Unfortunately the data are not as full as is necessary for a reliable test of the formula. In most of the cases nothing is said as to the length of time which elapsed between the driving and the load ing of the pile,—a factor that often has a very marked effect upon the bearing power (see the first paragraph of 778). Further, the condition of the head of the pile when the test blow was struck is not stated in any of the cases, which also is an important factor (see Table 64, page 390).
On account of the lack of information on both of the above matters, the comparison in Table 65 is not entirely conclusive, but it is the best that can be done with such experiments as have been made. Careful and comprehensive experiments on the actual sup porting power of piles are very much needed.
However, notice that the computed bearing power is safe, that is, is less than the actual bearing power in all cases except for the second trial for No. 3, and for the first trial for No. 8; and hence the preponderance of the evidence is in favor of the safety of the formula, although in a few cases it gives results extravagantly safe.
Again the blows of the friction-clutch pile-driver are usually more effective than those of the nipper driver, because the former are more rapid; and therefore, if the penetration is taken when driving ceases, piles driven with a series of rapid blows will ultimately have a greater bearing power than those driven with a series of slower blows, other things being the same.
There is still another reason why the penetration should not be taken immediately after driving ceases, for some seeming unimportant condition may affect the ease of driving but not the ultimate bearing power. For example, a stream of water discharged against the pile
at the ground line materially increases the penetration. In driving piles for a foundation in Chicago, the piles were "snaked" out of the river and allowed to lie upon the ground for about half an hour. A 40-foot pine pile driven in the ordinary way required 295 blows of a drop hammer to drive it, and a similar 45-foot pile required only 164 blows, the only difference being that a garden hose discharged a gentle stream of water at the surface of the ground against the latter pile while it was being driven. The soil is a moderately soft blue clay; and this example seems to be fairly representative of experience under similar conditions. Tests with a hammer after 24 hours seemed to show no difference in the supporting power of a pile driven "wet" and of one driven "dry." A pool of tar or of clay puddle around the base of the pile is said to be used in Russia to lubricate the pile while being driven.
In driving piles with a friction-clutch driver care should be taken that the operator does not hold the hammer to reduce the apparent penetration. It needs close observation to detect this trick. In making the test blow with a friction-clutch pile-driver, the hoisting cable should be detached from the hammer to give a free fall.
If the hammer bounces to any considerable extent, the fall is too great, or the pile has struck a solid obstacle, or the hammer is too light. Under such circumstances, careful trials and discriminating judgment are required to determine the cause of the bouncing. Frequently, decreasing the fall will decrease the bouncing and also increase the effectiveness of the blow. If the pile has struck an impenetrable stratum, and the driving is continued, it is probable that there will be a small and continuous apparent penetration due to the mashing of the foot of the pile. Not infrequently when piles are dug out or pulled up, the foot is found badly bruised, and sometimes the body of the pile is crushed. Of course, after the point is bruised or the body crushed, further driving is useless. In hard driving there is likely to be a little rebound of the hammer, owing to the elastic compression of the pile; buy in making the test blow there should be only a very little bouncing.