the birth of the art is as old as the emergence of man from savagery.
The savage who hollows out a log of wood in order to construct a canoe
has taken the first step in the art of shipbuilding; and when he has
constructed a hut, however rude, to take the place, as an abode, of
the cave hollowed out by nature, he has moved one step nearer to those
triumphs of building construction which satisfy man’s necessities,
comforts, and æsthetic desires. From this standpoint civil engineering
is as old as the oldest of the arts and sciences. Not only is civil
engineering an ancient art, but when the archæologist points to some
of the masterpieces of building construction which have been literally
hidden from view by the débris of centuries, and describes the old
roads which the disintegrating forces of nature, working for centuries,
have not been able to destroy, it is natural to assume that in many
features the civil engineering of the present day is but a copy of
ancient work, or, at least, that there has been comparatively little
real progress. It may be claimed that bridges are very old, that
canals, lighthouses, and roads antedate the Christian era, and that
even the ancient Egyptians knew that the earth is round, and had made
a rough computation of its diameter. But it will be shown that even
in these cases there has been an enormous advance, not only in the
character and magnitude of the work done, but also in another feature
of civil engineering which is frequently overlooked, namely, the
_economy_ of labor and material. Civil engineering has been defined as
the art of doing well with one dollar what any bungler can do somehow
with two dollars. This definition, although very loose and one-sided,
nevertheless contains a very important truth. If by improved methods a
canal or a bridge can be constructed for one half to one third of what
it would have cost by older methods, then the world has advanced, in
that it may have two or three canals or bridges at the same cost of
labor as would have been previously required for the construction of
one. When we add to this a vast improvement in quality, an improvement
that would have been previously impossible at any cost, the world’s
advance is hardly measurable by any standard. It is a well-known fact
that many engineering works, justly considered masterpieces at the
time of their construction, could now be replaced by a much better
structure for a comparatively small part of their original cost. This
statement not only applies to very old constructions, but even to some
of the great engineering works of the latter half of this century. Some
of these reconstructions have actually occurred, as is illustrated in
the Victoria tubular bridge at Montreal, or the Roebling suspension
bridge at Niagara Falls,—described later. In fact, the progress in
civil engineering during the nineteenth century is chiefly made up of
the enormous advances which have been made during the latter half of
the century. It should not be argued that these recent constructions
are cheaper, because “everything is cheaper now.” The general scale
of wages has advanced, and the total cost of construction is cheaper,
only because improved methods of work have reduced the labor required
to produce finished building material from the raw product and to erect
that material into a structure. Therefore in considering in detail
the construction of the great masterpieces of this century, we should
not lose sight of the enormous advance in general methods of work,
which has rendered it possible to have all of these structures which
so minister to the prosperity of the world, at such a reduced cost in
labor.
A complete discussion of the century’s progress in civil engineering
would require a treatise on all modern practice as well as a
description of nearly all of the great engineering masterpieces in
existence, but the limitations of this article utterly preclude the
possibility of even a short discussion of all the branches of the
science, to say nothing of a detailed description of all of the
examples. The following discussion will therefore be confined to those
branches in which the advance has been most notable, even to the
unscientific reader, the progress being illustrated by brief statements
regarding the most typical constructions.
II. BRIDGES.
Not only is there evidence that bridges of the simplest forms have
been used from prehistoric times, but the engineering world has been
frequently surprised at the discovery, in semi-barbarous lands where
there was evidently no scientific knowledge of bridge construction,
of a bridge which, in its mechanical analysis, is a rude example of
some one of the more complicated types now in use. But these bridges
are always small, and are constructed with an utter disregard of that
economy of construction which is one of the great triumphs of modern
bridge engineering, being uselessly strong in some parts, considering
their weakness in others. At the beginning of this century there was
not a wrought-iron or steel bridge in existence. Disregarding stone
arches for the present, all other bridges were made of wood—with the
exception of a few bridges of cast iron, which were constructed during
the latter part of the eighteenth century. But cast-iron is unsuitable
for pieces requiring tensile strength; it is also difficult to cast
very large pieces with any assurance of uniformity. The best existing
examples of cast-iron bridges are, therefore, those of the arch type;
but these are very heavy in proportion to their real strength, and
would now be much more costly than, as well as inferior to, steel
bridges of equal strength. Therefore the great advance in bridge
work during this century consists in the development of steel bridge
construction, and a brief description will be given of a few bridges
which represent the chief types.
