732. Piles may be made of concrete either with or without steel reinforcement. In the former case they are built in place, but where steel is used the piles are usually driven after they have been prepared in suitable molds. Concrete is also employed to protect from decay, or from the ravages of the teredo, wooden piles already in service.
Concrete piles are much more durable than wooden piles, and may be used without reference to the water line. A saving may thus be made under certain conditions, as the use of concrete piles may obviate the necessity of excavating to the water line and building up with masonry resting on a wooden pile foundation. As the diameter of the pile is not limited, a much greater load per pile may be provided for. There are of course many places where piles of concrete are not as suitable as wooden piles; they are not as well adapted to withstand certain kinds of hard usage, such as violent shocks, and they are much less flexible.
In certain kinds of soil, such as stiff clay, a wooden pile, or dummy, of the proper length may be driven and withdrawn, the hole left being at once filled with concrete. The application of this crude method is very limited, as it is seldom that the soil will stand until the hole is filled with concrete.
For the building of piles without reinforcement, Mr. A. A. Raymond 1 has patented a system by which a thin steel shell or casing is driven to the desired depth and then filled with concrete in place. A shell is first slipped over a steel pile core made to fit it, and the shell and core are driven by a pile driver in the ordinary manner. The core is then slightly shrunken in diameter, by a simple device, and withdrawn, leaving the shell in the ground. The core is hoisted in the pile driver leaders, another shell is lowered into the one just driven and then slipped up on the core, after which the driver is shifted to the next location, and this shell is driven in the same manner as the first. The filling of the shells with concrete is done as soon as convenient. While the shape of the shells may be varied to suit conditions, the ordinary size is about twenty inches diameter at the top and six inches at the bottom, and such a shell twenty feet long weighs about seventy pounds.
734. The same company has a system of sinking shells in sand by the water jet. For this purpose the shells are in conical telescopic sections about eight feet in length. A two and one-half inch pipe with three-quarter inch nozzle is attached to the center of a cast iron point fixed to the inner section. Water forced through the pipe causes the shell to settle, and as the inner shell descends, its upper end engages with the lower end of the second section, so that when fully lowered the sections form a continuous cone. The concrete is filled in simultaneously with the sinking, imbedding the two and one-half inch pipe which remains permanently in the center of the concrete pile.
1 Raymond Concrete Pile Co., Chicago, 111.
Piles of concrete-steel usually have three or more steel rods of about one square inch cross-section imbedded longitudinally in the pile, and connected by smaller rods or wires at intervals of six to ten inches. Molds are so made that they may readily be detached and used again. At least one side of the mold should also be in short sections that may be put in place as needed, in order to facilitate placing the concrete. The molds should be set up vertically with the longitudinal steel rods in position. Enough concrete is put in the molds to fill six to ten inches in length, when a set of transverse tie rods or wires is placed, then another layer of concrete, etc. The concrete, which is of Portland cement, should be mixed rather wet, as thorough tamping is difficult in the confined space. The piles should be provided with a cast iron shoe at the bottom, or a steel plate covering to protect the point. At the top, one of the main rods is bent over to form a ring to facilitate handling the piles.
When the concrete has hardened sufficiently, say at the end of four to eight days, the mold should be removed, and the pile allowed to remain in its original position twenty to twenty-five days longer, sprinkling it occasionally. When thoroughly set, they may be driven with an ordinary pile driver, using a heavy hammer and short drop. A steam hammer is preferred, however, and a special cap must be used to prevent injury to the pile head. Such a special cap may well be made of cast steel, fitting over the head of the pile like a helmet. The space between the lower end of the cap and the side of the pile is calked with clay and rope yarn or other suitable material. Through a hole provided in the top of the helmet, the space between the pile and cap is then completely filled with dry sand. Such a cushion cap effectually protects the pile head, distributing the pressure to the entire head. Caps in the form of a steel ring filled with sawdust surmounted by a wooden block, and also caps made of alternate layers of lead, wood and iron plates have been successfully used.