foundations at great depths is, of course, hardly known to the
thousands who constantly see the structures supported on those
foundations, but in any fair consideration of such engineering
achievements this must not be omitted. The beautiful bridge
built by Captain Eads over the Mississippi River at St. Louis,
bold in its design and excellent in its execution, is an object
of admiration to all who visit it, but the impression of its
importance would be greatly magnified if the part below the surface
of the water, which bears the massive towers, and which extends to
a depth twice as great as the height of the pier above the water,
could be visible.
[Illustration: Railway Pass at Rocky Point in the Rocky Mountains.]
The simplest and most effective foundation is, of course, on solid
rock. In many localities reliable foundations are built upon
earth, when it exists at a suitable depth and of such a character
as properly to sustain the weight. Foundations under water, when
rock or good material occurs at moderate depth, are constructed
frequently by means of the coffer-dam, which is simply an enclosure
made water-tight and properly connected with the bottom of the
stream. The water is then pumped out and the foundation and
masonry built within this temporary dam. When the material is not
of a character to sustain the weight, the next expedient is the
use of piles, which are driven into the ground, often to a very
considerable depth, and sustain the load placed upon them by the
friction upon the sides of the piles of the material in which they
are driven. It is seldom that dependence is placed upon the load
being transferred from the top to the point of the pile, even
though the point may have penetrated to a comparatively solid
material. Wood is generally used for piles, and where the ground
is permanently saturated there seems to be hardly any known limit
to its durability. The substructure of foundations, where it is
certain that they will always be in contact with water, can be,
and generally is, of wood, and the permanency of such foundations
is well established. An exception to this, however, occurs in
salt-water, particularly in warmer countries, where the ravages
of the minute _Teredo Navalis_, and of the still more minute
_Limnoria Terebrans_, destroy the wood in a very short period of
time. These insects, however, do not work below the ground-line or
bed of the water. In many special cases hollow iron piles are used
successfully.
[Illustration: Bridge Pier Founded on Piles.]
The ordinary method of forcing a pile into the ground is by
repeated blows of a hammer of moderate weight; better success
being obtained by frequent blows of the hammer, lifted to a slight
elevation, than results from a greater fall, there being danger
also in the latter case of injuring the material of the pile. The
use of the water-jet for sinking piles, particularly in sand, is
interesting. A tube, generally of ordinary gas-pipe, open at the
lower end, is fastened to the pile; the upper end is connected by
a hose to a powerful pump and, the pile being placed in position
on the surface of the sand, water is forced through the tube and
excavates a passage for the pile, which, by the application of
very light pressure, descends rapidly to the desired depth. The
stream of water must be continuous, as it rises along the side of
the pile and keeps the sand in a mobile state. Immediately upon
the cessation of pumping, the sand settles about the pile, and it
is sometimes quite impossible to afterward move it. The water-jet
is used in sinking iron piles by conducting the water through the
interior of the hollow pile and out of a hole at its point. The
piles of the great iron pier at Coney Island were sunk with great
celerity in this way. The illustration opposite shows one of the
piers of a bridge founded upon wooden piling.
In many cases it would be impossible to drive piling in such a way
as to insure the durability of the structure above it. This is
particularly true of the foundations of structures crossing many
of our rivers, where the bottom is of material which, in time of
flood, sometimes scours to very remarkable depths; the material
often being replaced when the flood has subsided. The expedient
adopted is the pneumatic tube, or the caisson. Both are merely
applications of the well-known principle of the diving-bell. In
the former case hollow iron tubes, open at the bottom, are sunk to
considerable depths, the water being expelled by air pumped into
the tubes at a pressure sufficient to resist the weight of the
water. Entrance to the tubes is obtained by an air-lock at the top,
the material is excavated from the inside, and sufficient weight
placed upon the tube to force it gradually to the desired depth.
When that depth is attained, the tubes are filled with concrete,
and thus solid pillars of hydraulic concrete, surrounded by
cast-iron tubing, are obtained.
The pneumatic caisson is an enlargement of this idea of the
diving-bell. The caisson is simply a great chamber or box, open
at the bottom; the outside bottom edges are shod and cased with
iron so as to give a cutting surface; the roof and sides are made
of timber, thoroughly bolted together, and of such strength as to
resist the pressure of the structure to be finally founded upon it.
The chamber in the open bottom is of sufficient height to enable
the laborers to work comfortably in it. This caisson is generally
constructed upon the shore in the vicinity of the structure and
towed to the point where the foundation is to be sunk. Air is
supplied by powerful pumps and is forced into the working chamber.
The pressure of the air of course increases constantly as the
caisson descends; it must always be sufficient to overbalance the
weight of the water and thus prevent the water from entering the
chamber.
