THE EAST RIVER SUSPENSION-BRIDGE.
America Illustrated, 1877
NEVER was a more important engineering
work undertaken, nor one which, notwithstanding its immense cost,
is more likely to be prolific of rich results by the increased
value given to real estate in, all parts, and particularly the
outskirts, of Brooklyn, than the construction of the bridge across
the East River. Geographically the greater portion of the city
of Brooklyn is nearer the business centre of New York than any
part of the latter city above Fourteenth Street, and were it not
for the isolationcaused by the necessity of twice daily
going through le mauvais quart d'heure consumed in crossing
the East River in the crowded ferry-boats, there is no doubt but
that the growth of "The City of Churches" would have
been even more rapid and extraordinary than that which has actually
The act to incorporate the New York Bridge Co., for the purpose
of building and maintaining a bridge across the East river between
New York and Brooklyn, was passed by the Legislature of New York
on the 1st of April, 1867, and shortly after this date the work
on the structure began. The capital stock of the company was placed
at five millions of dollars, to be divided up into shares of one
hundred dollars each. By a clause in the charter the corporations
of the cities of New York and Brooklyn are empowered to take possession
of the bridge and its appurtenances at any time on payment to
the bridge corporation of its cost and thirty-three and one-third
per cent additional but if advantage be taken of this proviso,
the bridge is to be made free to travelers and vehicles. It was
further enacted, that no pier should be constructed in the river
beyond the pier lines laid down in the Act of authorization, and
that the bridge should be built, at an elevation of not less than
one hundred and thirty feet above the river at high tide, thus
leaving the navigation of the East river unobstructed. It begins
at or near the junction of Main and Fulton Streets, in the city
of Brooklyn, and will cross the river as directly as possible
to some point at or below Chatham Square, not south of the junction
of Nassau and Chatham Streets, in the city of New York. In addition
to the private subscriptions toward the capital stock, New York
city is to pay five hundred thousand dollars in each of the years
1874 and 1875, and the city of Brooklyn the sum of one million
dollars during the same time.
The work of construction was carried on under the supervision
of the late John A. Roebling, the well-known engineer and architect
of the Cincinnati Suspension-Bridge, up to the time of his decease.
This onerous position was then transferred to his son, Mr. W.
A. Roebling, and at this writing is still held by him. The plans
and specifications adopted at the inception of the enterprise
anticipated many of the leading features which have since contributed
to the success of the great bridge at St. Louis. To cross a river
1,600 feet in width with a single span had, up to this time, been
thought beyond the limits of engineering skill. But the difficulties
in this case were much enhanced by the fact that the towers to
support, the immense structure necessary for this purpose had
to be constructed under circumstances that rendered the usual
modes of sinking piers impracticable. It was, therefore, determined
to employ the method of working by compressed air. Caissons were
made having the horizontal dimensions of the two piers, that on
the New York side being 102 feet by 172. Each caisson was, in
effect, a wooden box turned bottom upward, the interior space
being nine feet high. The roof of the New York caisson (bottom
of the box) was twenty-two feet thick of solid timber bolted together,
supported by frames running from side to side. These frames, together
with the edges of the box, and the upward pressure of the condensed
air within the caisson, were to sustain the vast superincumbent
weight. The area of the structure was about 17,500 square feet.
The caissons having been built on ways, were launched after
the manner of a ship, and towed to the points where the piers
were to be located. Courses of granite blocks were then laid upon
the top of the caisson, by which it was sunk to the bed of the
river. Air was then forced into the chambers from the shore until
the water in the interior space was entirely displaced, the engines
working night and day to maintain the pressure. The workmen obtained
access to the chamber by means of two shafts extending above the
surface of the water. At the bottom of each shaft were two air-locks,
simply ante-chambers constructed of iron, into which the men entered
from the shafts, and, closing an air-tight door behind them, admitted
the compressed air from the caisson by means of a cock. When the
pressure in the last ante-chamber was equal to that in the caisson,
a communicating door was opened, and the men passed into the chamber
below. In going out this process was reversed, the compressed
air in the lock being allowed to blow off through a cock into
the open shaft. Great care has to be taken in regulating the compression
of the air, as work in the caisson is attended with considerable
danger. Two men have already died, and a score or more have been
prostrated, owing to its injurious effects.
