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.
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.
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.