Descent to the caisson is made through a tube, generally of wrought
iron, and having, at a suitable point, an air-lock, which is
substantially an enlargement of the tube, forming a chamber, and of
sufficient size to accommodate a number of men. This air-lock is
provided with doors or valves at the top and at the bottom, both
opening downward, and also with small tubes connecting the air-lock
with the chamber below and with the external air above. Entrance
to the caisson is effected through this air-lock. The lower door,
or valve, being at the bottom, closes and is kept closed by the
pressure of the air in the caisson below. After the air-lock is
entered the upper door or valve is shut, and held shut a few
moments, and the tube connecting with the outer air is closed; a
small valve in the tube connecting with the caisson is then opened
gradually and the pressure in the air-lock becomes the same as
that in the chamber below; as soon as this is effected the valve,
or door, at the bottom of the air-lock falls open and the air-lock
becomes really a part of the caisson.
[Illustration: Pneumatic Caisson.]
A sufficient force of men is employed in the chamber to gradually
excavate the material from its whole surface and from under the
cutting edge, and the masonry structure is founded upon the top
of the caisson and built gradually, so as to give constantly a
sufficient weight to carry the whole construction down to its final
location upon the stable foundation, which may be the bed-rock or
may be some strata of permanent character.
The problem of lighting the chamber was until recently of
considerable difficulty. The rapid combustion under great pressure
made the use of lamps and candles very troublesome, particularly on
account of the dense smoke and large production of lampblack.
The introduction of the electric light has greatly aided in the
more comfortable prosecution of pneumatic foundation work.
[Illustration: Transverse Section of Pneumatic Caisson.]
The removal of rock, or any large mass, from the caisson is
effected through the air-chamber; but the removal of finer
material, as sand or earth, is accomplished by the sand pump or
by the pressure of the air. A tube, extending from the top of
the masonry and kept above the surface by additions, as may be
required, enters the working chamber and is controlled by proper
valves. Lines of tubing and hose extend to all portions of the
chamber. A slight excavation is made and kept filled with water.
The bottom of the tube, or the hose connected with it, is placed in
this excavation, and, the material being agitated so as to be in
suspension in the water, the valve is opened, and the pressure of
the air throws the water and the material held in suspension to the
surface, through the tube, from the end of which it is projected
with great velocity and may be deposited at any desired adjacent
point. This method, however, exhausts the air from the caisson too
rapidly for continuous service. The Eads sand-pump is therefore
generally used. This is an ingenious apparatus, somewhat the same
in principle as the injector which forces water into steam-boilers.
A stream of water is thrown by a powerful pump through a tube
which, at a point near the inlet for the excavated material, is
enlarged so as to surround another tube. The water is forced upward
with great velocity into the second tube, through a conical annular
opening, and, expelling the atmosphere, carries with it to the
surface a continuous stream of sand and water from the bottom of
the excavation.
This system has been used successfully in the foundations of piers
and abutments of bridges in all parts of the world. The rapidity
of the descent of the caisson varies with the material through
which it has to pass. The speed with which such foundations are
executed is remarkable, when one remembers with what delicacy and
intelligent supervision they have to be balanced and controlled. In
some instances it has been necessary to carry them to great depths,
one at St. Louis being 107 feet below ordinary water level in the
river.
The pressure of air in caissons at these depths is very great; at
110 feet below the surface of the water it would be 50 pounds to
the square inch. Its effect upon the men entering and working in
the caisson has been carefully noted in various works, and these
effects are sometimes very serious; the frequency of respiration
is increased, the action of the heart becomes excited, and many
persons become affected by what is known as the "caisson disease,"
which is accompanied by extreme pain and in some cases results
in more or less complete paralysis. The careful observations of
eminent physicians who have given this disease special attention
have resulted in the formulation of rules which have reduced the
danger to a minimum.
[Illustration: At Work in a Pneumatic Caisson--fifty feet below the
surface of the water.]
The execution of work within a deep pneumatic caisson is worth a
moment's consideration. Just above the surface of the water is a
busy force engaged in laying the solid blocks of masonry which are
to support the structure. Great derricks lift the stones and lay
them in their proper position. Powerful pumps are forcing air,
regularly and at uniform pressure, through tubes to the chamber
below. Occasionally a stream of sand and water issues with such
velocity from the discharge pipe that, in the night, the friction
of the particles causes it to look like a stream of living fire.
Far below is another busy force. Under the great pressure and
abnormal supply of oxygen they work with an energy which makes it
impossible to remain there more than a few hours. The water from
without is only kept from entering by the steady action of the
pumps far above and beyond their control. An irregular settlement
might overturn the structure. Should the descent of the caisson
be arrested by any solid under its edge, immediate and judicious
action must be taken. If the obstruction be a log, it must be cut
off outside the edge and pulled into the chamber. Boulders must be
undermined and often must be broken up by blasting. The excavation
must be systematic and regular. A constant danger menaces the lives
of these workers, and the wonderful success with which they have
accomplished what they have undertaken is entitled to notice and
admiration.
