New-York tribune., 1913-07-27
EIGHTY FEET TO BEDROCK
StOne Base for New Equitable
Quickly Found.
PLAN GIANT CoFFERDAM
Structure Will Have Larger
Floor Area than Any Business
Building in World.

Great structures nave arisen so rapidly
q, the last quarter of century in this
.r,v that residents are not surprised TO
,se yreckers tearing down old buildings
- make way for new. To the truo
,o,,noist the sounds of boring machinery
o,d the blasting of foundations or bed
-ock whether in the day or night. are
,ndi,atlve Of the city's great progress,
commercially and otherwise.
One of the busiest spots in the city to-
aov Is the block bounded by Broadway.
Nassau. Pine and Cedar streets, the site
~ the largest Office structure in the
world. the new Equitable Building, which
will rear itself from the ashes OF the old
Eouitable Building, burned nearly two
years ago.
A few weeks ago the ORourke En-
rineering and Construction Corporation,
under orders from the Thompson-Sta
Company. the building contractors for
the new Equitable structure. began the
serious work of boring and blasting the
vav TO rock base and removing old
granite and concrete remains of former
foundations TO make room for the mag
niacent pile OF carved and polished stones.
marble and steel, which will constitute, in
area. the largest Office structure in the
world. with over 100000 square feet Of
rentable floor space and towering aloft to
the height Of thirty-siX stories. Such a
mighty structure must needs have a great
rock foundation. and when It is compre-
bended that II will weigh more than
9N0nO tons, as against 10000 tons of the
Woolworth Building. $500 tons Of the
Metropolitan Building, lSS.5OO tons Of the
Municipal Building and s., $50 tons Of the
Singer Building, it will be granted that
the foundations must be absolutely per-
feet k&carry the burden. The steel frame
work F now being forged and rolled at
the mills and will weigh approximately
5000 tons when in position. as against
5000 tons of the Woolworth Building,
1900 tons Of the Municipal Building, 6,l0O
tons Of the Singer Building and 1000 tons
Of the Metropolitan Building.
The engineers of the new Equitable
Building fortunately found rock bed
eighty feet below the curb around the
entire site for the more important base
Of the foundation. 1t is on this bedrock
that the mighty cofferdam, which will
form the true middle of foundation
covering over 1900 square feet. will be
built On the Nassau street side 1t will
extend 80 feet. on the Broadway side l67
feet, on the Cedar street side 3l5 feet. on
the Pine street side 304 feet. or altogether
about 935 feet of giant caissons Of con-
crete and steel. 50 feet long. feet
Il mid ~, oCaOrAaUpa
with the requirements of construction.
Each Of these caissons will be sunk from
curb to bed rock. eighty feet below.
The cofferdam itself, when complete,
will be a solid concrete wall feet broad
and so feet deep, strengthened or rein
forced by heavy steel rods.
The plans Of the foundation call for a
retaining wall, extending from Cedar to
Pine street. This wall will add greatly
to the strength of the cofferdam, but it
will be built principally for the purpose
Of creating two levels for vaults and
cellars beneath the building.
The concrete caissons that will form the
great cofferdam wall are Of interest.
Formerly caissons were built Of wood. but
recently engineers have decided that re-
inforced concrete construction is prefer
able. as when the caisson is finally filled
with concrete II becomes a monolithic
mass, proof against all deterioration.
Where water cannot be kept out Of
foundation excavation by ordinary means
and IL Is necessary that work be done on
dry rock, the pneumatic caisson, or method
Of excavating by hand under compressed
air, is generally adopted. An airtight dia-
phragm is constructed across the caisson
and sunk into position, with its cutting
edge imbedded in the ground. Into the
compartment formed beneath the air is
forced through a pipe until the pressure is
slightly greater than that Of the greatest
head Of water. The water is thus drained
out Of the ground, leaving "A compara-
lively dry bottom upon which to work.
Access is obtained from the outer air
by an arlock, a chamber at the level on
the diaphragm. with two alright iron
doors, one opening into the lower compart-
ment and one into the upper valves con-
trolled from both inside the lock and from
each compartment, admitting air into the
lock or permitting its egress. Workmen
(often called sand hogs") go down into
the working chambers, or bottom compart-
ment and dig the soil, or whatever II 49,
which is taken out through the air lock
IN buckets. AS they dig the earth away
from under the cutting edge OF the Gals
son the resistance 10 its sinking is re-
dueed and the caisson settles down. IL
must be guided or kept vertical by digging
uniformly. or by more extensive digging
at the side which is TO be sunk most
rapidly.
Pneumatic caissons are usually sunk 10
bedrock. When the rock surface is lever
It is only necessary to clear IT thoroughly
have the loose rock removed and the
working chamber and alright shaft hiieo
with concrete, making the structure
solid. imperishable stone. "a monoiithic
mass" proof against time and decay
The pneumatic caisson. however is nm
ited by the air pressure which may pe
used. Men cannot do good work In an air
pressure exceeding fifty pounds square
inch above atmospheric. This is sufhcleht
to balance head of water OF ABOUT IIV "
ll5 feet. but when. as Is often the case
the air pressure in the caisson must DC
Kept equal TO the full hydrostatlc heao Vi
the ground water, the method Is practicai
only for depths up to 1lO feet below the
level Of the ground water.
A study OF the foundation plan Tor the
new Equitable Building and its immense
area, together with the great mass OF con-
crete caissons and piers firmly imbeoded
on rock eighty feet below curb, will sno"