A tunneling shield consists essentially of a tube or cylinder slightly
larger in diameter than the tunnel it is intended to build, which slides
over the exterior of the finished lining like the tubes of a telescope.
The front end of this cylindrical shield is provided with a diaphragm or
bulkhead in which are apertures which may be opened or closed at will.
Behind this diaphragm are placed a number of hydraulic jacks, so
arranged that by thrusting against the last erected iron ring the entire
shield is pushed forward. The hind end of the shield is simply a
continuation of the cylinder which forms the front end, and this hind
end, or tail, always overlaps the last few feet of the built-up
iron-shell tunnel.
When the openings in the bulkhead are closed, the tunnel is protected
from the inrush of water or soft ground, and the openings may be so
regulated that control is maintained over the material passed through.
After a ring of iron lining has been erected within the tail of the
shield, excavation is carried out ahead. When sufficient excavation has
been taken out, the jacks are again extended, thus pushing the shield
ahead, and another ring of iron is erected as before.
[Illustration: ONE OF THE SIXTY-SEVEN-TON TUNNEL SHIELDS]
For the erection of these heavy plates, a hydraulic swinging arm, called
the “Erector,” is mounted, either on the shield itself or on an
independent erector platform, according to conditions. This erector
approaches closely the faculties of the human arm. It is hydraulically
operated and can be moved in any desired direction. This method of
construction can be followed in almost every kind of ground that can be
met with, and it is especially valuable in dealing with soft, wet
grounds. In passing through materials saturated with water, the shield
is assisted by using compressed air in the working chamber.
[Illustration: CUTTING SHIELD HEAD]
The employment of compressed air under such conditions is really a
rather simple thing in itself, and means merely that the pressure of air
in the chamber where men are working is maintained at a point sufficient
to offset the pressure of the hydrostatic head of water and thereby
prevent its inflow. A crude comparison may be made by saying that if the
ceiling of a room was weak and threatening to fall--if we filled the
room with sufficient pressure of air, it would support the ceiling and
prevent it falling in. In tunnel work, air is supplied under compression
from the mechanical construction plant located on the surface, and the
pressure of air maintained in the working chamber is determined by the
depth of the work below tide level, as the hydrostatic head increases
with the depth.
Control of air pressure is never entrusted to any but the most reliable,
competent and experienced man, as it is of the utmost importance that
air pressure be maintained properly. The first impulse of an
inexperienced man, should he notice an inrush of water, would be to
increase the air pressure, which might be a very dangerous thing to do.
An experienced man, however, would very likely first lower his pressure
in such an emergency, and then put up with the nuisance and difficulty
of having a good deal of water in his working chamber. By doing this, he
would permit the greater external pressure to squeeze the soil into the
leaking pockets and thereby choke the leak.
[Illustration: APRON IN FRONT OF SHIELD, FIVE MINUTES BEFORE SHOVING]
To improperly or inopportunely raise the air pressure would be quite
likely to result in the air blowing a hole through the roof of the
tunnel heading, allowing all air pressure to escape, and permitting an
uncontrollable volume of water to rush in and flood the work.
The outer shell of the tunnel shield is composed of two- or three-ply
boiler plates, and the interior is braced with a system of steel
girders. The shields used weighed approximately sixty-seven tons each.
Sixteen or eighteen were used. To move the shield forward, each shield
was equipped with sixteen hydraulic jacks, arranged around the shield
circumferentially. These jacks were controlled by a series of valves,
which were so designed that any one jack or any set of jacks desired
could be operated. This was necessary as the direction of the shield
was, as it were, guided by the pressure of the jacks. When it was
desired to alter the direction of the shield, either upwards or
downwards, or to the right or left, the jacks on the opposite side to
which the shield was to point, were operated. The hydraulic pressure
operating these jacks was 5,000 pounds per square inch, and the total
energy, when all jacks were employed at the same time, was equivalent to
2,500 tons, which was equal to eleven tons per square foot of heading.
[Illustration: CUTTING EDGE OF SHIELD IN NORTH TUNNEL]
Air pressure used to prevent the inflow of water and soft dirt varied
from nothing up to forty-two pounds, although a fair average throughout
was thirty-two pounds. It varied, of course, according to the condition
encountered.
The working chamber is the space between the tunnel heading where work
is in progress and the air-lock. The air-lock is a device used for the
purpose of enabling workmen and materials to pass from the portion of
the tunnel where the atmospheric pressure is normal into the portion
where the air pressure is greater than normal; that is, the working
chamber. The air-lock is a cylinder, usually about six feet in diameter
and twenty feet in length, with a heavily constructed iron door at each
end. This lock is placed horizontally in the tunnel at such a level as
the conditions of the work necessitate, but usually near the bottom, and
around this cylinder, and completely filling the cross-section of the
tunnel, a concrete bulkhead is built and is known as the lock bulkhead.
