Bridges must have foundations for their piers. Up to the middle of the
nineteenth century engineers knew no better way of making them than
by laying bare the bed of the river by a pumped-out cofferdam, or by
driving piles into the sand, as Julius Cæsar did. About the middle of
the century, M. Triger, a French engineer, conceived the first plan of
a pneumatic foundation, which led to the present system of compressing
air by pumping it into an inverted box, called a caisson, with air
locks on top to enable men and materials to go in and out. After the
soft materials were removed, and the caisson sunk by its own weight to
the proper depth, it was filled with concrete. The limit of depth is
that in which men can work in compressed air without injury, and this
is not much over one hundred feet.
The foundations of the Brooklyn and St. Louis bridges were put down in
this manner.
In the construction of the Poughkeepsie bridge over the Hudson in
1887–88, it became necessary to go down 135 feet below tide-level
before hard bottom was reached. Another process was invented to take
the place of compressed air. Timber caissons were built, having double
sides, and the spaces between them filled with stone to give weight.
Their tops were left open and the American single-bucket dredge was
used. This bucket was lowered and lifted by a very long wire rope
worked by the engine, and with it the soft material was removed. By
moving this bucket to different parts of the caisson its sinking was
perfectly controlled, and the caisson finally placed in its exact
position, and perfectly vertical. The internal space was then filled
with concrete laid under water by the same bucket, and levelled by
divers when necessary.
While this work was going on, the government of New South Wales, in
Australia, called for both designs and tenders for a bridge over an
estuary of the sea called Hawkesbury. The conditions were the same as
at Poughkeepsie, except that the soft mud reached to a depth of 160
feet below tide-level.
The designs of the engineers of the Poughkeepsie bridge were accepted,
and the same method of sinking open caissons (in this case made of
iron) was carried out with perfect success.
The erection of this bridge involved another difficult problem. The mud
was too soft and deep for piles and staging, and the cantilever system
in this site would have increased the cost.
A staging was built on a large pontoon at the shore, and the span
erected upon it. The whole was then towed out to the bridge site at
high tide. As the tide fell, the pontoon was lowered and the steel
girder was placed gently on its piers. The whole operation was
completed within six hours. The other five spans were placed in the
same manner.
The same system was followed afterwards by the engineer of the Canadian
Pacific Railway in placing the spans of a bridge over the St. Lawrence,
in a very rapid current. It is now used in replacing old spans by new
ones, as it interrupts traffic for the least possible time.
The solution of the problems presented at Hawkesbury gave the second
introduction of American engineers to bridge building outside of
America. The first was in 1786, when an American carpenter or
shipwright built a bridge over Charles River at Boston, 1470 feet long
by forty-six feet wide. This bridge was of wood supported on piles. His
work gained for him such renown that he was called to Ireland and built
a similar bridge at Belfast.
Tunnelling by compressed air is a horizontal application of
compressed-air foundations. The earth is supported by an iron tube,
which is added to in rings, which are pushed forward by hydraulic jacks.
A tunnel is now being made under an arm of the sea between Boston and
East Boston, some 1400 feet long and sixty-five feet below tide. The
interior lining of iron tubing is not used. The tunnel is built of
concrete, reinforced by steel rods. This will effect a considerable
economy. Success in modern engineering means doing a thing in the most
economical way consistent with safety.
The Saint Clair tunnel, which carries the Grand Trunk Railway of Canada
under the outlet of Lake Huron, is a successful example of such work.
Had the North River tunnel, at New York, been designed on equally
scientific principles, it would probably have been finished, which now
seems problematical.
The construction of rapid-transit railways in cities is another branch
of engineering, covering structural, mechanical, and electrical
engineering. Some of these railways are elevated, and are merely
railway viaducts, but the favorite type now is that of subways. There
are two kinds, those near the surface, like the District railways of
London, the subways in Paris, Berlin, and Boston, and that now building
in New York. The South London and Central London, and other London
projects, are tubes sunk fifty to eighty feet below the surface and
requiring elevators for access. These are made on a plan devised by
Greathead, and consist of cast-iron tubes pushed forward by hydraulic
rams, and having the space outside of the tube filled with liquid
cement pumped into place.
The construction of the Boston subway was difficult on account of the
small width of the streets, their great traffic, and the necessity of
underpinning the foundations of buildings. All of this was successfully
done without disturbing the traffic for a single day, and reflects
great credit on the engineer. Owing to the great width of New York
streets, the problem is simpler in that respect, but requires skill in
design and organization to complete the work in a short time. Although
many times as long as the Boston subway, it will be built in nearly the
same time. The design, where in earth, may be compared to that of a
steel office building twenty miles long, laid flat on one of its sides.
The reduplication of parts saves time and labor, and is the key to the
anticipated rapid progress. Near the surface this subway is built in
open excavation, and tunnelling is confined to rock.
The construction of power-houses for developing energy from coal and
from falling water requires much structural besides electrical and
mechanical engineering ability. The Niagara power-house is intended to
develop 100,000 horse-power; that at the Sault Ste. Marie as much; that
on the St. Lawrence, at Massena, 70,000 horse-power. These are huge
works, requiring tunnels, rock-cut chambers, and masonry and concrete
in walls and dams. They cover large extents of territory.
The contrast in size of the coal-using power-houses is interesting.
The new power-house now building by the Manhattan Elevated Railway,
in New York, develops in the small space of 200 by 400 feet 100,000
horse-power, or as much power as that utilized at Niagara Falls.
One of the most useful materials which modern engineers now make use
of is concrete, which can be put into confined spaces and laid under
water. It costs less than masonry, while as strong. This is the revival
of the use of a material used by the Romans. The writer was once
allowed to climb a ladder and look at the construction of a dome of the
Pantheon, at Rome. He found it a monolithic mass of concrete, and hence
without thrust. It is a better piece of engineering construction than
the dome of St. Peter’s, built fifteen hundred years later. The dome of
Columbia College Library, in New York, is built of concrete.
Concrete is a mixture of broken stone or gravel, sand, and Portland
cement. Its virtue depends upon the uniform good quality of the cement.
The use of the rotary kiln, which exposes all the contained material to
a uniform and constant intense heat, has revolutionized the manufacture
of Portland cement. The engineer can now depend upon its uniformity of
strength.
Wheels, axles, bridges, and rails have all been strengthened to carry
their increased loads; but, strange to say, the splices which hold in
place the ends of the rails, and which are really short-span bridges,
are now the weakest part of a railway. The angle-bar splice has but
one-third of the strength of the rail, and its strength cannot be
increased, owing to its want of depth. Joints go down under every
passing wheel, and the ends of the rails wear out long before the rest.
This is not an insignificant detail. It has been estimated by the
officers of one of the trunk lines that a splice of proper design
and strength would save yearly enough in track labor (most of which
is expended in tamping up low joints) to buy all the new rails and
fastenings required in some time. It would save much more than that
in the wear of rolling-stock. A perfect joint would be an economic
device next in value to the Bessemer steel rail. Here is a place for
scientific and practical skill.