dotted lines in the cross-section, in which case the amount of concrete is increased, but the cost of form work is decreased somewhat. Also, in the latter case no additional reinforcement will be necessary for shear, while some, in the form of stirrups may be necessary in the former.

A further development of the combined footing is the raft footing. In this case there are more columns, or a whole building may be supported on one footing. The footing acts as an inverted floor

FIG. 72. Combined Reinforced-Concrete Column Footing.

carrying the upward pressure of the soil to the columns and is designed as such.

Where the foregoing types of foundations fail to develop sufficient resistance to support the loads upon them some other form of foundation must be used, such as piles or caissons.

REINFORCED CONCRETE PILES

Historical. The first concrete piles were made in France by Mr. F. Hennebique, in 1896. These piles were cast in a convenient location and driven with a pile-driver the same as an ordinary wood pile. About the same time, A. Raymond of Chicago, conceived the idea of a pile cast in place. The Simplex pile which was patented in 1903, is also a cast-in-place pile, although the method of casting is different than in the Raymond pile. Since the introduction of the above pile, various other piles have been developed, but are really little more than modifications of the Hennebique, Raymond, and Simplex piles.

Advantages.-An ordinary wood pile will carry a load of about 15 tons; a reinforced-concrete pile may be loaded with thirty to fifty tons, depending on the nature of the ground. Wood piles, unless continually saturated with water are subject to rapid decay,

and even when under water are subject to the attack of wood-borers. Reinforced-concrete piles are not subject to decay or to the attack of any destructive animal life. Reinforced-concrete piles may, therefore, be used where a wood pile would not be advisable. concrete pile may have the lower end continually submerged, its middle portion alternately wet and dry, and the top portion always

A

dry, without being in any way harmful to the pile, or in any way shortening its life. A timber pile under similar conditions would last but a very short time.

Disadvantages. The argument most frequently used against reinforced-concrete piles is their cost. While the cost per linear foot of a reinforced-concrete pile is certainly greater than that of a

wood pile, still when the much greater bearing power of the reinforced-concrete pile is considered the additional cost per linear foot is discounted; in fact, the reinforced-concrete pile is likely to prove the more economical.

Types of Piles.-Reinforced-concrete piles may be divided into two classes. First, the cast-and-driven pile; second, cast-in-place pile.

The Cast-and-Driven Pile.-Piles of this type are first cast in some convenient locality, and when thoroughly seasoned are transported to the desired site and driven. Piles of this type are always reinforced both with longitudinal rods and hooping. Great care is required in handling and driving these piles. A cap, to take up the shock of the hammer and prevent shattering of the head of the pile,

"Bar

"Pipe

FIG. 74.-The Chenoweth Concrete Pile.

is always used, and it is on some detail of this cap that the patent for the pile is usually based. In general, the cap will consist of a cushion of confined sand, rope, or rubber hose, upon which rests a false pile. This false pile receives the shock of the hammer, the shock being distributed, by the sand, uniformly over the head of the pile. Piles of this type are frequently driven by means of a water jet, and this is advisable for all but short piles. It is also advisable to fit the pile with a metal point so as to facilitate penetration.

The principal cast-and-driven piles are the Hennebique, Gilbreth, Chenoweth, and Williams.

The "Hennebique" was the first cast-and-driven pile. It is either rectangular, triangular, or circular in cross-sections, reinforced and driven as above.

In the "Gilbreth" pile the cross-section is an octagon with corrugated side. The object of the corrugation is to allow an outlet for the water from the water jet, and to increase the skin friction.

The ap

The "Chenoweth" pile is round and is made without forms by a special machine. It is reinforced with a coiled sheet of wire netting and longitudinal steel rods placed near the surface, at equal distances apart. paratus for rolling the pile consists of a travelling platform and a roller between which the pile is formed. In operation the steel wire netting, with the longitudinal rods attached, is spread on the platform and covered with a layer of concrete. One edge of the netting is attached to the edge of the platform and the other to the winding pipe or mandrel. The mandrel is then rotated and the netting and its covering of concrete is wound or coiled up. At the same time and as fast as the netting and concrete are coiled, the platform moves under the roll and the roll itself rotates, thus pressing the pile into shape.

FIG. 75.-Raymond Concrete Pile Showing Partly Collapsed Core and Shell and also Finished Pile.

The "Williams" pile is reinforced by

an I-beam. At the point, the web is cut away and the flanges forged to a point. The pile is further reinforced by hoops a short distance apart.

Piles Cast in Place.-Piles of this type may be divided again into two classes of which the Raymond and Simplex piles are typical.

In the "Raymond" pile, a collapsible core, the size and shape of the pile desired is enclosed in a thick, closely fitting, steel shell, and driven with a pile driver in the usual manner. After driving, the core is withdrawn, leaving the shell in the ground. This shell is then filled with concrete and should reinforcement be used, which is seldom with this pile, it is placed before the filling commences. The shell of these piles must be of sufficient strength to hold its shape after the withdrawal of the core.

The "Simplex" system consists in driving an extra heavy iron pipe into the ground with a special point to exclude the dirt; when this pipe is driven to the proper bearing, a drop-bottom bucket, filled with concrete, is lowered to the bottom of the pipe and dumped. The bucket is then removed, a heavy weight lowered into the pipe, and the pipe raised nearly to the top of the concrete, the weight being repeatedly dropped on the concrete, thus forcing it out of the end of the pipe. Another bucket of concrete is then placed in the pipe and the operation repeated until the pile is formed.

Two kinds of points are used, depending upon the character of soil through which the pipe is driven. If in soft soil, a castiron point closes the end of the pipe, while in stiff clay a pair of jaws are used. These jaws are attached by hinges to the bottom of the pipe, and automatically open to permit the concrete to flow through them as the pipe is raised. The pipes used are extra heavy steel, banded where necessary, and are made up in sections of varying lengths to suit the length of pile required.

Relative Advantages and Disadvantages.-The chief objection to the cast-and-driven pile is that it may possibly be injured in driving. Careful driving will, however, prevent this. The advantages of piles of this type are that the piles may be of any length, and can be thoroughly inspected during manufacture.

The chief objections to the cast-in-place pile, where a shell remains, are: First, the danger of the light shell collapsing; second, the dropping of the concrete through such a height may cause separation. The first objection is overcome by using sufficiently heavy shells and inspecting with an electric light before filling begins. As the concrete is usually a very wet mixture and deposited in very small quantities, the second objection is more fancied than real.