The two doors open in the same direction; the one at the normal pressure
end opening into the cylinder, and the one at the heading end opening
away from the cylinder. One door is always closed, and both doors are
closed during the operation of entering or leaving the air-pressure
section.
Going into the air pressure, the door at the heading end is held closed
by the pressure of air against it while one is entering the lock, after
which the outer door is also closed. A valve is then opened which
permits the air to flow from the working chamber into the lock, until
the lock becomes filled with air of the same pressure as exists in the
heading. As soon as the pressure is thus equalized, the door at the
heading end can be opened and the workmen pass into the heading. Going
out, the operations are simply reversed. After the heading door is
closed, with the workmen in the air-lock, a valve is opened which
permits the air in the lock to exhaust into the normal air, until the
pressure within the lock reduces to the same as that outside, when the
outer door can be opened and persons inside the lock pass out. Both
operations must be gradual, as a sudden change from normal to high
pressure, or _vice versa_, would be very dangerous to anyone.
[Illustration: SHIELD CUTTING EDGE BREAKING THROUGH WALL AT SIXTH AVENUE
AND TWELFTH STREET, LOOKING SOUTH, OCTOBER 23, 1907]
In tunneling under the river, nearly every conceivable combination of
rocks and soils were met, but for the most part the material was silt.
In such material, with a pressure of 5,000 pounds per square inch on the
shield jacks, the shield was pushed through the ground as though one
pushed a stick into a heap of snow, pushing aside the silt, and thus
obviating the necessity of removing any excavated material. Sand or
gravel, or any material which would not flow or become displaced by the
shield, of course, had to be excavated ahead of the shield, and removed
from the heading prior to pushing it forward. In the silt the most
satisfactory and economic progress was attained, and a record was made
of seventy-two feet of finished tunnel, completely lined with iron, in
one day of twenty-four hours.
The most difficult combination that had to be dealt with under the river
was when the bottom consisted of rock and the top of silt and wet sand.
In such cases, and there were many of them, the upper section of soft
ground was first excavated and the exposed face securely supported with
timbers ahead of the shield, and the rock underlying then drilled and
blasted. This was very tedious and expensive work. Exceedingly small
charges of dynamite had to be used and the procedure conducted with the
utmost caution.
In the course of their progress, the shields were subjected to the most
intense strains and hard usage, as may well be imagined. One of the
shields is illustrated. It was used to construct the south tunnel of the
up-town pair of tubes, and passed from under the Hudson River, through
Morton, Greenwich and Christopher Streets, into Sixth Avenue, and north
to Twelfth Street, a total distance of 4,525 feet, of which 2,075 feet
was through rock overlaid with wet sand. During the progress of this
shield, 26,000 sticks of dynamite were exploded in front of the cutting
edge, causing great damage to the structure of the shield, so that when
it arrived at its destination at Sixth Avenue and Twelfth Street, it was
in such a condition of distortion that it was with difficulty that the
tunnel lining could be erected behind it.
[Illustration: NORTH TUNNEL, SHOWING COMMENCEMENT OF NEW WORK]
In pushing a shield forward with the battery of powerful hydraulic
jacks, each advance is of two feet, and must be followed immediately by
installation of the permanent lining in the rear. In the early days,
brick work was used for lining, and in recent years it has also been
used to some extent, but even with the use of quick-setting Portland
cement, neither brick work nor concrete has proved successful for
subaqueous work, as the cement cannot reach the required strength within
the time it is feasible to leave the shield standing before advancing it
again.
[Illustration: HOLE BROKEN THROUGH THE SOUTH TUBE OF THE NEW YORK AND
JERSEY TUNNEL LOOKING WEST]
During the early work on the north tube of the uptown tunnels, a point
was reached where the rock was sixteen feet above the bottom of the
tunnel, and the overlying silt was in a semi-fluid state. Five barges of
clay had been dumped in the river over this point to make a roof for the
tunnel, but the fluid clay could not be controlled, and crept through
the doors of the shield. After trying all known methods to get through,
it was decided to bake this wet clay by means of intense heat. Two large
barges of kerosene were sent into the tunnel, and an air pipe connected
to them. Fine blow-pipes were also attached, and the fire from the
blow-pipes was impinged on the exposed clay until it became caked
sufficiently dry and hard to overcome slipping. It required eight hours
of this baking to dry the clay hard, and, during this period, water had
to be played continuously on the shield to avoid damage due to the high
temperature. It is believed that this was the first time that soft
material met with in tunneling under a river has been solidified by
means of fire. Seven days after passing this troublesome point, the rock
suddenly disappeared and the work proceeded without further trouble.