[Illustration: Pier of Hawkesbury Bridge, Australia.]
Another process, which has succeeded in carrying a foundation to
greater depths than is possible with compressed air, is by building
a crib or caisson, with chambers entirely open at the top, but
having the alternate ones closed at the bottom and furnished with
cutting edges. These closed chambers are weighted with stone or
gravel until the structure rests upon the bottom of the river;
the material is then excavated from the bottom through the open
chambers, by means of dredges, thus permitting the structure
to sink by its weight to the desired depth. When that depth is
reached, the chambers which have been used for dredging are filled
with concrete, and the masonry is constructed upon the top of this
structure. The use of this system has enabled the engineer to place
foundations deeper than has been accomplished by any other device,
one recently built in Australia being 175 feet below the surface of
the water. The illustrations above and on page 76 show this method
of construction.
[Illustration: Foundation Crib of the Poughkeepsie Bridge.]
Even more remarkable than the pneumatic caisson is this method
of sinking these great foundations. The removal of material must
be made with such systematic regularity that the structure shall
descend evenly and always maintain its upright position. The dredge
is handled and operated entirely from the surface. The very idea
is startling, of managing an excavation more than a hundred feet
below the operator, entirely by means of the ropes which connect
with the dredge, and doing it with such delicacy that the movement
of an enormous structure, weighing many tons, is absolutely
controlled. This is one of the latest and most interesting advances
of engineering skill.
* * * * *
[Illustration: Transverse Section of the same.]
While it is true that the avoidance of large expenditure, when
possible, is a mark of the best engineering, yet great structures
often become absolutely necessary in the development of railway
communication. Wide rivers must be crossed, deep valleys must be
spanned, and much study has been given to the best methods of
accomplishing these results. In the early history of railways
in Europe substantial viaducts of brick and stone masonry were
generally built; and in this country there are notable instances of
such constructions. The approach to the depot of the Pennsylvania
Railroad, in the city of Philadelphia, is an excellent example.
Each street crossed by the viaduct is spanned by a bold arch of
brick. Upon a number of our railways there are heavy masonry arches
and culverts, and at some places these are of a very interesting
character. The arches in the approach to the bridge over the Harlem
Valley (recently completed) are shown above. They are of granite,
having a span of 60 feet. The illustration shows also the method
of supporting the stone work of such arches during construction.
Braced timbers form what is called the centre, and support the
curved frame of plank upon which the masonry is built, which, of
course, cannot be self-supporting until the keystone is in place;
then the centre is lowered by a loosening of the wedges which
support it, and the stone work of the arch is permitted to assume
its final bearing. It is generally considered that where it is
practicable to construct masonry arches under railways there is a
fair assurance of their permanency, but some engineers of great
experience in railway construction advance the theory that the
constant jar and tremor produced by passing railway trains is
really more destructive to masonry work than has been supposed,
and that it may be true that the elements of the best economy will
be found in metal structures rather than in masonry. It is a fact
that repairs and renewals of metal bridges are much more easily
accomplished than of masonry constructions.
[Illustration: Granite Arched Approach to Harlem River Bridge in
Process of Construction.]
In this country the wooden bridge has been an important, in fact an
essential element in the successful building of our railways.
Timber is also used extensively in railroad construction in the
form of trestles; one example of which has been alluded to on
page 50. There were also constructed, years ago, some very bold
viaducts in wood. One of the most interesting is shown above, being
the viaduct at Portage, N. Y. This construction was over 800 feet
long, and 234 feet high from the bed of the river to the rail. The
masonry foundations were 30 feet high, the trestles 190 feet, and
the truss 14 feet; it contained more than a million and a half
feet, board measure, of timber. The timber piers, which were 50
feet apart, are formed by three trestles, grouped together. It was
framed so that defective pieces could be taken out and replaced
at any time. This bridge was finished in 1852 and was completely
destroyed by fire in 1875. The new metal structure which took
its place is shown on the opposite page, and is an interesting
example of the American method of metal viaduct construction, an
essential feature of that construction being the concentration of
the material into the least possible number of parts. This bridge
has ten spans of 50 feet, two of 100 feet, and one of 118 feet. The
trusses are of what is called the Pratt pattern, and are supported
by wrought-iron columns, two pairs of columns forming a skeleton
tower 20 feet wide and 50 feet long on the top. There are six of
these towers, one of which has a total height from the masonry to
the rail of 203 feet 8 inches. There are over 1,300,000 pounds of
iron in this structure.
[Illustration: The Old Portage Viaduct, Erie Railway, N. Y.]
