CONCRETE WITH STEEL BEMIS.
The foundation is prepared by first laying a bed of concrete to a depth of from 4 to 12 inches and then placing upon it a row of I-beams at right angles to the face of the wall. In the case of heavy piers, the beams may be crossed in two directions. Their distance apart, from center to center, may vary from 9 to 24 inches, according to circumstances, i.e., length of their projection beyond the masonry, thickness of concrete, estimated pressure per square foot, etc. They should be placed far enough apart to permit the introduction of the concrete filling and its proper tamping.
Hollow Cylinders of cast iron or plate steel, commonly called caissons, are frequently used with advantage. The cylinders are made in short lengths with internal flanges and are bolted together as each preceding length is lowered. They are sunk by excavating the natural soil from the interior. When the stratum on which they are to rest has been reached they are filled with concrete.
Cofferdams. There are many circumstances under which it becomes necessary to expose the bottom and have it dry before commencing operations. This is done by enclosing the site of the foundation with a water-tight wall. The great difficulties in the construction of a cofferdam in deep water are, first, to keep it water tight, and, second, to support the sides against the pressure of the water outside. Fig. 12 shows the simplest form; it consists of two rows of piles driven closely and filled with clay puddle. In shallow water and on land sheet piling is sometimes sufficient.
Sheet Piles are flat piles, usually of plank, either tongued and grooved or grooved only, into which a strip of tongue is driven; or they may be of squared timber, in which case they are called "close piles," or of sheet iron. The timber ones are of any breadth that can be procured, and from 2 to 10 inches thick, and are shaped at the lower end to an edge wholly from one side; this point being placed next to the last pile driven tends to crowd them together and make tighter joints (the angle formed at the point should be 30°). In stony ground they are shod with iron.
When a space is to be enclosed with sheet piling two rows of guide piles are first driven at regular intervals of from 6 to 10 feet, and to opposite sides of these near the top are notched or bolted a pair of parallel string pieces or "wales, " from 5 to 10 inches square, so fastened to the guide piles as to leave between the wales equal to the thickness of the sheet piles.
If the sheeting is to stand more than 8 or 10 feet above the ground, a second pair of wales is required near the level of the ground. The sheet piles are driven between the wales, working from each end towards the middle of the space between a pair of guide piles, so that the last or central pile acts as a wedge to tighten the whole.
Sheet piles are driven either by mauls wielded by men or by a pile-driving machine. Ordinary planks are also used for sheet piling, being driven with a lap; such piling is designated as "single lap," "double lap," and "triple lap." The latter is also known as the "Wakefield" triple-lap sheet piling, shown in Fig. 13.
Cribs are boxes constructed of round or square timber, divided by partitions of solid timber into square or rectangular cells. The cells are floored with planks, placed a little above the lower edge so as not to prevent the crib from settling slightly into the soil, and thus coming to a full bearing on the bottom. After it has been sunk the cells are filled with sand 'and stone. On uneven rock bottom it may be necessary to scribe the bottom of the crib to fit the rock. In some cases rip-rap is deposited outside around the crib to prevent under mining by the current. A crib with only an outside row of cells for sinking it is sometimes used, with an interior chamber in which con crete is laid under water and the masonry started thereon. Cribs are sometimes sunk into and then piles are driven in the cells, which are afterward filled with concrete or broken stone. The masonry may then rest on the piles only, which in turn will be protected by the crib. If the bottom is liable to scour, sheet piles or rip-rap may be placed outside around the base of the crib. Cribs with only an outer row of cells for puddling may be used as a coffer dam, the joints between the outer timbers being well calked, and care taken by means of outside pile planks to prevent water from entering beneath it. .