[Illustration: BROOKLYN SUSPENSION BRIDGE.]
BROOKLYN BRIDGE.—The suspension bridge between New York and Brooklyn
is the largest bridge of its kind in existence, and, until the
construction of the “Forth” bridge, was the longest clear span ever
built. Every one is so familiar with this stupendous structure that
only a few statements will be made, which may give a better idea of
the unprecedented problem which confronted the great engineer, John
A. Roebling. When looking at the exceedingly graceful design of the
towers, one is apt to forget that a large part of the structure
of each tower is hidden from view. The bottom of the foundation of
the pier, on the New York side, is 78 feet below mean high tide, and
spreads over an area 172 feet long and 102 feet wide. The pressure
exerted by the caisson on its base is about 114,000 tons, or 6½ tons
per square foot. This great area, 354 feet below the parapet of the
towers, is a surface consisting partly of bed-rock and partly of a
material so compact that it was found, to be almost impossible to drive
an iron bar into it. Down below the mud, below all danger of scour,
far below the depth where the dreaded _teredo navalis_ can destroy the
timber in the caissons, these piers rest on an immovable foundation,
and are an imperishable monument of man’s skill. The floor of the
bridge is supported by four cables, each containing 6300 wires. Each
wire is supposed to be subjected to a stress of about 570 pounds, and
to have an ultimate strength of 3400 pounds. To say that each cable
is pulled by a force of 3,591,000 pounds conveys but little real
impression to the mind—as little as to say that it would require a
pull of over 21,000,000 pounds to break it. And there are four such
cables! The main span, including the weight of the cables, weighs about
5000 tons. Some interesting facts concerning the caissons under the
piers of this bridge will be given under the heading of “Caissons.”
[Illustration: THE NIAGARA RAILWAY ARCH.]
NIAGARA RAILWAY ARCH.—The railway suspension bridge, constructed by
Mr. John A. Roebling across the Niagara gorge in 1853–55, was justly
considered a monument to the skill of a great engineer, a monument of
the world’s progress; and yet so rapid has been the advance in the art
of bridge engineering, that this great structure is already a thing
of the past, and has now been replaced by another bridge which better
fulfills the increased requirements. It was not that Roebling’s bridge
was an engineering failure, but that the large increase in the weight
and length of trains now requires a much stronger bridge. There were
several formidable conditions confronting the engineer who designed the
steel arch which has now replaced the suspension bridge. For one thing,
a heavy railroad traffic was using the old bridge. The interruption
of railroad traffic for even a few day’s is a serious matter. Extend
the time to several months, and the consequences are too serious for
toleration. And thus it became necessary to so plan and construct the
arch that both structures would occupy the same site, not interfere
with each other, and not interfere with the running of trains. It is
an amazing, almost inconceivable, triumph of constructive skill that
this was accomplished so that “_not a single train was delayed_, and
traffic on the highway floor was suspended only for about two hours
each day, while the upper floor system was being put in.” The second
rigid requirement was the necessity for constructing the arch without
any “false works” underneath. Of course it was not practicable to
suspend the various members of the arch during construction, from the
old bridge, as it was not designed for such a load. Nor would it have
been possible to plant false works in the deep and swift current of
the Niagara River. And so it became necessary to make each half of
the bridge self-supporting, as it hung out over the raging torrent a
distance of about 275 feet from the abutments, until the two projecting
arms could be joined in the centre. The illustration does not show the
independence of the arch from the old bridge. If the old bridge had
not been there (as was virtually the case, so far as support given by
it is concerned), the independence of those arms reaching out over
the river would have been more apparent. Add to all these rigorous
conditions the marvelous fact that the erection of this great arch was
begun on September 17, 1896, and that the bridge was tested on July 29,
1897 (only 315 days afterward), and we have here one of the greatest
triumphs of engineering which could be imagined.
PECOS RIVER VIADUCT.—The original location of the Galveston,
Harrisburg, and San Antonio Railway included a section of about 25
miles which was very difficult to operate, on account of its very heavy
grades and sharp curvature. After some years of study and surveying,
a line was found which would save 11.2 miles in distance, 378 feet
of rise and fall, and 1933 degrees of curvature, besides being free
from land slides which threatened the old line at many points. But the
great economic advantages in the expenses of operating could only be
obtained at the cost of an almost unprecedented structure,—a viaduct
2180 feet long, which should cross the Pecos River at an elevation of
320 feet 10½ inches above the water surface. There are two bridges in
Europe which span very deep gorges by _arches_, which are higher above
the water than this viaduct, but in such cases the depth of gorge is of
no engineering importance. There is also a viaduct, for a narrow-gauge
railway in Bolivia, 800 feet long and with a height of 336 feet from
the rails to the water. But the Pecos viaduct is built to carry
standard-gauge railway traffic over a valley nearly half a mile wide,
and at such a height that a train moving over it appears diminutive.