The fundamental idea of a bridge is a simple beam of wood. If metal
is substituted it is still a beam with all superfluous parts cut
away. This results in what is called an I beam. When greater loads
have to be carried, the I beam is enlarged and built up of metal
plates riveted together and thus becomes a plate girder. These are
used for all short railway spans. For greater spans the truss must
be employed.
[Illustration: The New Portage Viaduct.]
Before referring, however, to examples of truss bridges, a
description should be given of the Britannia Bridge, built by
Robert Stephenson in 1850, over the Menai Straits. This great
construction carries two lines of rails and is built of two square
tubes, side by side, each being continuous, 1,511 feet long,
supported at each extremity and at three intermediate points,
and having two spans of 460 feet each and two spans of 230 feet
each. The towers which support this structure are of very massive
masonry, and rise considerably above the top of the tubes. These
tubes are each 27 feet high and 14 feet 8 inches wide; they are
built up of plate iron, the top and bottom being cellular in
construction, and the sides of a single thickness of iron. The
tubes for the long spans were built on shore and floated to the
side of the bridge and then lifted by hydraulic presses to their
final position. The rapid current, and other considerations, made
the erection of false works for these spans impracticable. The
beautiful suspension bridge, built by Telford in 1820, over the
Menai Straits, is only a mile away from this Britannia Bridge, but,
at the time of the construction of the latter, it was not deemed
possible by English engineers to erect a suspension bridge of
sufficient strength and stability to accommodate railway traffic.
[Illustration: The Britannia Tubular Bridge over the Menai Straits,
North Wales.]
The Victoria Bridge at Montreal is of the same general character
of construction as the Britannia Bridge, but is built only for a
single line of rails; this bridge also was built by Mr. Stephenson,
in 1859. These two structures were enormous works; their strength
is undoubted, but they lack that element of permanent economy which
has been spoken of in this article; their cost was very great, and
the expense of maintenance is also very great. A very large amount
of rust is taken from these tubes every year; they require very
frequent painting, and there are on the Victoria Bridge 30 acres of
iron surface to be thus painted.
[Illustration: Old Stone Towers of the Niagara Suspension Bridge.]
A remarkable and interesting contrast to these heavy tubes of
iron is the Niagara Falls railway suspension bridge, completed in
March, 1855. The span of this bridge is 821 feet, and the track
is 245 feet above the water surface. It is supported by 4 cables
which rested on the tops of two masonry towers at each end of
the central span, the ends of the cables being carried to and
anchored in the solid rock. The suspended superstructure has two
floors, one above the other, connected together at each side by
posts and truss rods, inclined in such a manner as to form an open
trussed tube, not intended to support the load, but to prevent
excessive undulations. The floors are suspended from the cables
by wire ropes, the upper floor carrying the railroad track, and
the lower forming a foot and carriage way. Each cable has 3,640
iron wires. This bridge carried successfully a heavy traffic for
26 years; it was then found that some repairs to the cable were
required at the anchorage, the portions of the cables exposed to
the air being in excellent condition. These repairs were made,
and the anchorage was substantially reinforced. At the same time
it was found that the wooden suspended superstructure was in
bad condition, and this was entirely removed and replaced by a
structure of iron, built and adjusted in such a manner as to secure
the best possible results. For some time it had been noticed that
the stone towers which supported the great cables of the bridge
showed evidences of disintegration at the surface, and a careful
engineering examination in 1885 showed that these towers were in
a really dangerous condition. The reason for this was that the
saddles over which the cables pass on the top of the towers had
not the freedom of motion which was required for the action of the
cables, caused by differences of temperature and by passing loads.
These saddles had been placed upon rollers but, at some period,
cement had been allowed to be put between these rollers, thus
preventing their free motion. The result was a bending strain upon
the towers which was too great for the strength and cohesion of
the stone. A most interesting and successful feat was accomplished
in the substitution of iron towers for these stone towers, without
interrupting the traffic across the bridge. This was accomplished
within a year or two by building a skeleton iron tower outside of
the stone tower, and transferring the cables from the stone to the
iron tower by a most ingenious arrangement of hydraulic jacks.
The stone towers were then removed. Thus, by the renewal of its
suspended structure and the replacing of its towers, the bridge has
been given a new lease of life and is in excellent condition to-day.
[Illustration: The New Iron Towers of the Same.]
This Niagara railway suspension bridge has been so long in
successful operation that it is difficult now to appreciate the
general disbelief in the possibility of its success as a railway
bridge, when it was undertaken. It was projected and executed
by the late John A. Roebling. Before it was finished, Robert
Stephenson said to him, "If your bridge succeeds, mine is a
magnificent blunder." The Niagara bridge did succeed.
[Illustration: Below the Brooklyn Bridge.
From a painting by J. H. Twachtman.]