MESSAGE
| DATE | 2005-08-08 |
| FROM | Ruben Safir
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| SUBJECT | Subject: [NYLXS - HANGOUT] Worth Reading Part 1
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This piece, written in 1972, looks at the construction of the World
Trade Center's twin towers, at a time when they were a symbol of
possibility.
Soaring above the lower end of Manhattan Island is the world's largest
cluster of tall buildings, whose oblongs, spires, and turrets have,
since this century began, given New York the most spectacular skyline
anywhere. Each one of the towers whose upper extremities pierce the
clouds is rooted, below the city's surface, in a huge, unseen structure
that may itself be the size of a ten-story building. The finishing
touches are now being put on the biggest foundation in the world, which
is below what are, as of now, the highest pair of buildings in the
world. These are the twin hundred-and-ten-story towers of the World
Trade Center, built for the Port Authority of New York and New Jersey.
Nine or ten good-sized office or apartment buildings could have been
fitted into the hole that was dug for the Trade Center, and the
foundation proper is six times as large as that of the usual fifty-story
skyscraper and four times as large as its closest competitorâthe
basement of the neighboring sixty-story Chase Manhattan Bank Building.
On a hot, dry summer day nearly seven years ago, I went down to the
corner of West and Cortlandt Streets to witness the initial tests of
some basic equipment for the construction of the main foundation walls
of what would be the biggest building job ever attemptedâin height of
the structures, size of the foundation and excavation, and almost
everything else. I was there to witness the test with Robert E. White,
who is executive vice-president of Spencer, White & Prentis, a firm of
foundation experts that is almost always called in whenever architects
and builders think anything complicated or unexpected may occur below
ground. Robert White and his brother Edward, president of that firm, are
old friends of mine, and when I had mentioned not long before that I had
always wondered what kept the tall buildings in New York anchored to the
ground or whatever was underneath it, they suggested that I observe some
of the steps in the construction of the foundations for the Trade
Center. I knew that besides the two skyscrapers, each thirteen hundred
and fifty feet high, at least three other buildings would be erected on
the sixteen-acre site: an eight-story structure for the United States
Customs Bureau and two nine-story ones for exhibits, meetings, and trade
activities, around a five-acre plaza. To accommodate this extraordinary
new assemblage within fourteen blocks of jammed lower Manhattan, the
Port Authority had condemned a hundred and sixty-four buildings then
standing on the siteâincluding the big, rambling headquarters of the old
Hudson and Manhattan Railroad, now the Port Authority Trans-Hudson
System, known as PATHâand had closed off parts of five streets that ran
through it. I had studied the map, and knew their namesâCortlandt, Dey,
Fulton, Washington, and Greenwich. I also knew the names of the four
streets bordering the site: West Street, running parallel to the dock
area and beneath the West Side Highway along the bank of the Hudson
River; Liberty Street, to the south, which is only a brief walk from the
Battery; Church Street, a step from Trinity Church, on the east; and, to
the north, Vesey Street, where the New York Telephone Company has its
headquarters. The city had made a neat bargain with the Port Authority.
In return for municipal assistance in obtaining the huge site, the Port
Authority was going to add about twenty-four acres of new real estate to
New York City by dumping the dirt and rock that would be excavatedâmore
than a million cubic yards, or enough to make a pile about a mile high
and seventy-five feet squareâinside a great riverside cofferdam, or
bulkhead. On this man-made peninsula, with a base extending from the old
Pier 7 to the old Pier 11 and the Central Railroad of New Jersey ferry
slip along the Hudson, plans called for streets, parking facilities,
sewers, mains for water, electricity, and steam, and, eventually,
apartment houses, stores, and various other buildingsâparts of a complex
to be known as Battery Park City.
The backbone of Manhattan is a rock ledge, which actually can be seen in
Central Park and a number of other places. Starting at Fourteenth
Street, it goes gradually beneath sea level, and extends under
Governor's Island, Staten Island, New Jersey, and possibly as far as
Pennsylvania, where similar rockâknown here as Manhattan Schistâhas been
found. At the World Trade Center site, the rock is seventy feet below
sea level, and above it is a nightmare for all construction
engineersâfilled land. Two hundred years ago, New York City was a little
colonial town at the tip of the island, with docks and piers reaching
like fingers into the rivers on either side. As the city grew, the dirt
and rock dug out for cellars and, later, for subways and other
underground installations, was dumped into the rivers to create new real
estate, and it is estimated that the island's shoreline was pushed out
about seven hundred feet in the area around the Trade Center. When a
heavy building rests on bedrock, its engineers can sleep peacefully.
Once they have dug to such rock, that's as far as they have to go, which
made Manhattan a superb location for many of the first skyscrapers. The
bedrock is so close to the surface in midtownâonly eight feet down at
Rockefeller Centerâthat sometimes it has to be blasted out for
basements.