The stone towers in the illustration appear small, but they are
constructed to a height of over 50 feet above the ordinary level of the
water, to allow for possible floods. The longest “bents” have a height
of 241 feet 0¾ inches. No “false works” were used in erecting the
bridge. The “traveler,” shown in the illustration, had an arm 124 feet
6 inches long. After completing the construction on one side of the
river (including one half of the “suspended” span immediately over the
river), the traveler was taken apart, loaded on cars and transported by
rail a distance of nearly 40 miles, in order to reach the other side
of the valley. Then the construction was carried on as before, until
the two halves of the suspended span met in the centre. The work of
erection began November 3, 1891, and on February 20, 1892 (only 108
days later), the two halves of the suspended span were connected. A
portion even of this time was lost by inclement weather and unavoidable
delays. This light “spider-web” method of construction for crossing
very high valleys was originated by American engineers, the first
notable instance of it being the construction of the “Kinzua” viaduct,
on the N. Y. L. E. & W. R. R., which has a length of 2050 feet and a
height of 302 feet above the water—figures which are only slightly
less than the above.
[Illustration: THE FIRTH OF FORTH BRIDGE. GENERAL VIEW.]
FORTH BRIDGE.—The next type of bridge to be considered has for its
example the largest bridge in the world—the “cantilever” crossing the
Firth of Forth, in Scotland. The economic design of bridges of this
type, on the basis of the mechanical principles involved, is not only
an achievement of this century, but of the latter part of the century.
Nevertheless, we may find illustrations of the fundamental principle
in the stone lintels in an Egyptian temple; in a rough wooden bridge
erected by Indians in Canada, near the line of the Canadian Pacific
Railroad; and in a bridge erected over two hundred years ago in
Thibet, and discovered in 1783 by Lieutenant Davis, of the English
embassy to the court of the Teshoo Lama. The principle of these bridges
is very graphically shown by a photograph made at the time of the
construction of the Forth bridge.
[Illustration: PECOS RIVER VIADUCT.]
This bridge joins two sections of Scotland which had been previously
separated by an arm of the sea, which could only be crossed by a
tedious ferry. Even this ferry was frequently tied up by fog or by
the strong gales which so often blow up the channel. The prevalence
of heavy wind pressure demanded that special attention should be
given to this feature, and the most elaborate tests ever made of the
effect of wind on a bridge structure formed a part of the preliminary
work. The estuary, for a distance of nearly fifty miles, is never
less than two miles wide, except at this one place, where it is but
little more than one mile wide, with the added advantage of having the
island of Inchgarvie nearly in the centre of the channel. The channel
on both sides is about two hundred feet deep, which would forbid the
location of a pier at any place except on this island, which, being
composed of basaltic trap rock, furnished a sufficient foundation at
a comparatively slight depth below the surface. To secure the maximum
rigidity consistent with economy in weight, the “vertical columns”
of the towers were spaced 120 feet apart at the base, but only 33
feet apart at the top. The towers are 330 feet high. As shown in
the illustration, the cross-sectional dimensions of the cantilevers
diminish rapidly both in width and height, so that although the weight
of the steel per running foot at the towers is 23 tons, it becomes
only a little over two tons per foot at the centre. The structure is
exceptionally rigid.
The picture of any gigantic structure, especially when well
proportioned, utterly fails to give an adequate idea of the size of its
component parts. It is difficult to realize from the illustration that
the four tubular “vertical columns” on each main pier are twelve feet
each in diameter at the base—large enough for “a coach and four” to
drive into, if they were laid horizontally. Over 50,000 tons of steel
were used in the main spans. The total cost of the whole structure was
over £3,200,000 ($16,000,000).
