Puslapio vaizdai

1608, a tunnel which at that period was a very bold piece of engineering. It was designed to drain the Valley of Mexico, which has no natural outlet. This tunnel was more than six miles long and ten feet wide. It was driven through the formation called tepetate, a peculiar earth with strata of sand and marl. It was finished in eleven months. At first excavated without a lining, it was afterward faced with masonry. It was not entirely protected when a great flood came, the dikes above gave way, and the tunnel became obstructed. The City of Mexico was flooded, and it was decided that, instead of repairing the tunnel an open cut should be made. The engineer who had constructed the tunnel, Enrico Martinez, was put in charge of this enormous undertaking, and others took his place after his death. The cut is believed to be the largest ever made in the world. For more than a century the work was continued. Its greatest depth is now 200 feet. It was cut deeper, but has partially filled with the washings from the slopes. The cost was enormous, more than 6,000,000 dollars in silver having been actually disbursed! Wages for workmen were then from 9 to 12 cents a day. All convicts sentenced to hard labor were put at work in the great cut. The loss of life was very great. Writers of the time state that more than 100,000 Indians perished while engaged in the work.

The Nochistongo Cut, Mexican Central Railway.

When a line of railway encountered a grade too steep for ascent by the traction of the locomotive, the earlier engineers adopted the inclined plane. Such planes were in use at important

points during many years. Notable instances were those by which traffic was carried across the Alleghany Mountains, connecting on each side with the Pennsylvania railway lines. These old planes


are still visible from the present Pennsylvania Railroad where it crosses the summit west of Altoona. The planes were operated by stationary engines acting upon cables attached to the cars. These cables passed around drums at the head of the planes, the weight of the cars on one track partially balancing those on the other. Similar planes were in use also at Albany, Schenectady, and other places.

Another effective expedient is the central rack rail. No better or more successful example of this method of construction can be given than the Mount Washington Railway [illustrated p. 12]. The road was completed in 1869. Its length is 3 miles and its total rise 3,625 feet. Its steepest grade is about 1 foot rise in every 3 feet in length; the average grade is 1 in 4. It is built of heavy timber, well bolted to the rock. Low places are spanned by substantial trestle work. The gauge of the road is 4 feet 7 inches, and it is provided with the two ordinary rails and also the central rack rail, which is really like an iron ladder, the sides being of angle iron and the cross-pieces of round iron

14 inch in diameter and 4 inches apart. Into these plays the central cog-wheel on the locomotive, which thus climbs this iron ladder with entire safety. Very complete arrangements are made to prevent the descent of the train in case of

accident to the machinery. The locomotive is always below the train, and pushes it up the mountain. Many thousands of passengers have been transported every year without accident.

The rack railroad ascending the Righi, in Switzerland, was copied after the Mount Washington line. Some improvements in the construction of the rack rail and attachments have been introduced upon mountain roads in Germany, and this system seems very advantageous for use in exceptionally steep locations.


When a line of railway meets in its course a barrier of rock, it is often best to cut directly through. If the grade is not too far below the surface of the rock, the cut is made like a great trench with the sides as steep as the nature of the material will allow. Very deep cuts are, however, not de

The Mount Washington Rack Railroad.

sirable. The rains bring down upon their slopes the softer material from above, and the frost detaches pieces of rock which, falling, may result in serious accidents to trains. Snow lodges in these deep cuts, at times entirely stopping traffic, as in the recent experience near New York. A tunnel, therefore, while perhaps greater in first cost than a moderately deep cut, is really often the more economical expedient.

And here is as good a place, perhaps, as any other in this article, to say that true engineering is the economical adaptation of the means and opportunities existing, to the end desired. Civil engineering was defined. by one of the greatest of England's engineers, as "the art of directing the great sources of power in nature for the use and convenience of man," and that definition was adopted as a fundamental idea in the charter of the English Institution of Civil Engineers. But the development of engineering works in America has been

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Trestle on Portland and Ogdensburg Railway, Crawford Notch, White Mountains.

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tive ability, the late Ashbel Welch, said, in discussing a great undertaking proposed by an eminent Frenchman: "That is the best engineering, not which makes the most splendid, or even the most perfect, work, but that which makes a work that answers the purpose well, at the least cost." And it may be remarked, as to the project which he was then discussing, that after a very large expenditure and an experience of eight years since that discussion, the plans of the work have been modified and the identical suggestions made by Mr. Welch of a radical economical change have been this year adopted.* Another eminent American engineer, whose practical experience has been gained in the construction and engineering supervision of more than five thousand miles of railway, said, in his address as President of the American

Society of Civil Engineers: "The high

Reference is made to the substitution locks in the

Panama Canal for the original project of a canal at the sea-level.

VOL. IV.-2

object of our profession is to consider and determine the most economic use of time, power, and matter."

That true economy, which finally secures in a completed work the best results from the investment of capital, in


first cost and continued maintenance, is an essential element in the consideration of any really great engineering feat.

The difficulties involved in the construction of a tunnel, after the line and dimensions have been determined, depend generally upon the nature of the material found as the work advances. Solid rock presents really the fewest difficulties, but it is seldom that tunnels of considerable length occur without meeting material which requires special provision for successful treatment. In where the roof of the tunnel is to be, some cases great portions of the rock, press downward with enormous weight, being detached from the adjacent mass by the occurrence of natural seams. This was the case at the tunnel excavated for the West Shore Railroad near the bank of the Hudson River under the Military Reservation of West Point. The time occupied and the cost of building this tunnel were greatly increased by this unexpected obstacle.

encountered, and the passage then is atAt other places soft material may be tended with great difficulty. Temporary supports, generally of timber, and of

great strength, have often to be used at every foot of progress to prevent the material from forcing its way into the excavation already made.

In long tunnels the ventilation is a difficult problem, although the use of compressed air drills has aided greatly in its solution.




Plan of St. Gothard Spiral Tunnels.

not prosecuted continuously; it was completed in 1876.
These tunnels are notable chiefly on account of their
great length; there are others of more moderate extent
which have peculiar features;
one, illustrated on the preced-
ing page, is unique. This tun-
nel is a portion of the St. Goth-
ard Railway, and not very far
distant from the great tunnel
referred to above. In the de-
scent of the mountain it was
absolutely necessary to se-
cure a longer distance than a
straight line or an ordinary
curve would give;
the line was
doubly curv-
ed upon it-
self. It enters ---
the mountain

at a high ele-
vation, de-
scribes a cir-
cle through
the rock and,
under itself
at the side;

still descending, it enters the mountain at another
point and continues in another circular tunnel until
it finally emerges again, under itself, but at a com-
paratively short horizontal distance from its first en-
try, having gained the required descent by a con-
tinued grade through the tunnels. The profile above
shows the descent, upon a greatly reduced scale, the
heavy lines marking where the line is in the tunnel.

The remarkable success achieved by engineers in


Among the great tunnels which have been excavated the St. Gothard is the most remarkable. It is 94 miles long, with a section 26 feet wide by 19 feet high. The work on this tunnel was continuous, and it required 9 years for its completion.

The Mont Cenis tunnel, 8 miles in length, was completed in 12 years.

The Hoosac Tunnel, 43 miles in length, 26 feet wide and 21 feet high, was


Profile of the Same.

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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.

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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

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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 their durability. The substructure of foundations generally, 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

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60 Feet.

Transverse Section of the Same.

water. In many special cases hollow iron piles are used successfully.

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

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