Caisson (Fr. caisse, a case or chest), in architecture, a panel sunk below the surface in soffits or ceilings. In civil engineering, the term is applied, first, to a hollow floating box, usually of iron, which serves to close the entrances of. docks and basins; and second, to a box-like structure used in constructing or sinking the foundation of piers under water. Of the latter there are at least three different varieties: the ordinary, the bottomless or open, and the inverted, which includes the pneumatic. 1. The ordinary caisson is a large box with bottom and sides, made of timbers or planks, in which masonry is built and sunk to its desired position under water. The first caissons of this description of which we have any account were used in laying the foundations of the Westminster bridge, England, in 1738-'40, by Charles La-belye, a Swiss. Frere Romain had in 1685 laid the foundation of the bridge of the Tuile-ries in what has sometimes been called a caisson, but it answered more nearly to what is now termed a crib, the stones being cramped together with timbers and sunk by the aid of guide piles.

Baskets and even barges filled with stones had been sunk at various places; but as the idea of making a tight box in which masonry could be properly laid, and the sinking of it done gradually and under full control, seems to have originated with Labelye, it is mentioned as a new system of laying foundations in deep water. These caissons, of which there were twelve, were oblong and pointed at each end. They were 80 ft. long from point to point, 30 ft. wide, and 18 ft. high. The bottoms were formed of timbers 12 in. square laid lengthwise and close together. Under these were a course of planks 3 in. thick, and over them a course of timber 9 in. square, both laid across the first course and secured to it. The sides were built of fir timbers laid up horizontally on each other and pinned together with oak treenails. All the corners excepting at the ends were framed' together, and further secured by three oak knees each; the two points were secured by irons, which were capable of being unfastened, so that the sides could be removed and used for the other caissons. When the masonry was built in these caissons the water inside was controlled by pumps so as to lower the whole gradually to its proper position.

De Cessart had just invented a saw for cutting off piles under water, and was about to use it at Saumur on the Loire, when the success of Labelye caused him to change his plans, and he used caissons not only at Saumur, but later at Dieppe, Toulon, and Rouen. Bayeux used caissons at Tours on the Loire in 1755, Bellecour at Lyons on the Saone in 1789, Deschamps at Bordeaux, and Beaupre at Sevres, besides many other distinguished engineers down to the present time. 2. Open or bottomless caissons. Curbs or a species of movable coffer dam have been used of a variety of forms and sizes, and as many of these have been called caissons by the best engineers, they are included under this head. The most prevalent form of these curbs or caissons has been cylindrical, and they have usually been made of iron. The usual method of sinking them has been to lower them down so that they stand vertically on their lower edge; then, by weights or building on flanges, to force them as far as possible into the bed of the stream.

When by dredges or pumps the material on the inside has been excavated and the whole gradually lowered till a bed has been reached so impervious as to permit the water to be all removed from the inside, workmen have completed the excavation and filled the interior with masonry or concrete as desired, the whole forming a portion of the pier. In 1842-4 the Royal Terrace pier at Milton below Gravesend, England, was so constructed, iron cylinders being used by Mr. Redman. At Peterborough, in 1851, William Cubit sunk cast-iron caissons 6 ft. square. Hawkshaw at Londonderry and at Charing Cross used cylinders of cast iron, which at the latter place were 14 ft. in diameter at the bottom. At Parnitz cylinders 26 ft. in diameter were used, and at the new Victoria bridge cast-iron cylinders of 21 ft. The new Blackfriars bridge piers were each placed on six caissons, four rectangular and two pointed; the rectangular were 36 by 18 ft. At Point du Jour, Paris, large wooden caissons were used. They were also used by Chanute at Kansas City, as large as 70 by 22 ft., and 67 by 30 ft., besides many others similar, in this and other countries. At the dock in Glasgow Mr. Bateman sunk cylinders of brick laid in cement. In India brick cylinders have been very generally used for foundations.

In Hungary stone has been substituted, and Mr. Butler proposes Ransome's artificial stone for the same purpose. - The introduction of compressed air as an agent in constructing subaqueous foundations has enlarged the use of caissons, which are inverted and sunk to the bed of the stream, with the open space beneath filled by means of air pumps or compressors with air of sufficient density to expel and keep out the water, and admit of workmen being employed in excavating under them. This method dates back to about the year 1841, when M. Triger, a French engineer, sunk a shaft under the bed of the river Loire to a coal stratum, which made more fully known the capabilities of the method; but it had been fully described and patented ten years before by Lord Cochrane in England, including even the principles of the air lock as now used. The air lock is a small anteroom through which men and materials pass to and from the air chamber with only a moderate loss of compressed air. It is usually an upright closed cylinder of iron made air-tight, having a door opening into it from the outside above, and another door opening from it into the chamber of compressed air below.