As Robert White and I were walking toward the test site, he said, "Some
foundations, like those of the Empire State Building, are so routine
they aren't interesting. A one-story service station built on a swamp
could be more exciting. But on this kind of filled land there is nothing
but trouble," he said, looking pleased. "For a typical downtown New York
skyscraper, you normally dig down thirty or forty feet, but this
foundation will have to go anywhere from sixty to a hundred feet. Around
here, there's usually ten or fifteen feet of fill near the
surfaceârubble, old bricks, old anything. Then you have five to
twenty-five feet of Hudson River siltâblack, oozy mud, often covering
old docks and ships. Down here we may hit parts of an old Dutch vessel
called the Tijger, which burned off Manhattan in 1614. Below the silt,
there's maybe a dozen feet of red sand called bull's liver, which is
really quicksandâthe bugbear of all excavating. The more you dig in it
the more everything oozes into the hole. We expect to find it here, but
we know how to deal with it. Under that is hardpanâclay that was
squeezed dry by the glacier and its accompanying boulders. Finally,
beneath the hardpan, there's Manhattan Schist."
As we entered the lot where the test was to take place, I noticed water
running from a small pump into the gutter. White said, "People passing
by complained that we were wasting water, so a city inspector came
around. He laughed when he found that we were pumping out tidewater.
We're working below the level of the Hudson, with the same tides as the
Batteryâvarying from two to six feet." The lot was bare except for an
enormous green crane on red wheels, a pile of bags marked "Bentonite,"
several oxygen tanks, a large gray hydraulic jack, and a blue box, which
White said was for cutting wires. A dozen men, some in business suits
and some in grimy work clothes, all wearing hardhats, stood next to a
strip of concrete about twenty-five feet long and the width of a small
sidewalk. White explained that the concrete strip was the top of a
sample piece of foundation wall, extending about ten feet below the
ground, and that what I was going to see demonstrated was part of an
unusual system of foundation construction that would be used on this
job. Projecting from the concrete slab, at a forty-five-degree angle,
were three pipe casings, inside which, he explained, groups of rods,
wires, or cables, all known as tiebacks, extended, unseen, a hundred and
thirty feet down, where they were anchored to bedrock. Concrete for the
permanent walls making up the foundation's perimeter, thirty-one hundred
feet long and about seventy feet deep, would be poured into trenches dug
to the walls' width and depth before any other excavation began. Then
workmen would dig down to install tiebacks, such as those here, and they
would be stretched to a tension greater than the underground pressure
behind the walls, to hold them up while all the earth, rock, and other
matter they enclosed was removed. Ultimately, a complex system of
interior concrete partitions and floors, extending six stories below the
level of the street, would take over the job of supporting the outside
foundation walls. Then the fifteen hundred tiebacks would be de-stressed
and cut off. The purpose of today's tests, said White, was to determine
which of the three different sorts of rods, wires, and cables inside the
pipe casings would make the most effective tiebacks.
The group gazing at the section of wall opened ranks for us. Several of
the men proved to be Port Authority officials: John M. Kyle, the Chief
Engineer, who was a short man in a white helmet inscribed "Lincoln
Tunnel, Third Tube, Last Bolt, June 28, 1956;" Arne Lier, the
structural-design expert; Martin S. Kapp, head of the soils division, a
big friendly engineer in overalls; Harry Druding, the engineer in charge
at the site; and Leon Katz, the Port Authority's information officer.
(Both Kyle and Kapp, who succeeded him two years ago as Chief Engineer,
died of heart attacks before they could see the Trade Center completed.)
The rest of the men were from West Street Associatesâa group of five
heavy-construction companies (of which the Whites' outfit was one) that
had banded together with Slattery Associates, Inc., the lead contractor,
to take on most of the technically tricky, financially risky
twenty-seven-million-dollar job of underpinning the World Trade Center.
Protruding from one pipe casing were more than a dozen cables, now
slack, whose far ends were cemented underground. Each of these was now
to be pulled taut separately by the hydraulic jack exerting as much as
twenty tons of tension to test the tieback's strength. This machine was
moved into a position where its wedge-shaped jaws could grip one of the
cables, and it began drawing the cable up tight. As we watched, the
pressure gauge on the jack moved to two thousand pounds per square
inchâwhich represented a quarter of the cable's theoretical breaking
pointâand White explained that the steel cable would be stretched about
eight inches, like a rubber band. The needle crept to three thousand
pounds, then four thousand. When it was at five thousand, someone said,
"Get back in case the wire snaps." Everybody moved back a few feet, and
White said, "I saw two steel rods break during this sort of test in West
Virginia, and they shot like spears halfway across the Monongahela
River." Everyone moved back a step farther. Then Kyle called out,
"Sixty-seven hundred pounds per square inch! That should be enough.
