Evening star. [volume] (Washington, D.C.) 1854-1972, 1900-09-08
DESCENT INTO A CAISSON
NEW YORK;. TJMEs REPORTER
MAKES. OBSERVATIONS.
The Eject of Compressed Air-ExDeri-
ence a. . Sand Hog, Many
Feet Under Ground.

From the New York Times
Come down here some morning in the
| oldest and worst-looking suit of clothes
that you can get hold of, and well make
| sand hog out of you. Sand hog is the
| trade name for workman In caisson
| and the foregoing remark was by way of an
| invitation to a Times reporter to go down
| in one of the caissons that are being sunk
| for the foundations of the new Exchange
| building, at Broad street and Exchange
place.
Aside from the part played by compressed
air in caissoR construction, the working
principle is very simple, and were 1t not for
the compressed air descent into caisson
would not be a much more novel or inter-
esting experience than going down into the
cellar The caisson, or working chamber,
as 1t Is sometimes called Is nothing more
than huge steel box, without any bottom,
and having the same area as the foundation
pillar or column that is to be carried down
to bedrock The caisson at the beginning
of the work rests on the surface of the
ground. Men inside of 1t begin excavatinS,
and at the same time another gang OF men
begins to build up the masonry or concrete
on top of It, the added weight causing 1t to
settle as rapidly as the materials are re-
moved from underneath 1t, and the men at
the surface building up the masonry as rap
idly as the settling takes place
leaving the working chamber must be main
tanned and this is done by shaft about
three feet in diameter leading upward
from hole Of the same diameter in the
roof Of the caisson. This shaft is built up
of sections of steel cylinder, sc that as the
caisson sinks the top Of the shaft may al
ways be kept above the surface. Where
the foundation column is to be built of con-
crete instead of masonry there must also
be an outer cylinder of steel of the same
area as the roof Of the caisson. This outer
shell. however, simply acts as mold to
confine the concrete, and plays no essential
part in the general principle of caisson con-
struction.
Work of Compressed Air.
Now for the important part which COM
pressed air plays in the work. If rock
could be reached ten or fifteen feet below
the surface, the process as already out
lined might be all satisfactory. But where
forty or fifty feet or even greater depth
OF soil, mud, water and nobody knows what
must be penetrated, the problem presents
new difficulties. A stratum of Quicksand
might be encountered, the caisson or work
Ing chamber would fill in few minutes,
and probably nobody in it would live to
tell what happened. Moreover, the pres
sure at a depth of forty or fifty feet is
great that even where the material en-
countered is of fiTmer character it would
work up underneath the edges of the cais-
son and would have tendency to crush
I'm its walls, while it's roof might also col
lapse under the great weight of masonry
resting upon 1t.
To counteract this the caisson itself and
the inner cylinder shaft leading to it are
constantly kept filled with compressed air,
supplied by a large steam pump. The
pressure Is raised as the caisson reaches
lower depths, beginning with perhaps only
three or four pounds to the square inch
just below the surface, and increasing to
twenty or twenty-five pounds at depth of
fifty feet. To maintain this uniform pres
sure in the working chamber and the shaft
and still allow the latter to be opened as
frequently as may be desired, so that men
and materials may pass up and down, is a
problem which the caisson workers have
solved by very ingenious and at the same
time simple device.
To the top of the inner cylinder or shaft
there IS always attached an affair that
looks much like the ordinary boiler found
In the kitchen of every residence, although
somewhat larger, II being perhaps four
feet In diameter and six feet high This
is known as the lock. 1t has door or
nap In It's top, opening downward, and a
similar door at the bottom, which also
opens downward into the shaft Now sup
pose man wants to go down to the work
Ing chamber. The door In the top of the
lock is open and the air inside Is at the
normal pressure of the outside atmosphere
The lower door, however, is closed, and
below 1t in the shaft there is a pressure
Of say, twenty-five pounds to the square
inch The man climbs into the lock and
making it absolutely alright Then a
valve connecting the lock with the shaft Is
opened, allowing rapid influx of the COM
pressed air from the shaft below into the
lock and equalizing the pressure I'm both
The lower door of the lock NAV then he
opened and the descent made to the cais-
son itself.
Accepting an invitation to go through q
sand-hogging experience, a Times re-
porter went down to Broad street and ex-
change place one morning last week and
was taken in charge by John CahilI SUI
perintendent of the caisson work for tho
Engineering Contract Company, which con-
Germ has the subcontract for the founda-
tions of the Exchange building Mr.. Cahill
decided the reporters apparel, although be-
dragged and torn in places was much too
sumptuous. and a suit of oilskins was pro-
vided as additional protection.
Pressure Let in Gradually.
You are here at a very favorable time"
said Mr.. Cahill. 1--WE have just reached
the rock In one of the by IA caissons over
next to the Lords Court building"
1t took but a minute to scramble I'D the
short ladder to the top of the lock and to
get down into it.
Now said Mr.. Cahill as he closed the
upper door, and making the lock as dark
a pocket, being you're new at this
game, I'm going to let the pressure in here
gradually AS l do keep your mouth closed
tight, hold your nose and breathe out as
The reason for this is to prevent injury
to the ear drums. The great increase in
pressure from without is liable to cause
serious trouble in the ears, unless by blow
ing with the mouth and nose closed. the
air is forced up through the eustachian
tubes to the inner chamber of the ear, thus
In manner equaliZing and counteracting
the pressure from without.
There a hiss, of escaping steam
which developed almost to a roar as the
valve was opened and the compressed air
rushed in from the lower shaft sufficiently
to fill the lock. With all his blowing there
was sensation in the novice's ears that
was anything but agreeable; but it was all
over in a few seconds and down went the
little door in the bottom of the lock- show
ing few dim candle lights far below and
with a little rope ladder the only means of
getting down to them.
you are now in an air having pressure
of twenty pounds to gthe square inch" said
Mr.. Cahill The novice was gratifed by
this bit of ten, because if he had
been asked to guess on the subject he would
have estimated the pressure at about seven
tons-three tons on his right ear and four
on his left. liz z
The strange feeling wore off very rapidly,
however. and the oppressive sensation was
replaced by one Oft exhilaration, owing,
doubtless, to the. greater density of the air
and the correspandingly increased amount