St. Johnsbury Caledonian. [volume] (St. Johnsbury, Vt.) 1867-1919, 1883-06-01
The Brooklyn Bridge
Greatest in the World.
THE WORK OF CONSTRUCTION

THE WORK OF CONSTRUCTION
began on January 3, 1870. The great
est difficulty was to secure firm fOunda-
tions for the towers. To build these
foundations below the level of the bed
of the river was work of great mag
nitude. Wooden caissons were used to
support the towers of the bridge. A
caiSsOn is merely great box bottom
turned up. The Brooklyn caissOn was
fifteen feet thick on the top, of large,
solid yellow pine timbers, and the sides
were nine feet thick and nine feet high.
The measurement across the box from
edge to edge was l02Xl6S feet. 1t was
divided into six compartments, which
were connected by means of doors.
This great box was made water-tight
and then anchored on the spot on
which the tower was to stand. The
caisson was protected from the river by
a cOtfer-dam of piles and sheeting.
The work of building the tower on the
caiSsOn was at once begun and cOntin
ned until enough weight was on top of
it to keep il on the river bottom. There
were large wrOught-irOn tubes or shafts
in the roof of the caisson, through
which earth and stones eXcavated on
the inside could be taken out, and there
was an air-lock for persons to enter the
caiSSon.
When it was sunk on the river bot
tom the water was forced out of the
caisson by compressed air, and men
could 80 in and work. One pound of
air pressure equals two feet of tide
water, so for every two feet the caisson
was lowered, one pound had to be add
ed to the air pressure inside. Guages
in the cngine-rOOm above indicated the
| height of the tide and the pressure of
air. The highest pressure attained in
! the caissOn was 81 pounds to the square
! inch, in addition to atmospheric pres
sure. At that pressure IA man could
not whistle, and a candle blown out
would Immediately ignite again. Fresh
water springs appeared. Beneath this
caisSOn blasting and excavation went
on for months, when a depth of 45 feet
was reached, and the caissOn was then
SETTLED on rocky foundation. The in
side was then filled with broken stone
and concrete until the whole thing was
il solid mass; and then the work Ot
erecting the tower went on rapidly.
Greater difficulty was encountered
on the New York side, and the caissOn
was made stronger and bigger, and had
to he sunk to depth of seventy eight
feet Even at that depth solid rock
was not discovered. But Jagged point,
of rocks stuck up here and there, amiG
IL bed of quicksand. The points of til,
ledge, which appeared only under one
end of the caiSsOn, were levelled off
! II Il IL,
Il wall of concrete, the whole area o
the caissOn was filled with the same
material, after which the tower wa
erected and finished like the one il
Brooklyn.
THE TOWERS, BUILT OF GRANITE,
brought from Maine, are 276 feet 9 inch
es above high water mark. The Brook
, lyn tower reached its full height il
May, 1875, and its New York mate il
: July, 1876. The New York tOwe
above the top of the caissOn weigh
\ $4,000 tons. The tirmDess of til
! foundation is shown in the fact that tn
5 tower has not settled two inches. y
c IA height of 10 feet there are two arch
. ed openings in each tower, through
= which will pass the streams of travel
= plates, huge iron castings, on whic
t rest the weight of the cables.
e The cables are fastened in anchor
: ages $50 feet distant from the tower,
The anchOrages are built of stone, an
Il are l29 by 10 feet at the base and 3
e feet high. Over the top of them ru
II the roadways. Imbedded in the tw
V anchorages are huge plates of irO
h weighing twentyth tons each, t
e which are fastened iron bars forme
= into chains, at the end of which are tn
:, cables that hold up the suspended pan
e of the bridge. There are vaults i'll tn
e anchOrages, where the cables join til
e anchor chains, and they can be inspect
e ed at any time by the engineers. Th
o remaining stone work of the bridg
9 consists of granite approaches, whic
It are lofty viaducts. he Brooklyn al
e proach begins at Sand street, and it
,. 81 feet long to the anchorage. III
II grade is two feet nine inches i'll eac
d 100 feet. In this distance PrOspec
d Main and York streets are spanned L
e iron bridges. The New York apprOaC
X is 1862 feet long from Chatham stree
a to the anchorage. The streets a.
x spanned by stone arches, except ;
ir Franklin square, where an iron bridl
= crosses over the station of the elevate
le railway. Beneath the arches of Il
II, approaches are spaces which will