Triumphs and Wonders of the 19th Century: The True Mirror of a Phenomenal Era / A volume of original, entertaining and instructive historic and descriptive writings, showing the many and marvellous achievements which distinguish an hundred years of material, intellectual, social and moral progress

by James P. Boyd

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.

MANCHESTER SHIP CANAL.