The Salt Lake herald. [volume] (Salt Lake City [Utah]) 1870-1909, 1897-06-28
Greathead. who recently died-a loss to
the whole world.
SHIELD THAT DUG THE WAY.


rear part of il there were 81 hydraulic
rams. each 10 inches in diameter, whose
duty it was 10 push the face of the
shield forward into the uneXcavated
material lying in front. These rams
rested for their pushing support on the
already built up rings of the tunnel,
just described. They exerted a total
pressing force, when all were in use,
of over 5000 tons. They were worked
by water pressure from two hydraulic
engines of 70-horse power In the front
Of the shield doors were arranged
through which the workmen could pick
and shovel into the material. or which
could be closed against soft, running
stuff, and little doors in them could
then be opened. by which the sand or
mud or gravel, little by little. could be
taken out, and, by means of chuteS in
the center of the shield, run into little
cars on the tracks below.
There were also provided in the shield
'erectors," worked by hydraulic pres
sure. They were simply arms, capable
IKE human arm, Of picking up a
weight and putting it in place. just as
the bricklayer picks up a brick and
sets il in place on the wall. But this
erector arm picked up a ton weight. q
whole segment of the circular ring, and
put it up in its proper place, above,
one side or below, and held it and
pressed it there until the workmen
bolted it securely, md then it picked
up another and did the same.
The details of all this shield work are
shown in the two sketches.
The shield with its appurtenances
was certainly giant, doing the work
OF more men than could have been
crowded into the confined space where
It did its duty from day to day and
from night to night.
HOW THE WATER WAS KEPT OUT.
All so far seems easy and simple,
but now comes to our minds
conditions that make success we'll
nigh impossible; for ii occurs
TO us naturally to ask how
the water which filled the soil was kept
out CT the way so that the men could
work safely under a river so wide and
deep. We know that water presses
equally in all directions and that a vol
ume OF water one-eighth of an inch
wide VIII exert just as much pressure
as body of water an ocean wide.
That is what is called the ''hydrostatic
paradox simply because it does not
seem reasonable. We know also that
water is well-nigh incomDressible. If
we turn a tumbler bottom side up and
put it into a tub of water and push 1t
down there under the surface, we notice
that there is still air in the tumbler
II row we put the tumbler down to the
bottom of the tub we see that the air
is still there, but occupying less space
than before The knowledge of this
fact led TO the use of the diving bell, a
great iron box. say feet wide, Is feet
long and 10 feet deep. turned bottom
up like the tumbler, with thick circular
glass windows set into the top, and
lowered down into the bottom of a
river or sea so that men could stand in
the top OF it inside and work. Then it
was discovered that air. compressed by
means of pumps. could be forced into
the top o. this bell and the water
driven out to the very bottom of it, so
that men could stand dry shod on the
rock or ground and work as on shore.
This principle was applied to the Black
wall tunnel y wall or bulkhead of
brick. u feet thick was built across the
tunnel; then air compressors, driven by
engines of about 1.000-horse power.
forced the air down through pipes into
the working section of the tunnel,
which lag between this wall and the
shield at the other end; and the men
worked there. in the dry. The total air
pressure thus formed against the bulk
head was 1000 tons. The plant, that is
the pumps of various kinds, air and
water, were placed above ground near
a shaft already described, and the air
and water under pressure was led by
pipes down the shaft and through the
bulkhead to the point in the working
chamber. where the power of various
kinds were needed.
WORKMEN SUBJECTED TO AIR
PRESSURE.
The pressure of water is in proportion
TO its depth. and is about 2-5 of a
pound TO every foot of depth. Eighty
feet of depth would therefore make a
pressure of % pounds more than the
ordinary pressure under which we
usually live at sea level, which is Is
pounds. In other words, the men had
10 work in an air pressure of more than
twice that of the air above. By going
slowly from the outer air into the con-
densed air one can live and work; and
so what is called an air lock was built
into the brick wall. The men stepped
into it and shut the outer door.
valve let the compressed air slowly
into the lock from the inner working
chamber This lock is a horizontal
steel tube 10 feet long and six feet in
diameter. The air, in rushing in, sounds
just like that escaping from the cylin-
der OF an airbrake under a railroad
passenger car. When the pressure in
the lock is the same as that in the
working chamber of the tunnel the in
ner door is opened, and the men go out
of the lock into the chamber.
Now, this extraordinary pressure that
overbalances the water pressure and
drives the water out. also endeavors by
every possible means to escape from its
confinement. and such was the loose
nature of the porous material in the
bed of the river that there was a con-
slant escape of the air into the Thames
above. 1t required the dumping of
great clay blanket several feet thick
over the whole of this part of the river
bed, and even then the air in the tun-
mel blew out through it twice, throwing
up water at one time % feet high over
an area of so feet in diameter. so that
the boatmen above thought a great
torpedo had exploded.
Another serious condition had to be
wisely met. We have stated the facts
about the water pressure, that it in
creases 2-5ths of a pound with every
foot of increased depth. The tunnel
was over % feet in diameter. This
would give about II pounds per square
inch greater pressure at the bottom
than at the top of the tunnel. Now.
if sufficient air pressure were used to
drive the water back out of the bot
tom. it would be stronger than was
necessary at the top of the tunnel by
u pounds per square inch, and there
would, of course, be an increased ten
dency to blow the river bed up into
the water. 1t was decided, therefore,
not to attempt to keep the water out
of the bottom, but to take care of some
of it by pumping. That which came
in was blown by air pressure back
ward through the bulkhead and car
by ordinary pumps. pumping to the
ton and diScharCinS into the river.