NYC Environmental Protection
Inside the concrete
It only required 40 years, but massive tunneling deep beneath New York City for a vital new water-carrying conduit is nearing completion. A whale-sized digging machine, some tough construction workers and megatons of concrete have done the job.
The $6 billion dig — the city’s largest capital project ever — involves tunneling out miles of bedrock and building a tube in the tunnel using millions of cubic feet of concrete so the city of 8.3 million people will have an insurance policy against failure of two older subterranean water lines.
“This project represents one of the most significant investments in the future of the city’s drinking water system,” said Cas Holloway, the city’s environmental protection commissioner. His remarks came in May at the conclusion of the paving phase of Tunnel 3 under Manhattan. The final phase involves installation of piping, mechanical and electrical systems, a process that won’t be completed until sometime in 2013 and will cost another $176 million.
As construction projects go, Tunnel 3 is one of the more curious in engineering annals. For the most part it has been ongoing for decades almost completely out of sight of millions of people living within a few hundred feet of it. Other ongoing underground NYC construction projects – including mass transit ventures like the East Side access and Second Avenue subway – are significant, but they don’t measure up, or down, to Tunnel 3.
Tunnel 3 is incomparable in terms of its duration through six city administrations, its massive cost – the Manhattan tunneling phase alone cost $680 million – and its demanding logistics, requiring hardhat labor at depths of 400 to 800 ft. (121 to 242 m) for miles and miles on end.
It’s All About Water
“We can live without a lot of things, water is not one of them,” Mayor Michael Bloomberg has said in backing the tunnel project. The mayor alluded to the city’s responsibility to furnish some 1.5 billion gal. of water each day to 9 million NYC metropolitan area residents for drinking, cooking and bathing. Almost as important, the water is a necessity for firefighting.
City fathers found all this to be true in the 19th century when a series of epidemics stemming from bad water ran through the city and then a devastating fire struck it. Consequently, a series of aqueducts from upstate New York were dug to supply greater volumes of water to the city from watersheds as far as 125 mi. (200 km) away.
Then early in the 20th century, two tunnels were created to disperse the gravity-fed water to the five city boroughs.
Tunnel 1 was completed in 1917 and Tunnel 2 in 1936. They were constructed painstakingly — literally, with some tunneling workers dying from the “bends” as a consequence of their work in underground pressurized caissons. Their work was not in vain, however, for the two tunnels have been an unqualified success.
That’s the problem: They are indispensable. They have never been out of service – and need to be. Like any other infrastructure, the rub of constant utilization erodes and debilitates surfaces and wears out mechanical and electrical systems. Construction of Tunnel 3 will let water be diverted from Tunnels 1 and then 2 so that much-needed inspection and repair of the two older tunnels will be possible.
Mayor Bloomberg has among other degrees a bachelor of engineering from John S. Hopkins University. He made completion of the tunnel a city funding priority.
“I looked at our water tunnels and, in the past six years we have dedicated more money in this area than did the previous five administrations combined,” he told an interviewer for Leaders Magazine in 2008. “A third tunnel will give us redundancy and more capacity and because of the way it’s designed, it will let us turn off each of the two old ones, which nobody has looked at before. We’re afraid to turn them off because we think we may not be able to turn them back on, and there’s a worry that without water in them, they will collapse.”
While there is as yet no repair schedule, Tunnel 1 will be the first one drained and scrutinized. It is the oldest and most vital of the two. The city’s Department of Environmental Protection, which manages the New York City water supply, will send inspectors into the tunnels for a meticulous assessment of tunnel walls and valves.
When Tunnel 3 is completely functioning and inspection of the other two tunnels proceeds, the water will flow unabated. Everything is tied to keeping the water constantly moving – hundreds of millions of gallons a day – from one tunnel or another, up through vertical shafts spaced periodically along the way, into 7,000 mi. (11,200 km) of water mains and finally into lines serving homes and commercial buildings.
Building the Tunnel
Construction of Tunnel 3 involved a variety of general contractors through the years, but it utilized the same union of craftspeople who gave the city the Holland and Lincoln tunnels and mile upon mile of functioning subway and sewer tunnels. They are members of Laborers Local Union No. 147, affectionately called “Sandhogs.” The name came from their first job rooting around in sand during construction of footings for the Brooklyn Bridge.
They are categorized as miners – urban miners – because they often dig through subterranean rock and soil. They differ from other miners in that their tunneling is the purpose of their labor. But as in other mining work, it can be dangerous: 24 people have died in the course of Tunnel 3’s construction, though none in recent years.