STONE ARCHES.—The nineteenth century has but little to claim as
to the development of stone arches. The mechanical theory of their
stresses is perhaps better understood now than ever, and the largest
masonry arch in existence (the Cabin John arch, having a span of 220
feet, carrying the Washington aqueduct over a creek) is a piece of
American work of this century. But it should not be forgotten that
more than five hundred years ago there was constructed at Trezzo,
Italy, a granite arch of 251 feet span. This arch was unfortunately
destroyed in 1427. One of the most remarkable arches in existence was
designed and built by an “uneducated” stone-mason at Pont-y-Prydd,
Wales, in 1750. A rigorous analysis of its strains—of which the
designer probably knew nothing—shows that the “line of resistance”
passes almost exactly through the centre of the arch ring. The most
highly educated engineer of the present day could do no better. On
the other hand, the development of the theory has been shown by the
successful construction of an exceedingly bold design for a bridge on
the Bourbonnais Railway, in France. The span is 124 feet, and the rise
only 6.92 feet. The design was considered so very bold that a model of
the arch was first constructed and tested before the design was finally
adopted. The extension of the use of stone arches, especially those
of very large size, is doubtless prevented by their excessive initial
cost over the cost of a steel structure of equal span and strength.
Since a stone arch is generally considered more beautiful than a steel
bridge, the æsthetical element often demands the construction of stone
arches in public parks in situations where a metal structure would be
more economical. The great reduction in the cost of steel during the
past few years, due to improved processes of manufacture, generally
renders the cost of a steel bridge, even with a proper allowance for
maintenance, repairs, and renewals, cheaper than a stone arch, unless
the span is short.
III. CAISSONS.
The use of compressed air to keep back the water that would naturally
flow through the soil into a deep excavation is a comparatively recent
idea. In 1839 M. Triger, a French engineer, conceived the idea of
sinking an iron cylinder through twenty metres of quicksand in the
valley of the Loire River, in order to reach a valuable coal deposit
which was known to be located beneath the river. A chamber with doors,
such as is now called an air-lock, was constructed at the top of the
cylinder. To pass into the cylinder the lower door, opening downward,
was closed, and when the air in the chamber was at atmospheric
pressure, the upper door, also opening downward, was opened. Upon
entering the chamber the upper door was shut, and air was pumped in
until the pressure equaled the pressure in the cylinder underneath,
which was also the pressure necessary to keep back the water from
the excavation. The lower door could then be opened and the working
chamber entered. To pass out, the reverse process in inverse order was
necessary. This was the first pneumatic caisson ever sunk, although
such plans had been proposed and even patented in England several years
before. The idea was essentially the present plan, but the process has
been improved and enlarged. The required pressure is substantially that
due to the weight of a column of water as high as the depth of the base
of the caisson below the water surface. In the case of the St. Louis
bridge, the bottom of the caisson was sunk to 109 feet 8½ inches below
the water surface, which required an air pressure of about 47 pounds
per square inch in the working chamber. Such a pressure is dangerous to
those working in it. The men literally “live fast.” Great exertion is
easily made, but is followed by corresponding exhaustion after leaving
the caisson. Those having heart disease, or who have been debilitated
by previous excesses, are liable to be seriously affected—generally
by a form of paralysis which has been specifically named by physicians
the “caisson disease.” At the St. Louis bridge, when working at the
greatest depths, the men were only worked four hours per day, in
two-hour shifts. Facilities were likewise provided to have them bathe,
rest, and take hot coffee on coming out of the working chamber. Healthy
men, who observed these and similar precautions, were not permanently
affected by the work.
[Illustration: FORMAL OPENING OF SUEZ CANAL.
Procession of Ships in Canal, November 16, 1869.]
The caissons of the New York and Brooklyn suspension bridge are the
largest ever constructed, and a bald account of some of the experiences
encountered is fairly dramatic. Under such air pressures the flame
of a candle will return when blown out, and so the danger of fire
inside the wooden caissons became very serious. One evening a fire
was discovered in one of the caissons, caused presumably by a workman
holding a candle temporarily against the wooden roof while searching
for his dinner pail. When discovered it was apparent that the fire
had burned out a cavity in the solid timber roof, and the supply of
compressed air was fast turning those timbers into a mass of living
coal. Two pipes capable of throwing one and one half inch streams had
been provided for this express contingency, and the two streams were
turned on as quickly as possible. All night the fight went on. At 4
A. M., when the water was pouring out of the orifice of the cavity as
fast as it was sent in by the hose, it seemed as if the cavity must
have been thoroughly flooded and the fire out. To make sure of the
absolute extinction of the fire, borings were made, which showed that
the fire had worked its way along individual timbers, especially those
which were “fat” with resin, and that the fourth roof course was still
a mass of burning timber. It was then decided that the caisson must be
flooded, which was done by pumping in 1,350,000 gallons of water. After
flooding the caisson for two and one half days, it was pumped out and
the work examined. It required the services of eighteen carpenters,
working day and night for two months, to repair the damage caused by
that fire.
