Page:The American Cyclopædia (1879) Volume III.djvu/561

 CAISSON 555 masonry is built and gunk to its desired position under water. The first caissons of this descrip- tion 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 Komain had in 1685 laid the foundation of the bridge of the Tuile- rios in what has sometimes been called a caisson, but it answered more nearly to what is now termed a crib, the stones being cramped togeth- er 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 mason- ry could be properly laid, and the sinking of it done gradually and under full control, seems to have originated with Labelye, it is mention- ed 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 to- gether, 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 control- led by pumps so as to lower the whole gradually to its proper position. De Cessart had just invented a saw for cutting oft' 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 en- gineers 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 preva- lent 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 lower- ed till a bed has been reached so impervious as to permit the water to be all removed from the inside, workmen have completed the excava- tion and filled the interior with masonry or concrete as desired, the whole forming a por- tion 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. Ilawkshaw at Londonderry and at Charing Cross used cylinders of cast iron, which at the latter place were 14 ft. in diameter at the bot- tom. At Parnitz cylinders 20 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 rectan- gular 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 cyl- inders of brick laid in cement. In India brick cylinders have been very generally used for foundations. In Hungary stone has been sub- stituted, and Mr. Butler proposes Ransome's artificial stone for the same purpose. The in- troduction of compressed air as an agent in con- structing subaqueous foundations has enlarged the use of caissons, which are inverted and sunk to the bed of the stream, with the open space be- neath filled by means of air pumps or compres- sors 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 be opened, and entrance is gained to the main cham- ber, the pressure being transferred to the upper door. In returning, the lower door and the cock are closed, while another cock communi- cating with the outside is opened, and the air soon becomes rarefied so that the upper door can be opened and the exit made. M. Tri- ger ascertained that by making an aperture through a pipe some distance up from the bot- tom 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