PROGRESS IN CIVIL ENGINEERING
By WALTER LORING WEBB, C.E.,
Assistant Prof. of Civil Engineering, University of Pennsylvania.
I. AN INTRODUCTORY VIEW.
If we broadly define civil engineering as the art of construction, then 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.
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.”
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.
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.
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.
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.
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.
Suez Canal.—This great work permits a reduction of about 3750 miles in the length of a voyage from Western Europe to India. Compared with some of the other great canals of the world, its construction was easy. The total length between termini is about 101 statute miles, of which about nine miles required no excavation; sixteen miles more required only a slight excavation to make the channel of sufficient depth through existing dry depressions, called “lakes;” and the remaining seventy-six miles of excavation were cut chiefly through a soft alluvial soil. At only one point did the excavation reach fifty or sixty feet in depth, and here also was found the only instance of rock excavation. Even this rock (gypsum) was so soft that part of it was excavated by the steam shovels. About 80,000,000 cubic yards of material were removed. If this material had been loaded on to cars carrying twenty-five cubic yards per car, made up into trains of twenty cars per train, and the trains were strung along at the rate of five per mile, it would have required 32,000 miles of such trains to transport the material that was excavated. Work was actually begun in 1800. The Viceroy of Egypt originally agreed to furnish the laborers required, and at one time about 30,000 laborers were thus employed. On a change of administration in Egypt, the new Viceroy refused to furnish the native labor, and it then became necessary to import labor from Europe, and to supplement this insufficient and high-priced supply of labor by very large dredging machines, or steam shovels, of which about sixty were employed. The task of supplying water for the vast army of workmen was an engineering feat of no mean character and cost, as the entire route lies through an arid desert. A system of waterworks, having its source at Cairo, on the Nile, and distributing the water throughout the length of the canal, was therefore constructed. In the latter part of 1869, the waters of the Red and Mediterranean seas were joined, large arid depressions had been transformed into great lakes, and ocean-going vessels were sailing through what had been a desert. The canal is 26 feet deep, 72 feet wide at the bottom, the sides sloping variably, according to the nature of the material, the resulting width at the top varying from 190 to 328 feet. Although not deep enough for the very largest vessels afloat, it will accommodate the great bulk of ocean travel, including war vessels. The total cost of this work, including the breakwaters, lighthouses, etc., at each terminus, was, approximately, £20,000,000, or $100,000,000.
COMPLETE ROCK CUT. CHICAGO DRAINAGE CANAL.
(Depth 35 feet.)
Unlike most canals, the Suez canal has no locks. The original plan of the Panama canal did not include locks, but the revised plan provided for them, in order to save excessive cutting. The Nicaragua canal scheme necessarily includes locks. The water for the Suez canal comes directly from the seas which are connected. A canal with locks necessarily requires an ample water supply from some river or fresh-water lake. If the Suez canal had been constructed at a higher level than the Mediterranean and Red seas, had been supplied with water from the Nile, and had, therefore, been constructed with suitable locks at each end (as was actually recommended by some engineers), the cost of construction, as well as the perpetual expense of maintenance, would have been greatly in excess of its actual cost. And so the fact that it was possible to construct the canal without locks, and without providing for a supply of water, was a great advantage that facilitated the promotion of the enterprise.