To obtain access to the air chamber,. it is first necessary to enter the lock, close the outer door, and open a cock which permits the compressed air to come in from the chamber and fill the lock until it becomes of the same density, when the lower door can De opened, and entrance is gained to the main chamber, the pressure being transferred to the upper door. In returning, the lower door and the cock are closed, while another cock communicating with the outside is opened, and the air soon becomes rarefied so that the upper door can be opened and the exit made. M. Triger ascertained that by making an aperture through a pipe some distance up from the bottom of the chamber, the current of air thus escaping would carry out a column of water twice as high as was due to the pressure of air in the chamber. He therefore arranged a cock which served the purpose of ventilation also. In the same year (1841) William Bush patented in England a method of sinking a caisson by excavating within and beneath it in compressed air, the caisson becoming a part of the pier.

A sectional view of his caisson is shown in fig. 1, which represents the air chamber A below a second air chamber B B, in which is the air lock C, leading to the air shaft D. The problem of disposing of the excavated material, which is always in such cases a serious one, was solved by using the second air chamber B B as an anteroom or receptacle, it being of considerable size and provided with a door above and below. The small air lock 0 was for the passage of men without the loss of so much air as the opening of the large lock would occasion, and also to serve as a lock when in process of filling it became desirable to remove the diaphragm or partition between A and B B. Dr. Potts about 1847 patented a process of sinking hollow piles by air pressure by exhausting the air within. This was sold to Messrs. Fox and Henderson, who used it successfully at Anglesea with piles that were 12 in. in diameter; it was also used at Windsor and at Huntingdon; but on attempting the same process at Rochester in 1851, with cylinders 7 ft. in diameter, it proved unsuccessful, and the opposite or plenum process which is above described was adopted, and two air locks designed by Mr. Hughes were used.

These locks, placed at the top of the air shaft, were D-shaped, and extended into the shaft so that the lower door opened on the side; the earth was raised in buckets and swung into the locks. The same plan was pursued at Chepstow on the Wye. Brunei used the same locks in sinking caissons on the Saltash 37 ft. in diameter; he also used pipes and pumps for removing the water, so as to require less pressure of air. In 1854 Pfan-muller presented a design for a caisson at Mentz on the Rhine, which was to be constructed entirely of iron. It had supply shafts represented about 20 in. in diameter, running through the top of the caisson, with a door at each end for the purpose of conveying down the materials necessary for filling in the air chamber; it represented the air lock near the air chamber. In 1855 Mr. L. J. Flemming recommended Potts's process to be used on the Great Pedee river; but encountering a log, he with Major Gwynne used the plenum process. He had two pipes, one for air, the other for removing water. In 1857 the same arrangement was used on the Santee river. Similar arrangements were used by Stephenson about the same time on the Nile, where was also used a caisson 28 by 19 ft., which would hold 40 men.

In 1857 a caisson was sunk at Szegedin, Hungary, on the Theiss, which had a siphon pump for the removal of water, with a lock extending into the air chamber, as at Chepstow. In 1858 this method was also used near St. Germain des Fosses, France, on the Allier. In 1859 the caisson for the Kehl bridge over the Rhine was sunk; it had two shafts with air locks at the top, but provided with doors at the lower end of the shaft, converting the whole into a lock when required. A chain dredge running in a water shaft raised the materials excavated. This work was executed by Castor, who afterward sunk the foundations at Argenteuil. The same year a caisson was sunk at Kovno, Russia, on the Niemen, with two separate air shafts and locks, arranged so that when a bucket passed up one shaft another passed down the other. The same year, also, Gen. William Sooy Smith sunk several cylinders on the Savannah, Ga., in which he made two very important improvements. The first was a spout or trough extending out through the side of the air lock, through which by means of valves and cocks he could send out the material brought up into the lock expeditiously, and with little waste of air. But his most valuable improvement was the method of blowing the sand out through pipes by means of the compressed air.