Regardless of the dangers involved, in some NYC families Sandhogs go back for several generations. It is a testament to the unique appeal of the work, to Local 147’s attractive wages and to the pride that comes from laying the foundation, almost literally, for many of the monumental commercial and infrastructure projects in the city. At the height of work on Tunnel 3, some 350 Sandhogs and auxiliary workers worked their way through their subterranean passage behind a massive 450-ton (408 t) boring machine – lowered into the tunnel and then assembled – that chewed up bedrock at an average rate of 80 linear ft. (25 m) a day. The machine’s rotating front edge has 27 cutting blades that chip away at the solid rock found at that depth.
The chipped debris is automatically conveyed to the rear where it is loaded into rail cars and hauled to an outlet shaft.
This boring process — which was completed in Tunnel 3 in 2006 — replaced the traditional method of drilling holes for explosives, detonating them and clearing away whatever was blown loose.
The boring machine removed from the path of Tunnel 3 approximately 82 million cu. ft. (2.5 million cu m) of rock, all of which was crushed and sent to the surface for utilization on many other construction projects. City public affairs specialists compute the volume of debris to be the equivalent of rock filling the Empire State Building and the new Yankee Stadium.
Tunnel 3 meanders underground for about 60 mi. (96 km), beginning at Hillview Reservoir on the north edge of the city proper. The first stage of the tunnel parallels Tunnel 1 for much of its length down through the Bronx before jogging west and running half the length of Manhattan, then turning east to continue under the East River into Queens and Brooklyn. This 13-mi. (21-km) leg of the tunnel, costing $1 billion, was placed in service in 1998.
The second stage of the tunnel runs almost another 11 mi. (18 km), cutting deeper into Queens and then turning south to slice through Brooklyn. Concrete lining of that portion of the tunnel was completed in 2001. The final stage of the tunnel juts off from the first leg in Mid-Manhattan and runs on into Lower Manhattan, with two fingers of it curling around northward before dead-ending. The average depth of the Manhattan leg is 540 ft. (164 m).
Water Pipes and Valves
Tunneling through rock is just half the job. Water for city residents doesn’t actually rush through the exposed bedrock. Rather, a cylinder of concrete is constructed inside the tunnel so that engineers can more uniformly contain the flow of the water and maintain its sterility. Consequently, the urban miners don’t just tunnel, they build, and now have completed the concrete work in the tunnel.
They used some 500 ft. (152 ms) of collapsible cylindrical steel forms to shape the piping, which ranges in diameter from 24 ft. (7.3 m) to 10 ft. (300 cm), the pipe narrowing as it progresses. They would pour one section, then a second section, before breaking down the first section and leapfrogging it ahead of the second section to repeat the process. On average, about 145 linear ft. (44 m) of concrete pipe was poured daily.
The concrete work was both exacting and tedious. With the pressure of 14 in. (36 cm) of concrete bearing down on the forms in which the men work, it also can be dangerous. How the concrete was handled evolved as the project moved along. In the first two stages of tunnel construction, concrete was premixed and piped down to the tunnel floor where it was remixed and hosed into the cylindrical forms. On the final Manhattan leg of the tunnel, however, the concrete was pumped directly from above onto the forms.
How much concrete did the tunnel consume? Engineers said the entire Tunnel 3 project required about 30 million cu. ft. (900,000 cu m) — enough to fill up 80 percent of the Empire State Building.
Vertical shafts, which feed the water upward to the city’s water mains, were bored using a raise bore method in which the boring machine is pulled upward through a pilot shaft, with the debris falling down into the tunnel for removal. For reasons of pressure, the shafts widen as they come closer to the surface. The Manhattan stretch of the tunnel has 10 of these shafts.
The flow of the water in the tunnels will be regulated using valve chambers, which also are carved out deep underground in the path of the pipes. The rooms are boxy, concrete facilities measuring about 40 ft. (13 m) in width and height. Numerous stainless steel valves and flow meters are set up in the chambers and electronically monitored and controlled. Engineers can regulate flow and divert the water as required for maintenance or emergency situations.
The largest of the chambers is more than 600 ft. (182 m) long.
The probable final stage of water supply tunneling is a 16-mi. (26-km) segment running from a reservoir in Westchester County to a valve chamber in the Bronx. It would give the system more pressure and an additional water aqueduct to the city. One more tunnel feeding water into additional areas of the Bronx and Queens is possible.
The entire tunneling project is expected to be complete by 2020.