Manchester Canal.—This canal, having a total length of only thirty-five and one half miles, has transformed the city of Manchester, England, from an inland city to a seaport. Actual excavation was begun in November, 1887, and just six years afterwards the whole canal was filled with water. It has a depth of 26 feet, and a width at the bottom of from 120 to 170 feet, thus giving a greater capacity than the Suez canal or the proposed Panama canal. Some of the greatest difficulties involved arose from the necessity of providing for the existing canals and railroads with which that busy portion of England is so crowded. Perhaps the most interesting feat of engineering was the drawbridge carrying the Duke of Bridgewater’s canal at Barton. This small canal, having originally a depth of only four and one half feet, here crosses the River Irwell. It was justly considered a great feat of engineering when James Brindley constructed the canal, during the eighteenth century, so that it crossed the river on a viaduct. A waterway crossing a waterway on a viaduct was then a new idea. But this old canal was constructed considerably above the desired level of the Manchester canal, and yet, of course, not so high that a masted ship might pass under it. Therefore a draw became necessary. To add to the complication, the water supply of the small canal being somewhat limited, it was considered very undesirable to lose a troughful of water (roughly, 200,000 gallons) each time the draw was opened. To allow this water to flow into a tank and then pump it back would consume too much time, to say nothing of the expense. Therefore the bridge must swing with the trough full of water. That required gates at each end of the draw, as well as at the ends of the canal on each abutment. These gates were comparatively simple; but the difficult problem was to ensure a water-tight joint between the ends of the draw trough and the corresponding ends of the canal. Temperature changes, as well as many other considerations, would preclude the possibility of making even a fairly tight joint by swinging the draw to a close fit with the abutments. The desired result was accomplished by placing at each end of the draw a very short U-shaped structure, having the same cross section as the cross section of the trough, and having beveled ends fitting corresponding bevels on the ends of the trough. These beveled ends are faced with rubber. To open the draw the gates are closed, the water between the gates at each end (a comparatively small amount) is drained off and wasted, the U-shaped wedges are raised, and the draw is then free to turn. The wedges are operated by hydraulic rams.
Chicago Drainage Canal.—It will probably be a surprise to many people to learn that this “drainage” canal has a greater cross section throughout the “earth-work” sections than any ship canal in existence, and is only exceeded through the rock sections by the Manchester canal. The city of Chicago obtains its water supply from Lake Michigan. The “intake” pipe was at first located comparatively near the shore. As the population of the city grew and the volume of its sewage increased, it was observed that the water supply was becoming contaminated. The Chicago River, into which the sewage was emptied, became so foul that the odor was intolerable. The very evident fact of this odor probably had more to do with the promotion and accomplishment of the means of relief adopted than the far less evident but very dangerous pollution of the water supply. An extension of the intake pipe to a point several miles from shore by means of a tunnel (which was in itself a notable feat of engineering) only deferred the time when the water supply would again be fatally contaminated if the sewage continued to flow into the lake. It was accordingly determined to dispose of the sewage by discharging it into an artificial channel where it might become diluted with water from Lake Michigan, and thence pass from the watershed of the Great Lakes to the watershed of the Mississippi. The level of Lake Michigan is so high that there was no trouble about obtaining the requisite grade, and the divide between the watersheds is so low that the depth of the required cutting at the summit was not forbidding. But why have such a large canal? It was required that the sewage should be diluted, so as not to become offensive to the inhabitants of the region through which the canal must pass. The law under which the work was authorized required that the flow should be 600,000 cubic feet per minute, and that the minimum width at the bottom of the channel must be 160 feet. According to the well-known laws of hydraulics, it was seen that a deep canal would have a greater capacity per unit of excavation than a very wide shallow canal. This is especially true through the sections of deepest cut, since excavation above the water line adds nothing whatever to the capacity for flow. The sections adopted called for a depth of water of 22 feet. The side walls in rock are practically vertical, the width of channel being 160 feet at the bottom and 162 feet at the top. In earthwork the cross section is larger than in rock, thus reducing the velocity of flow and danger of scouring the banks. The width of channel at the bottom is 202 feet, the width at the water surface being 290 feet, and the side slopes 2 horizontal to 1 vertical.
A very expensive feature of this great work was the necessity for constructing a diversion channel for the Desplaines River throughout that portion of the river valley occupied by the canal. Lack of space forbids a further discussion of this feature. The canal drains into the Desplaines River at a point where the slope of the river is so great that there will never be danger that a strong west wind or an unusual lowering of the level of Lake Michigan can possibly cause the current to flow eastward.
Work on the canal was commenced only after many years of discussion, planning, legislation, litigation, and bitter opposition by the varied interests which considered themselves more or less injured. But the work was actually commenced in July, 1892. The estimated excavation was approximately 40,000,000 cubic yards—about one half that of the Suez canal; but the length is only 29 miles, compared with 101 miles for the Suez canal. The total cost was estimated at something over $27,000,000. On August 22, 1900, the Congressional River and Harbor Committee approved the work as far as completed.
