by use of the water-jet for loosening the material about the foot of the pile is frequently found exceedingly advan tageous in facilitating the sinking not only of wooden piles, but also of screw-piles and of the largest cylinders. In this method, by means of a steam-pump, a jet of water is forcibly impelled through a flexible tube around the bottom of the pile. I3y this simple device piles may be sunk through sand, gravel, and loose soil with astonishing rapidity.
Timber Cribs.—ln cases where the solid stratum cannot be reached by driving- piles the timber " crib " 5o, fig. 3) affords an excellent sub stitute. This is built of heavy squared timbers laid crosswise, notched at their intersections, so as to be bound strongly together, and enclosed at the sides with a solid wooden wall, the bottom being left open. The structure is thus divided into a number of rectangular cells. This timber frame work is put together afloat at some convenient place, and, after being towed to the position destined for it, is sunk by weighting- it with stone thrown into several of the cells, which have been provided with platforms to retain it. When sunk, the interstices of the crib are filled to the top with stone; or the crib may be built with only an outer row of cells for sinking it with stone, and the interior may then be filled with concrete under water. Where the bottom is irregular, a level foundation of broken stone may be prepared for the crib to rest upon, or the bottom edg-e of the crib may have to be shaped to conform to the inequalities, etc., so that the concrete shall not spread beyond the proper limits. When such a concrete foundation has been brought up within a few feet of the surface and properly levelled off, the masonry may be laid upon it directly. Where the structure will be exposed to scouring, the outside should be protected by sheet-piling or by a pile of heavy stones deposited around its base, as shown in the Figure.
Subaqueous Deposition of Conerete.—The usual proportions adopted for a concrete mixture are one part cement, two parts sand, and four parts broken stone. For depositing the concrete upon the river-bottom special arrangements have been contrived. Sometimes a box is employed, which, being filled with the mixture, is brought over the desired spot by means of a swinging crane, and is then lowered to the bottom, when, by pulling a rope reaching to the surface, a bolt is drawn, and, one of the sides of the box being opened, the load is discharged. The empty box is then drawn up, and these operations are repeated. Or the device known as the " tremie " is employed, which consists of a box, round or square, made of wood or of iron, of sufficient length to reach to the bottom and furnished with a hopper at tbe top, which projects above the water. This apparatus is first filled entirely with concrete by using some such device as the box first mentioned, and is kept filled by constantly putting into the hopper additional concrete as fast as the material passes out at the bottom. Some
device of the kind here described is necessary for the purpose of depositing the concrete on the bottom, since otherwise the cement would be washed out of the mortar in falling through the water, and its solidifying power would be destroyed.
The Caisson.—Another method of laying subaqueous foundations is that of the caisson. This device is a watertight timber box whose sides are so arranged that when it has served its purpose they can be detached from the bottom, which constitutes the platform on which the masonry is built, and can be used in the construction of other caissons. The finished caisson is floated over the desired spot, and is then sunk by weighting with stones. The bottom, which forms a substantial platform for the inasonrr, rests either on the natural bottom of the stream (previously levelled), or on a foundation of piling prepared by sawing off the tops to a uniform height and consolidating the same by a stone or cement filling. After the masonry' has been built to the desired height the side walls of the caisson are de tached from the bottom and removed, while the bottom remains as a firm platform under the masonry. Figure ISa, b (151. 41) exhibits a form of floating caisson used in the sinking, of the foundations of the Victoria Bridge, over the St. Lawrence at 'Montreal.
Coffi-r-a'anis.—A modification of the method just described consists in the use of the coffer-clam. This is a strong- timber crib open above and below and covered outside with closely-laid squared planks, the interstices between being well calked. The structure is strengthened by the inser tion of cross-braces, to protect it against crushing by pressure when the water is pumped out of the interior. The coffer-dam is launched, floated to its destined position, and sunk to the bottom of the stream by weight ing it with stone placed on a temporary platform made for the purpose. Proper measures must be taken to prevent leakage under the bottom and through the sides. Leakage under the bottom may be controlled either by a row of sheet-piling around the 1.valls, projecting some distance above the bottom, by the introduction of a laver of concrete, or by other expedients; and leakage through the walls may be controlled by calking defective joints in the framework. The water is then pumped out of the enclosure, and the masonry, which is started on the dry concrete bed at the bottom, is carried up to the required height. Coffer-dams with exterior walls of wrought iron have been used, as at the Marne Bridge at Nogent, the Weser Bridge at Bremen, etc.