To secure proper foundations for the two great towers was the
principal engineering problem to be overcome, and now that these
have been built and the towers erected on them, the bridge scheme
may be said to stand on a firm footing and its construction almost
regarded as an accomplished fact. The caisson on the Brooklyn
side of the river was sunk into its place during the year 1871.
Up to 1872 the borings for foundations on the New York side had
been confined to a small area covered by an old pier, owing to
the fact that the adjacent ferry slips could not be immediately
vacated. These were evacuated by the lessees at so late a date,
that experiments could only be made with four bore-holes before
the caisson was ready to be sunk.
The bore-holes developed the fact that there was an extreme
difference of twelve feet in the level of the bed rock, the hole
of the least depth touching hard-pan at a depth of eighty feet
below high water, and the deepest at ninety-two feet. The strata
consisted in the main of a black mud deposit of twelve feet, followed
by a layer of coarse sand of six feet in depth, which overlaid
a gravel bed of the same thickness. Beneath the gravel was a very
heavy deposit of quicksand of a depth varying from fifteen to
twenty feet, and abounding with boulders in its lower portion.
The quicksand extended usually to within a few feet of the rock,
and, in some instances, to the rock itself. But the immediate
rock surface was covered with a compact layer of material through
which it was impossible to drive a six-inch pipe without shattering
it. To drive the pipe only one inch required thirty blows of a
five hundred pound hammer, falling from a height of twenty feet.
When the sinking of the caisson commenced, it was not determined
by the engineers whether to go to rock or to remain above it.
Further investigation into the character of the river bed convinced
them that no single plan of operation would be adequate to remove
all the material that would have to be displaced. The immediate
river bed consisted of logs and loose dock stones followed by
a sticky black clay. These materials could be most conveniently
displaced by dredging. The river sand, and the firm gravel underneath
it, could be more easily removed through pipes, either by means
of pumps or by the air-pressure direct. The coarser gravel would
go to water shafts, while the fine quicksand could be blown out
through the pipes, until the preponderance of the boulders and
small rounded stones compelled a return to the water shafts. It
was at first determined to use the dredge for this latter purpose,
but it was found by experience that the dredge had not sufficient
capacity to remove stones imbedded in quicksand. After much tedious
investigation a satisfactory foundation was reached in May, 1872.
The probable cost of the bridge has been estimated at $13,045,065.00.
From this amount, however, a large sum must be deducted for land
which will lie under, the bridge when completed, and be capable
of utilization. It is expected that the leases of these properties
will reimburse the Bridge Company $1,644,350.00.
The original plans have been considerably modified and altered.
Since the first estimates were made, it has been found necessary
to increase the width from eighty to eighty-five feet. Two out
of the three sidewalks that were contemplated have been given
up, and two additional horse-car tracks substituted. This change
involved an increase of seven per cent in the cost of the entire
bridge, including superstructures, tower foundations and anchorages.
The United States Government directed there should be an additional
elevation of five feet, an order which necessitated a change in
the trusses and some of the masonry.
When the cities of New York and Brooklyn take charge of the
management of the bridge, the Board of Directors will consist
of twenty members. Each city is to have the nomination of eight
members, the right of appointment being vested in the Mayor and
Comptroller, and the last-named gentlemen are to hold similar
positions by virtue of their office.
There is a strong inclination on the part of New York to throw
off the burden it has assumed in this matter. It has been freely
argued that the bridge will work to the detriment of the metropolis
by drawing off a large share of its population; but the city is
legally bound to fulfil its agreement, and although it may succeed
in delaying the opening of the bridge, it will ultimately be compelled
to pay its grant. When completed the bridge will be the largest
of its kind in the world. Work on it is rapidly progressing; but
although the act of incorporation demanded it should be opened
for travel during 1870, it will not be finished for three or four
years to come. The tower on the Brooklyn side has been carried
to a height of one hundred and sixty-two feet, and the tower on
the New York shore is also attaining lofty proportions. The Brooklyn
tower, at the springing line of the arches, will be two hundred
feet high, and the New York tower will require an elevation of
one hundred and twenty feet to bring it to the level of the roadways.
Our picture of the bridge is taken from the Brooklyn side.
The building near it is the office of the Fulton Ferry Co., whose
Brooklyn terminus adjoins the office of the New York Bridge Co.
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