In 1860 the same process was used at Harlem, New York, by W. J. McAlpine. In 1862, at Argenteuil, France, on the Seine, the double locks at the top of the two air shafts were connected with each other by a pipe so as to allow the air escaping from the one to partially fill the other. The caissons at Konigsberg, Prussia, on the Pregel, and at Lorient, France, on the Scorf, the former with working chambers 50 by 20 ft., and the latter 39 ft. 6 in. by 11 ft. 5 in., were sunk like that at Kehl; the latter were sunk in 1866 by Des-noyes, as were also those on the Loire near Nantes. About the same time a circular caisson 26 ft. in diameter was sunk at Stettin on the Parnitz, in which a siphon pipe 2 1/2 in. in diameter, with a cock 6 ft. above the bottom, was used for ventilation and removal of water. In 1868 Burmeister and Wain used a removable caisson in excavating for and building piers at Copenhagen. At Perpignan, France, on the Tet, an iron cap with an air lock attached was secured to the top of a cylinder of masonry, and so carried down. In 1867 Gen. Sooy Smith planned a caisson of an annular elliptical form, with two air locks by which the foundations of the lighthouse at Waugoshance were sunk. The cylinders of the Omaha bridge were sunk in 1868-9 by Mr. Sickles on Gen. Smith's plan.

In 1869-'70 Oapt. Eads sunk the foundations of the St. Louis bridge, using very large caissons and going to the great depth of 110 ft. below the surface of the water with one of them. The caisson of the east pier served a twofold purpose, a coffer dam being erected on the top of the inverted lower portion, in which the masonry was built. This caisson was made of iron, of a hexagonal form, with the air chamber under its whole area. The air locks were placed partly within the air chamber, to which access was had both by stairs and an elevator running down the air shafts. The excavation was made by a water siphon designed by Oapt. Eads, by which the sand was carried out by the force of water, which passed down one pipe and returned through another, bringing the sand with it. This is probably the most effective method of removing sand or soft material under such circumstances. Capt. Eads also introduced glass globes in which lights were burned under the normal air pressure, and the smoke conveyed out of the caisson.

He practically demonstrated the possibility of carrying down a larger mass of masonry to a greater depth than had ever before been accomplished; and when it is understood that the consequent maximum air pressure was 54 lbs. per square inch within the air chamber, the great hazard of the undertaking may be imagined; but it was entirely successful. Among the many other caissons worthy of note may be mentioned those at Leavenworth by Gen. Smith and Mr. Sickles, at St. Joseph's by Col. E. D. Mason, at St. Charles by C. Shaler Smith, all on the Missouri river; while on the Danube alone may be added those at Steyeregg, Mannshausen, and Nussdorf. - The largest cais-. son that has ever been sunk was for the New York tower of the East river bridge, by Col. W. A. Roebling, in 1872; and as it embraces a variety of features, a view of the longitudinal section is presented in fig. 2. Its base is rectangular, being 172 ft. long and 102 ft. wide, with an air chamber 9 1/2 ft. high, the roof 22 ft. thick, and the sides carried up to a height of 82 ft. from the extreme lower edge. It was used in a double capacity, having a coffer dam above as well as an air chamber below. It was built of timber and lined with thin boiler iron, the whole held together by angle irons and bolts.

It contained 4,200,000 ft. board measure of timber, 235 tons of iron, exclusive of 385 tons of bolts, and weighed when completed 13,271 tons, in which was already laid 30,000 tons of masonry. It had two double air locks extending into the air chamber, similar to those in the Saint Louis caisson, in which coils of steam pipe were introduced for keeping an equable temperature. Two air shafts extended up through well holes in the masonry, with an elevator in one and a double circular staircase in the other. Two water shafts, each 7 ft. 9 in. in diameter, extended below the level of the edge of the caisson, in which powerful dredges grappled the stones and coarser materials that were deposited beneath them, and raised them to cars above, which conveyed them away; while the sand was blown out by the air pressure, on Gen. Smith's plan, through pipes, of which there were more than 40 in various parts of the caisson. Gas was employed to illuminate the interior, which was forced down into tanks and from thence distributed by pipes below. Communication was constantly kept up with the interior by means of a mechanical telegraph.

Four supply shafts about 2 ft. in diameter, each having doors at top and bottom, with equalizing pipes and cocks, served as the avenues for the introduction of materials for the concrete with which the whole interior was finally filled. This caisson was carried to a depth of 78 ft. from mean high tide, requiring a maximum air pressure of about 34 lbs. above the normal pressure. To supply this immense amount of air, 13 large compressors were provided; the air was conveyed by mains and rubber hose to shafts which communicated with the interior. The sinking was successfully accomplished, as had been that of a caisson nearly as large on the Brooklyn side the year before. (See Beidge).

Bush's Caisson.

Fig. 1. - Bush's Caisson.

Caisson of East River Bridge.

Fig. 2. - Caisson of East River Bridge.