When the Brooklyn caisson was twenty-five feet below the water
level, the boulders encountered became so large that blasting became
necessary. But blasting inside of a caisson was hitherto an untried
experiment. It was feared that the men would be injured; that their
ear-drums would break by a sudden explosion in that confined space
under heavy air pressure; that a “blow out” might occur, i. e., that
the compressed air might suddenly escape past the edges, and that an
inflow of water would then drown the men. At first a pistol was fired,
gradually using heavier charges; then a small blast was set off.
Encouraged by their freedom from resulting complications, the blasts
were gradually increased, until they finally used as heavy blasts as
was desired, the men simply stepping into an adjoining chamber to avoid
flying fragments; and an increase in the rate of progress was at once
apparent, the caisson being lowered from twelve to eighteen inches,
rather than only six inches, per week.
The caissons of the bridge across the Firth of Forth, Scotland, are
examples of the great development of the caisson idea. The pneumatic
caisson of Triger, in 1839, had but one air lock, through which must
pass men, excavated material, and constructive material for linings,
etc. This plan meant slow and expensive work. The caissons of the
Brooklyn bridge were a vast improvement over this plan, both on the
score of economy and safety. In the Forth bridge the caissons were
made almost wholly of iron, thus avoiding the danger of the fire which
so nearly wrecked the caisson of the Brooklyn bridge. The careless or
premature opening of the doors of air locks, which once nearly caused a
serious accident on the Brooklyn caisson, was rendered impossible by a
very elaborate system of interlocking. The efficiency of the apparatus
for removing excavated material from the compressed air chamber was
also greatly increased. Electric lights were used instead of gas or
candles.
“FREEZING PROCESS.”—This process is mentioned here on account of the
analogy of its object to that of pneumatic caissons—sinking a shaft
through excessively soft wet soil. The process is very recent, it
having been invented by Dr. F. H. Poetsch, of Prussia, in 1883. It has
been used only in a very few cases up to the present time, but where
it has been used it has accomplished results which were practically
unattainable by ordinary methods. A very brief description of one
instance of its use will explain the general idea. For many years
engineers had been baffled in their attempts to sink a shaft through
107 feet of quicksand at the Centrum mine, near Berlin, Germany. Dr.
Poetsch sunk sixteen pipes in a circle around the proposed location
of the shaft, and in thirty-three days had succeeded in producing a
frozen circular wall six feet thick, within which the excavation was
readily made and the shaft suitably lined. The freezing is accomplished
by circulating a freezing liquid (chloride of calcium) through the
tubes. After the shaft is completed the pipes can be thawed loose from
the wall of ice by simply circulating a hot liquid instead of a cold
one. The pipes can then be redrawn uninjured, and used over again—a
consideration of no small advantage. The process is not cheap. It would
seldom, if ever, be used where the more common methods are practicable;
but for passing through very soft and wet soils it is frequently the
only possible method.
[Illustration: MANCHESTER SHIP CANAL.]
IV. CANALS.
History records the construction of a ship canal across the Suez
Isthmus as early as 600 B. C.; that it continued in use for about 1400
years and was then abandoned. It was very small; all traces of it are
now utterly lost. The authentic records of it are very meagre, and
they serve only to show the great antiquity of the canal idea. The
nineteenth-century progress on this line, therefore, consists in the
enormously greater magnitude of the works accomplished in the solution
of the great subsidiary problems involved, and in the improvement in
methods of work which has rendered these great structures possible. The
limitations of this article utterly forbid even a brief description of
all the great canals which have been constructed during this century,
and it must therefore be confined to a few statements regarding the
more important and typical constructions. It might be thought that
no discussion of nineteenth-century canals would be complete without
a mention of the Nicaragua and Panama canal projects. But these
stupendous works, which will eclipse anything of the kind which the
world has ever seen, are not yet accomplished facts. The twentieth
century will be well under way before a trip “around the Horn” will
become unnecessary. The successful completion of one of these canals
will, very probably, so reduce the demand for the other that its
construction will be indefinitely postponed. These canals will not be
further considered.