, control, and measure the flow of water in sections of the tunnel.
Special Technique for Drilling Supply Shafts
Fifteen vertical supply shafts, up to 35 ft. (10.7 m) in diameter and hundreds of feet deep, are being built to supply the city's water mains from the third tunnel.
"One thing we're doing differently is to use the raise bore technique, from the bottom up, to excavate the shafts, as opposed to conventional shaft sinking, which is from the top down," Huang said. "The tunnel doesn't have to be there for the conventional method, in which you excavate sand and rock and bring it to the top. In raise bore, you need to have the tunnel there. First, you drill a pilot hole to the tunnel. This permits the contractor to set up a machine on top and drill down with drill-strength steel. The drill is connected to a head, which has been brought to the tunnel. The machine on top pulls the head up as the head rotates.
"The rotating head behaves like a tunnel machine, turning and fracturing muck, which drops to the bottom of the tunnel, where it is picked up and taken out by train. The shaft diameter varies depending on depth; generally it is largest, about 26 feet at the top and steps down to the smallest diameter, about 12.5 feet, where it connects with the tunnel."
For more information visit www.nyc.gov/dep.
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