Piles are frequently tested by applying static loads until movement occurs. Usually a load is balanced over a single pile, although some times a platform resting upon several piles is loaded. The pile is allowed to stand under the load at least twenty-four hours before being examined for settlement. It is desirable that the load be added in increments, each being allowed to stand for twenty-four hours, until a load is obtained which produces settlement.
In any test of hearing power, it is essential that the pile be tested under the sane conditions that will afterward apply to the foundation. The determination of the requirements in any particular instance is largely a matter of judgment on the part of the engineer, but such judgment should be exercised with knowledge of all conditions that may be evaluated and in accordance with the principles underlying such work.
The spacing of piles in a foundation is a ,matter of importance because of its possible bearing, upon the supporting power of the individual piles. In general, piles should not be closer than 3 feet center to center, although they are sometimes driven 2 feet apart. When piles are closely spaced over the area of a foundation, a consid erable compression of the soil between them must result. The effect of this disturbance of the soil depends upon its character, but too close driving impairs the bearing capacity of all of the piles, and they can not be considered as individually carrying loads up to their normal bearing capacity.
196. Concrete Piles.—Timber piles in structures intended to be permanent must be cut off below the water line, while concrete may be used without reference to moisture conditions. In many instances, therefore, the use of concrete piles is more satisfactory and economical than that of wood, sometimes effecting large savings in excavation. They may be nude in any size considered desirable and are not subject to the limitations of wooden piles in this respect.
Concrete piles may be either molded in place or molded before placing and then driven like wooden piles. Those molded in place aregenerally not reinforced, while those to be driven after molding must be reinforced so as to resist the stresses brought upon them in handling and driving. The methods employed for molding piles in place are patented. and a number of forms of pre-molded piles are also patented.
The Raymond pile is made by driving into the ground a thin shell of sheet steel with a collapsible core which holds the shell to its form while driving. When the shell has been driven to the required pene tration, the core is withdrawn and the shell filled with concrete. It is trade tapering, usually 1S to 20 inches in diameter at the head and 6 to S inches at the foot, with a closed hoot of heavier steel. They are made in sections for convenience in shipping.
The taper adopted for these piles gives high bearing capacity under ordinary conditions of use. The interior of the forte may be in spected before placing the concrete. Difficulty is sometimes met in the collapsing of the thin shell when heavy hydrostatic pressure comes upon it, a fault sometimes corrected by driving a second shell inside the first one.
The Simplex pile is formed by driving into the ground a heavy steel pipe with the bottom closed by a special jaw. The pipe is driven to the depth required, and is then withdrawn as the hole is filled with concrete. The jaw opens as the pipe is raised, permitting the concrete to pass through, and the concrete is rammed into place so as to fill completely the hole below the end of the pipe, and press the concrete against the earth at the sides of the hole. Sometimes a cast-iron shoe is used at the bottom of the pipe and is left in the hole when the pipe is withdrawn.
In driving through soft material which will not retain its form after the pipe is withdrawn, it is sometimes necessary to place a form of thin sheet metal inside the pipe and fill it with concrete before withdrawing the pipe. The soft soil then fills around this form and does not mix with or replace the concrete.
The Pedestal pile is intended to give larger bearing surface at the bottom of the pile. A pipe, or casing, is driven into the ground with a core inside which extends 3 or 4 feet below the bottom of the pipe. The core is then removed and the Bole below the pipe is filled with concrete. The core is then rammed into this concrete, as shown in Fig. 112, so as to force the concrete into the earth at the sides of the hole and form an enlarged base upon which the pile may rest, which procedure is repeated until a sufficient volume of concrete has been forced into the base, the pipe being then with drawn and the hole filled with concrete.