That's twenty tons on the cable." It was announced that the wall was
taking the load well, having moved a mere seven-eighths of an inch. If
it had moved substantially, that would have meant that the tieback had
pulled away from its anchorage. The jack went to work on a second cable,
then a third, and within an hour or so it had been ascertained that all
the different tiebacks stood up well under the strain. The contractors
therefore decided on the one that they believed would be the most
economicalâa seven-wire cable about a half inch in diameter.
After the test, I accepted Kyle's invitation to accompany him to his
office at the Port Authority headquarters, at Eighth Avenue and
Fifteenth Street, for further enlightenment. I remember his saying, as
he showed me in, "Maybe the Walls of Jericho fell down because they
weren't built on good foundations." In his outer reception room, he
halted before an old topographical map of Manhattan Islandâpublished by
Egbert L. Viele in 1865 for the use of sewer engineersâwhich showed the
island's natural springs, streams, and marshes before they were covered
over by building projects. Kyle told me that this map has been a basic
reference source for all underground planning and construction in New
York for years. Kyle said, "Engineers make large-scale map blowups from
it of the areas where they're working. Every existing stream in
Manhattan shows up on the Viele map. There's the one at Forty-first
Street that we hit when we were building the Port Authority Bus
Terminal, and there's Minetta Creek, in Washington Square, which comes
out in a hotel lobby as a fountain. And there you can see that we have a
spring right under this building, whose water we use as a coolant in our
air-conditioning system to save city water, and to wash down the Holland
Tunnel during droughts."
Kyle told me that the World Trade Center foundation job was the most
difficult and most interesting he had ever faced, and went on to explain
why. Conventional deep foundations for tall buildings in New York, he
said, are built by excavating the site, driving steel sheeting down to
bedrock, propping it with heavy braces inside the cut, building wooden
wall forms next to the sheeting and pouring concrete into them, removing
the forms, installing basement floors, and, finally, removing the wall
braces and the sheeting. If the ground contains excessive moisture, a
mammoth, heavily reinforced slab of concrete, sometimes as thick as
fifteen feet, is poured as a bottom floor, to counteract any upward
pressure from the underground water. None of these orderly procedures
could be applied with reasonable economy to the Trade Center site, he
said. The original shoreline of Manhattan runs close to Greenwich
Street, which bisects the site north and south, and this is a dividing
line between good and bad foundation-building land. The area that had
concerned the engineers from the outset was the western half, beyond the
original shoreline, where borings eighty feet apart indicated that what
was below was Hudson River silt loaded with underground
obstructionsâwharves built with wooden cribbings, foundations of stone
and brick, crisscross timbers or piles, boulders, riprap, rocks that had
been used as ballast in sailing ships coming empty from Europe to take
on cargo, and even some of the ships themselves. Both of the Trade
Center's skyscrapers, the Customs Building, and half of the plaza would
occupy this part of the site, Kyle said, and their total weight would
come to a million and a quarter tons, or twelve times as much as the
weight of the George Washington Bridge, including its concrete decks,
its steel anchorages, and everything but vehicles.
In a foundation of orthodox design, skyscraper towers, with special
concrete-and-steel footings in the rock, would have sufficient weight to
stay put in ground as watery as the tidal fill around the Trade Center,
but the surrounding streets might fall in and the smaller buildings
might settle down, or, worse, pop up. Insufficiently anchored tunnels
have risen from river bottoms to haunt engineers who miscalculated the
force of underwater buoyancy, and buried gas tanks are notoriously
jumpy. However, it is doubtful whether conventional foundations could
have been built at the Trade Center without the contractors' going broke
from cave-ins and other difficulties, such as the slow pace of
excavating the entire site before starting construction, the need to
pump tidal water out the whole time, the tortuous process of driving
piles or steel sheeting down to bedrock through the accumulated debris,
and the obstacle for other construction imposed by the huge braces
needed to keep the peripheral walls from falling in until the inside
floors were in place.
In addition, Kyle said, the World Trade Center had a problem that, as
far as he knew, no foundation engineer had ever before faced: a railroad
had to be kept running inside the foundation area while digging went on
around and beneath its tracks. Actually, two railroads go through the
siteâthe I.R.T. local subway line to South Ferry and the PATH System, a
fourteen-mile line running between Newark, Hoboken, Jersey City, and
Manhattan through tunnels under the Hudson River. PATH crossed the site
in two five-hundred-foot cast-iron tubes, almost three-quarters of a
century old, resting on beds of mud. Some way had to be found to jack up
the tubes, through which about a thousand trains rumbled back and forth
daily, carrying more than eighty thousand passengers, without disturbing
either the trains or the passengers. Ultimately, the tracks would be
relocated under the Trade Center and the old tunnels removed.
