Piles Acting as Colum-ns.—When piles are driven through soft soil, offering slight resistance to lateral motion, and rest upon a hard substratum below, they may be considered as columns. They are fixed in position at the bottom with the top free to move laterally but held in vertical position by the caps joining them together. Piles driven in water and not braced depend for lateral stiffness upon being driven into the soil beneath to a sufficient depth to hold them firmly at the bottom. The length of the column in such a pile is to be taken from the cap to a_ point below the surface of the soil, a distance depending upon the firmness of the soil. In stiff soil a depth of 1 or 2 feet may be sufficient to firmly hold the pile. In less resistant soils, one-third to one-half the total penetration may be required.
When piles project into the air, they are braced laterally, so that no bending can take place and the strength of the pile is that of the compressive strength of the wood, or the resistance to penetration of the soil into which it is driven. The compressive resistance of wooden piles depends upon the kind of wood employed, but is taken at a low value, commonly about 600 When the pile acts as a column, this is reduced to 600(1–L/60d), in which L is the length of the column and d is the diameter at its middle point.
Piles Supported by Friction.—\umerous attempts have been made to state in a formula the relation between the penetration of a pile under a hammer blow of given energy and the load the pile may bear without yielding. The effective work clone upon the pile by the hammer in striking the blow should equal the work done by the resistances in stopping penetration. There are, however, so many indeterminate losses of energy in the operation of striking the blow that a rational formula is not feasible—there is loss of energy in the friction of the hammer in the guides; some energy is consumed in brooming the head of the pile; the elastic compression of the pile consumes a part of the energy; the effectiveness of the blow is affected by the height of fall and velocity of the hammer. The impossibility of evaluating these and other data affecting the resulting penetration renders any formula obtained by discussion of the theory of the subject rather useless. Mr. Ernest P. Goodrich has made a very elaborate and interesting study' of the subject in which is produced a formula of very complicated form. This formula is reduced by evaluating experimentally many of the terms, but the result seems to show that a usable rational formula cannot be produced.
Engineering News Formula.—This formula was suggested in 1888 by Mr. A. M. Wellington, the editor of Engineering News. When the drop-hammer is used this formula is P=2117//(s-1), in which the safe load in pounds, II" is the weight of the hammer, h is the height of fall in feet, and s the average penetration under the last blows in inches. When using a steam hammer the formula suggested by
Mr. Wellington is P=2Wh/ (s-0.1).
These formulas are the only ones in common use. They are em pirical formulas obtained by studying all available data derived from tests of bearing power. It is assumed that the blows have been struck upon sound wood and commonly it may be necessary to cut off the head of the pile to remove the wood splintered or broomed by previous driving before making the tests. There must he no visible rebound of the hammer in striking the blows, and if such rebound occurs, it indicates that the fall is too great or the hammer too light, and the full effect of the blow is not communicated to the pile. The hammer must always he heavier than the pile, and should be twice as heavy, in order to strike an effective blow. The formulas are supposed to give a factor of safety of about six.
Egtelwein's formula is frequently used for reinforced concrete piles, on account of the greater weight of such piles. This formula takes into account the relative weights of pile and hammer. With a factor of safety of six the formula is in which WI, is the weight of hammer, IV, the weight of pile, H the height of fall and s the penetration.
It is desirable that the blows used for measuring penetration be struck with a hammer having free fall, as considerable loss of velocity may result from the resistance of a. rope and friction drum. It is also necessary that the penetration under the last few blows be uniform and fairly represent the state of resistance of the pile. The penetra tion should not be less than one-half inch, as less penetration may indicate injury to the pile rather than resistance to penetration.
When piles are driven into soft or plastic materials, the resistance to penetration usually increases with time after the driving ceases. A rest of twenty-four hours may be sufficient to cause the material to settle against the surface of the pile so as to develop a resistance several times that existing when the material was disturbed by the operation of driving. Numerous instances are recorded in which it was found that the penetration under a blow had been decreased by a rest of a few days to from one-third to one-sixth of that at the end of the original driving. In case of driving into material of this kind, it is desirable to examine the effect of rest upon the bearing power and piles upon which tests are to be made should have a period of rest before the final test is made. Piles easily sunk by light blows or even by static pressure frequently carry loads a few days later much greater than those required to sink them. In coarse sand or gravel, the time effect is of Less importance, if it exists at all.