Bearing Power

BEARING POWER of FRICTION PII.ES. There are two general methods of determining the supporting power of ordinary bearing piles: (1) by considering the relation between the supporting power and the length and the size of the pile, the weight of the hammer, the height of fall, and the distance the pile was moved by the last blow; or (2) by applying a load or direct pressure to each of a number of piles, observing the amount each will support, and ex pressing the result in terms of the depth driven, size of pile, and kind of soil. The first method is applicable only to piles driven by the impact of a hammer; the second is applicable to any pile, no matter how driven.

1. If the relation between the supporting power and the length and size of pile, the weight of the hammer, the height of fall, and the distance the pile was moved by the last blow can be stated in a formula, the supporting power of a pile can be found by inserting these quantities in the formula and solving it. The relation between these quantities must be determined from a consideration of the theoretical conditions involved, and hence such a formula is a rational formula.

2.

By applying the second method to piles under all the con ditions likely to occur in practice, and noting the load supported, the kind of soil, the amount of surface of pile in contact with the soil, etc., data could be collected by which to determine the sup porting power of any pile. A formula expressing the supporting power in terms of these quantities is an empirical formula.

Rational Formulae.

Many attempts have been made to express the relation between the supporting power of a pile and the weight of the hammer, the height of the fall, and the penetration; but, owing to the uncertainty of the data involved, it is doubtful whether any theoretical formula can be deduced which will be of any practical value. Among the uncertain elements in the problem of deducing a rational formula are the following: 1. There is no way of determining the amount of energy lost by the friction of the hammer against the air and the guides. The per cent of loss will vary with the weight of the hammer, the height of fall, and the fit and the lubrication of the guides. It has been proved practically that there is no gain in effectiveness by making the height of fall more than 40 or 45 feet; and part of the greater loss with the high fall of the hammer is doubtless due to friction against the air and the guides.

2. With a friction-clutch pile driver, the drag of the rope materi ally retards the fall of the hammer, and under ordinary conditions decreases the effectiveness of the blow from I to 7i; but there is no way of determining this effect except by trial for each particular case, and hence this factor can not be included in a general formula.

3. Only a portion of the energy in the descending hammer when it strikes the head of the pile, is used in actually driving the pile into the ground; and there is no way of determining how much of this is lost in the elastic compression of the hammer and of the pile. In

hard driving considerable energy is thus consumed, as is shown by the rebound of the hammer from the head of the pile. Owing to this bouncing, the energy in the hammer is employed in striking several light blows instead of a single heavy one. There is doubtless some loss from this cause even though there is no visible bouncing; and in any case, the loss will vary with the weight and form of the hammer, the height of the fall, the length, diameter, and material of the pile, and the hardness of the driving. The loss due to the elastic com pression of the pile also depends upon whether the resistance to driving is chiefly friction on the sides of the pile or resistance to penetration at the foot of the pile; for if it is the former, the com pressing force upon the pile is a maximum at the head and zero at the foot of the pile, while if it is the latter the compressive force is uniform over the entire length of the pile. As illustrating the possible value of this element, mention may be made that in driving the piles for the public library in Chicago it was claimed to have been proved that a blow directly upon a pile 54 feet long having an average diameter of 13 inches driven 52 feet into a uniform bed of soft clay was twice as effective as when an oak follower (a stick probably 8 to 10 inches in diameter and 6 feet long) was used, whether the hammer was a steam one or a drop hammer.* 4. Some energy is usually consumed in crushing or brooming the head of the pile; and in hard driving this loss is very great. It is impossible to compute the amount of this loss; but Table 64 shows in a striking way the difference in effectiveness of a blow upon comparatively sound wood and upon wood that is badly bruised. The pile was green Norway pine, and it was driven with a steam hammer, the ram of which weighed 2,800 pounds. Notice that the average penetration per blow was 2i times greater during the 15th foot than during the 14th; and nearly 4 times greater in the 19th than in the 18th. It does not seem unreasonable to believe that the first blows after adzing off the head were correspondingly more effective than the later ones; consequently, it is probable that the first blows for the 15th foot of penetration were more than 5 times as efficient as the last ones for the 14th foot, and also that the first blows for the 19th foot were 8 or 10 times more efficient than the last ones for the 18th foot. Notice also that since the head was only "adzed off," it is highly probable that the spongy wood was not entirely removed; and therefore if the blow had been struck upon really sound wood, the useful effect would have been very much greater. The amount of brooming is greater with a drop hammer than with the steam hammer, and will vary with the weight of the hammer, the height of the fall, the diameter of the pile, and the hardness of the driving.