The solution to all this was a daring one: to build one huge basement,
sixty-five feet deep in some places, a hundred feet deep in others, that
would take the form of a watertight box (but what a box!) occupying
eight acres on the treacherous eight-block western half of the site.
What the engineers were really doing was building a four-sided dam
around the troublesome part of the site. The bottom of the box would be
bedrock, into which the walls would be tightly socketed to keep water
out. During the construction of the walls, water trapped inside could be
removed as the filled land was dug away and the bedrock emerged. Then
the work of digging beneath PATH and constructing the basement
areaâcontaining a new PATH terminal, the underground home of the Trade
Center's maintenance and air-conditioning equipment, an emergency
electric generating plant, garages, and truck docksâcould proceed with
maximum efficiency. As it took shape, this box, the largest single
basement that Kyle or any of the West Street Associates knew of, came to
be called the Big Bathtub, and, finally, just the Bathtub.
While planning the Trade Center foundation, Kyle told me, he inspected
subways and other underground work in Paris, Brussels, London, and,
finally, Milan, where a new subway was completed in the late
nineteen-sixties. In all these places, a Milanese firm named ICOS, which
specializes in building walls in wet areas to keep water from flooding
construction sites, had used a new process called the slurry-trench
method. In Montreal, Toronto, and at a dam in Pennsylvania, an affiliate
of the Italian company, the Icanda Corporation, Ltd., of Canada, was
doing the same kind of work, and Icanda was eventually hired to work
jointly with the West Street Associates and actually construct the
Bathtub wall. To a layman, the idea of building a multimillion-dollar
foundation wall anywhere from sixty-five to a hundred feet deep
underground, blindly, without excavating on either side of it, is bound
to seem the height of folly, and I told Kyle so that afternoon. He
laughed, and said he would try to explain why it was the most practical
solution. The key to the slurry-trench method is the use of a volcanic
ash, or clay, called bentoniteâafter Fort Benton, in Wyoming, where
deposits of it first were found, in the eighteen-forties. The peculiar
property of bentonite, a powdery clay, is its ability to absorb enormous
quantities of water in an excavation, after which it is strong enough to
hold back the surrounding earth. The petroleum industry began using
bentonite instead of metal casings in oil-drilling holes around 1900.
When mixed with water, bentonite creates a counter-pressure to the push
of surrounding earth and water, and prevents cave-insâjust how is not
understood, though some experts believe that an undetectable electric
charge may be involved. "If you dig a trench and put down bentonite in
the right mixture, it will hold up the banks," Kyle said. "The bentonite
that is attached to the earth will stay attached even when concrete goes
down and displaces the water in it. The stuff acts like a membrane, and
the part that sticks to the wall holds the wall up. It annoys hell out
of you when you can't figure out why that is, but, basically, we aren't
interested in the theory. The important thing is that it works." The
Bathtub wall, he said, was to be constructed in sections. Trench
segments twenty-two feet long, three feet wide, and seventy feet deep
would be dug and filled with slurryâa mixture of six per cent bentonite
and ninety-four per cent water. As dirt was removed it would
continuously be replaced by slurry, so that the trench would always
remain full and the sides would not fall in. The digging would continue
down into bedrock, with about two feet of the rock itself chipped away
to give the wall a proper footing. Then a great cage of steel
rodsâshaped to fit into the full length of each twenty-two-foot segment
of trench and holding a number of forty-five-degree-angle steel guides
for the installation of tiebacksâwould be dropped into the soupy mix to
reinforce the concrete. Next, the concrete itself would be poured into
each section. As the liquid concrete rose to the top of the trench, it
would displace the slurry, which would be pumped into the next section
of trench. As each section of concrete wall was completed, the workmen
would excavate on the inner side of it to install tiebacks reaching
diagonally down through the soil behind it. Then the earth inside that
section of the Bathtub could be removed. With ten or fifteen machines
moving simultaneously along the perimeter, Kyle figured, the outside
foundation would take about a year to complete.
Before I left him that day, Kyle remarked that, of course, all kinds of
sewer, water, and steam pipes and electric and telephone lines running
through the site would have to be rearranged, adding that such work is
normal in new construction, but this would be the biggest relocation job
in the history of the New York Telephone Company. The company's main
office was right next door to the Trade Center, at the corner of West
and Vesey Streets, he reminded me, and the principal trunk lines for all
phone communication between major cities in the United States and to the
world outsideâincluding the hot line to Moscowâwere under what had been
Greenwich Street, in the very middle of the site. Local telephone lines
customarily run under public thoroughfares, too, and these would all
have to be moved to the West Street boundary. As for the long-distance
lines, two huge manholes, or vaults, opening into the two tunnels of
PATH were to be constructed, to reconnect these lines inside the tunnels
for their route across the Hudson. The vaults underneath West Street
would also serve to pin the cast-iron PATH tubes to bedrock.
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