V. GEODESY.
It may be that many, who have read of the incredulity of all Europe when the voyages of navigators during the fifteenth and sixteenth centuries first demonstrated the sphericity of the earth, will be surprised to learn that this knowledge had been acquired almost two thousand years before, and had since then been forgotten. To Eratosthenes, a Grecian, belongs the honor of first making a measurement (about the year 230 B. C.) of the size of the earth, which, while very rude and inaccurate, used the same fundamental principle as is now employed by geodesists. But the appliances of those ancient Grecians and of the Arabians, who later carried on the work, were exceedingly crude. Even during the sixteenth and seventeenth centuries, when the French, English, and Dutch were working very hard on the problem, and were gradually obtaining results which came closer and closer to those now known to be correct, the appliances for measuring angles were so rough and inaccurate that it was only possible to assert that the earth is spherical, with a diameter of about 7900 miles. The seventeenth century was nearly past when Picard first used spider lines to determine the “line of collimation,” or the true line of sight, in a telescope. This marked a new era in methods of work, but the eighteenth century was about half gone when it was first authoritatively proven that the earth is not a sphere, but is more truly an “oblate spheroid,”—such a figure as would be obtained by flattening a sphere at the poles. Some idea of the accuracy of the work done, even at this stage, may be obtained by considering that the computed flattening is so slight that if we had a perfect reproduction of the earth, reduced to a diameter of 12 inches, the flattening would be less than 1/25 of an inch—almost imperceptible even to a trained eye. The very highest mountain would be considerably less than 1/100 of an inch in height on such a sphere.
The present marvelous state of the science is due to the great improvements which have been made in the construction and use of angle-measuring instruments and of “base bars;” also to the development of the mathematical theory and processes involved, notably that of the “method of least squares.” As an illustration of the accuracy attainable in the construction of theodolites, the writer recently made an elaborate test of the error of the centering of one of these angle-measuring instruments. Of course no direct measurement is possible. The result is based on a long series of observations, which, when combined according to certain mathematical principles, will give the desired result. The error was thus computed to be forty-two millionths of an inch. To realize what is meant when an angle is measured with a “probable error” of a few hundredths of a second of arc, it should be remembered that one second of arc on a circle 10 inches in diameter is less than 1/40000 of an inch. The accuracy which has been attained in the measurement of base lines is not easily realized by a layman. An engineer realizes the practical impossibility of measuring a line twice and obtaining precisely the same result to the finest unit of measurement. The initiated are therefore able to appreciate the achievement of measuring a base line having a length of over nine miles, with a “probable error” of less than one five-millionth of its length. The words “probable error,” as used above, have a scientifically exact meaning, but they may be taken by the uninitiated as representing a measure of the precision obtained.
At about the close of the last century the great mathematician, Laplace, had declared that the results of the surveys which had then been made were inconsistent with the theory that the form of the earth is exactly that of an oblate spheroid. That form would require that the equator and all parallels of latitude shall be true circles, and that all meridian sections shall be equal ellipses. Laplace showed that the discrepancies between the actual results obtained and the results which the theory would call for are too great to be considered as mere inaccuracies in the work done. With the extension, during this century, of the great geodetic surveys, carried on by the various governments of the world, more and more evidence has developed that the meridian sections of the earth are not equal, which is equivalent to saying that the equator is not a perfect circle. This has led to the next stage, which has been to prove that the form of the earth may be more closely represented by an “ellipsoid” than by a spheroid, that is, that every section of the earth is an ellipse. Several calculations have been made to determine the length and location of the principal axes of such a figure. But these calculations are considered unsatisfactory, because evidence has developed that the true form of the earth cannot be represented even by an ellipsoid. This figure is symmetrical above and below the equator. There are reasons for believing that the southern hemisphere of the earth is slightly larger than the northern, and that the form of the earth is more nearly that of an “ovaloid,”—a figure of which the ordinary hen’s egg is an exaggerated example.
