The Penn's Grove record., 1923-11-09
Penn-Fenwick Bridge
Worlds Longest Suspension COnstruc-
ton VIII Span The Della
ware River

coast will be able to aVOiG the tedious
wait now often experienced at Phlla-
delphia by rolling across one Of the
worlds longest suspension bridges.
Work was started on the structure
only a year ago, and by spring the
slender, 3S0-foot steel towers began
to rise on either side of the Della
ware. From an engineering point OF
view, the most difficult feature of the
structure has been nearly completed,
although there is as yet nothing to
show above water, save the bargeS
CARRYING materials, and the tops of
the c~aissons in which the sand hogs
work in the construction of the foun-
dations.
The visible part of the task-the
building of the towers and the spin
ming ofthe two huge cables that are
to carry the tremendous weight of the
bridge-will be far more spectacular
to spectators, but not nearly so dan
gerous and thrilling to the workers as
the things that must be done below
water level, ever below the level of
the river bottom, all the way down to
bed rock. There men must work
under such heavy atmospheric pres
sure that a great deal of the time for
which they are paid is spent in going
to and coming from work in the elan
crate system of air locks used to ac-
custom them to changes of pressure.
A caisson is nothing more nor less
than a huge box, with no bottom,
divided into two sections. 1t is built
on a shipway of timber with steel
braeing and foated to the point where
it is to sunk. Then concrete IS poured
into the top section, thus forcing 1t
down into the water. That process is
continued until the box stops sinking.
Then the sand hogs get to work. Far
down in the concrete of the top section
is a chamber. or lock, entered by
means of a shaft in which ladders are
set. Another shaft from the cham
Der leads down into the bottom Sec
ton. When the workers get there
they find q certain quantity of air, just
as air stays into a glass that is turned
upside down and thust into a pan of
water. More air is pumped in from
above to force out the water, SO that
the waters can get to the mud of the
river bottom.
Working at first in shifts of four
hours, the sand hogs shovel the mud
into buckets passing through another
shaft. Extraordinary care is neces-
sary to keep the air pressure in the
chamber where they at work equal
to the pressure of the water fltering
through the mud from the outside. In
that connection a close watch is kept
for leaks. A supply of sand bags Is
kept on hand. When the foreman or
one of the workmen notices a trickle
of water entering, a bag of sand is
thrown against the wall of the caisson
at that point, just where it rests on
the mud. Meantime more concrete IS
being poured into the top section, thus
adding to the weight and slowly fore
ing the eaisson toward bed rock.
The deeper the caisson, of course,
the greater the air pressure. At the
lowest levels men can work for As
minutes at a stretch. They must
spend an hour or more in the lock
above, where the pressure is slowly
decreased. Occasionally, even with
the greatest precaution, a mar leaves
the last chamber before his system
has became accustomed to the lessened
pressure. Then he gets what is known
"the bends" In reality it is air in
the system. He may walk some dis
away and then suddenly stagger
as though intoxicated and collapse.
The only known treatment is to rush
lim to the hospital lock and increase
he air pressure until he RECEIVERS,
when the process of decreasing it a
be started.
The caisson on the Philadelphia side
the river was the first in which
work was started for the Delaware
River bridge. 1t reached bed rock
aome time ago. The process then is to
eveI off the rock. Then the sand hogs
Jack down the concrete poured in from
above, backing up until there is no
onger room for them to work. The
final air space is filled up with cor
,rete, and the sub-structure of the
is in place. All that re
mains to be done is to put up the steel
md spin the cable.
1t will be a year or more before that
york begins, of course, since n of the
Jig jobs to be done first is to sink the
lnchorages. The cables are carried !
yer the towers and thence to huge
masonry anchors located well in from ;
he bulkhead line. Caissons are used
o sink the ~anchorages, but they are !
much smaller than the foundation !
aissons and present less engineering,
difficulties. The spinning of the cables ,
s done just about the same way a
pider spins its web, by a machine that
t a distance will look not unlike a
liant spider, travelling back and forth
etween the two towers. /
Heretofore, all the large suspension
in the world have been built
nth four cables. The Delaware river
ridge is to have only two, or which
dead weight of $4,000 tons of metal
yill be suspended. The cables will be
he largest ever made. Each will be
0 inches in diameter, as compared to
81 inches in the Brooklyn bridge and
OF inches in the Manhattan, the
heretofore manufactured. The
ables for the Delaware bridge are to t
re made up of $500 strands of high t
.rade carbon steel wire, each 0.192 of
n inch in diameter. 1t is estimated
hat about $4,000 miles will be used in ,
TN-