[Illustration: NEW YORK AND NEW JERSEY TUNNEL SHOWING SIGNAL AND CAR]
Another unusual situation occurred in the south tunnel of the uptown
tubes. When the shield had advanced 115 feet from the Jersey side, the
night superintendent in charge of the tunnel work, in his anxiety to
push the work, disobeyed instructions, and the tunnel got away from him
and was flooded, and his men had a narrow escape with their lives. In
order to regain the tunnel, several schemes were considered, including
that of sending a dredge through to dredge out the bed of the river just
in advance of the shield, a sufficient depth to enable a diver to go
down and timber up the exterior opening of the doorway, where the silt
and mud had come through and filled the tunnel. This plan had to be
abandoned, as the river above was almost entirely occupied by shipping
that could not be interrupted.
[Illustration: AN X-RAY VIEW OF A BUSY HALF-MILE UNDER THE GROUND ON THE
JERSEY SIDE OF THE HUDSON RIVER]
[Illustration: CROSS-SECTION ON SIXTH AVENUE AT THIRTY-THIRD STREET, NEW
YORK
1. FOOT PASSAGE
2. MANHATTAN ELEVATED RAILROAD
3. STREET SURFACE AND METROPOLITAN STREET RAILWAY
4. NEW RAPID TRANSIT SUBWAY
5. HUDSON AND MANHATTAN RAILROAD STATION
6. PENNSYLVANIA RAILROAD TUNNEL]
Finally the difficult situation was met by obtaining two large and heavy
mainsails, which made a double canvas cover measuring about sixty by
forty feet. This canvas cover was then spread on a flat barge, small
sections of pig iron being attached around the edges of it. Ropes were
carried to fixed points to hold it in exact position. The barge was then
withdrawn, and the canvas cover dropped to the bed of the river, and,
most fortunately, it settled over the point where the leak had occurred,
and a large number of bags of dirt were then deposited on it. An opening
was then made in the bulkhead of the tunnel below, and for eight days
material, under hydrostatic pressure, forced its way into the tunnel,
where it was loaded on cars, and finally the canvas was drawn into the
hole, stopping it up. Additional material was then deposited into the
river to fill the cavity, and finally the tunnel was recovered, pumped
out and work resumed. This event is of somewhat historical interest, in
that the two mainsails which were used were procured from the owner of
the famous American cup defender, the well-remembered “Reliance.”
Probably the most unique and interesting pieces of construction are the
three junctions on the Jersey side of the river, where the uptown
tunnels from New York diverge, north to Hoboken and south to Jersey City
and New York downtown. For safe and expeditious operation of trains,
where the schedule is only one and one-half minutes, it was imperative
that grade crossings should be avoided. By grade crossings is meant the
tracks of one service crossing the tracks of another service at the same
grade. At the point in question, this was a knotty problem to solve,
owing to the unusual operating conditions which had to be met, there
being six separate and distinct operating classes of trains to be
handled around this triangle.
To meet this situation, three massive reinforced concrete caissons were
built on the surface. They are practically large two-story houses, each
being over one hundred feet in length, about fifty feet in height, and
about forty-five feet in width at their widest point. The bottom edges
were sharp, and, with the use of air pressure and great weights, the
three structures were sunk in the ground to the same grade as the
intercepting tunnels, and the tunnels were then driven into them.
Particular attention should be given to the Jersey City to Hoboken tube,
in the lower part of the caisson in the foreground, in the accompanying
illustration, which curls around the Hoboken to Jersey City tube, and
rises to the elevation of, and connects into, the New York to Hoboken
tube, at the caisson in the background, at the left of the illustration.
Very few of the people who travel through the tube are probably aware of
such manipulation. At the same time, the arrangement absolutely avoids
any grade crossing whatever, and without such an arrangement of tracks
the road could not be operated with trains run so closely together as
under the prevailing system.
In constructing the river tunnels the work was carried on simultaneously
from opposite sides of the river, the tunnels meeting under the river,
and it is interesting, if not remarkable, when one considers the
difficulties under which the engineering work had to be carried on, to
note that the tunnels met with practically absolute accuracy.
* * * * *
What Causes Floating Islands?
A floating island consists generally of a mass of earth held together by
interlacing roots.
They occur on the Mississippi and other rivers, being portions of the
banks detached by the force of the current and carried down the stream,
often bearing trees. Sometimes such islands are large enough to serve as
pasture grounds.
Artificial floating islands have been formed by placing lake mud on
rafts of wicker-work covered with reeds. They were formerly used in the
waters around Mexico, and may be seen in Persia, India, and on the
borders of Tibet. On these the natives raise melons, cucumbers and other
vegetables which need much water.