All the above forms, the sphere, spheroid, ellipsoid, and ovaloid are geometrical forms which represent with more and more exactness the true form of the earth, but even this increasing exactness will not account for the discrepancies and irregularities which have been found at various places, and which cannot be explained on the ground of inaccurate work. Geodesists have been forced to the conclusion that the true form of the earth is not a regular geometrical form, but is a “geoid,” that is, like the earth and like nothing else, unless we admit the exaggerated comparison that it is “like a potato.” It should be understood that the words “form of the earth” do not refer to the actual surface of mountain, valley, or ocean bottom, but to the actual ocean surface, and to the surface which the free ocean would assume if it could penetrate into the heart of the continents. The astounding accuracy of the work done may be appreciated when we consider that the differences between the “geoid” and the more accurate mathematical forms are distances which should be measured in feet rather than in miles. For many purposes, it is sufficiently exact to consider the earth as a sphere. For some very precise work it is necessary to consider it as a spheroid. The more exact forms have little or no utilitarian value, and the vast amount of work that has been spent on these researches has been due to man’s thirst for knowledge as such,—due to the same enthusiasm which advances the sciences in fields which only broaden man’s knowledge of the world in which we live.
VI. RAILROADS.
The achievements of engineering skill on the line of bridges, canals, tunnels, etc., have been great, but their effect is insignificant compared with the social revolution that was created by the invention and development of railroads. The railroads of this country represent a value of about $12,000,000,000—one sixth of the national wealth. Their pay-rolls include about 850,000 employees—1/28 of the working population. They support, directly or indirectly, about 5,000,000 people. They collect an annual revenue of about $1,200,000,000, which is greater than the value of the combined products of gold, silver, iron, coal, and other minerals, wheat, rye, oats, barley, potatoes, and tobacco, produced by the entire nation. Such a stupendous social institution requires special discussion, and it will be found treated separately under the heading of “Evolution of the Railway.”
VII. TUNNELS.
Tunnels are of exceedingly ancient origin, if by tunnels we include all artificial underground excavations. From prehistoric times natural caves have been used as burial places, and, following this practice, tunnels and artificial rock chambers have been cut out by kings and rulers in Thebes, Nubia, and India during periods so ancient that we call the study of their history archæology. Nor were the ancient tunnels confined to tombs. The Babylonians constructed tunnels through material so soft that a lining of brick masonry had to be used to sustain the work. The Romans constructed a tunnel over three and one half miles long to drain the waters of Lake Fucino. About 30,000 laborers were occupied on this work for eleven years. The nineteenth century can hardly boast of works that represent a greater amount of labor (measured in mere days of work) than some of these ancient monuments of constructive skill, but the masterpieces of this century are works which have been greatly aided and even rendered possible by three modern inventions,—compressed-air drilling machines, modern explosives, and the compressed-air process used in subaqueous work. The advance in methods of tunnel surveying is as great and nearly as important. Progress in excavating tunnels is necessarily slow, because the working face is so small that only a few men can work there at a time, and the rate of advance depends upon them. As an illustration: although the Mont Cenis tunnel belongs to the latter half of this century, the first blast being made in 1857, yet for the first four years hand drilling was employed, when the average progress was about nine inches per day. Then machine drilling with compressed air was adopted, when the rate of advance was multiplied five times. The invention of compressed-air drills simultaneously solved two difficulties: (1) The compressed air furnishes an extremely convenient and safe form of power, which enables holes to be drilled much more rapidly than it is possible to drill them by hand. (2) The compressed air, after doing its work, is exhausted into the tunnel, and thus furnishes a continuous supply of fresh air. The necessity for ventilation has often required the construction and operation of expensive ventilating plants. Add to these improvements the lighting of the tunnel, even during construction, by electric lights which consume no oxygen, and the comparison between ancient and modern methods becomes especially marked. Before the invention of explosives, hard rock was sometimes broken by building wood fires next to the rock, and then, when the rock had become very hot, cooling it suddenly with water. The sudden contraction would split the rock. Ventilation was attempted by waving fans at the tunnel entrances. With torches and fires to consume the precious oxygen, and no effective ventilation, it is a wonder how those earlier tunnels were constructed. The compressed air methods for subaqueous work will be referred to under a special case. The essential principles have already been described under caissons.