A Treatise On The Principles And Practice Of Harbour Engineering

PILING. 77 vibration is of primary importance. Indeed, such is the care which has to be exercised to prevent rupture, that the pile head is capped in a very elaborate manner. A cast steel helmet completely envelops the head, its interior being filled with sawdust and sacking. Between the helmet and the ram of the pile-driver is also interposed a wooden dolly, so that a very con- sidérable proportion of the momentum of the blow is absorbed before it reaches the pile. A much more efficient implement, where conditions admit of its employ- ment, is the steam hammer. Blows can be delivered with greater rapidity and effect. Timber piles driven by an ordinary weight machine to the utmost capability of the ram have responded readily to the steam hammer and have been forced to a considerably increased depth. Steam hammers are of two types. In the first, the piston is maintained in constant contact with the pile head, while the blow is administered by means of a heavy cast-iron cylinder, moving up and down under steam pressure. An average cylinder will weigh a ton and its stroke will be 3 feet. In the second type the cylinder is affixed to the head of the pile and the hammer is attached to the piston. The disadvantage attaching to machines of the steam hammer type is the leakage of moisture from the cylinder, which softens the head of the pile under impact, and reduces it to a pulpy state. This necessitates cutting and dressing a fresh head, otherwise the power of producing penetration is much impaired. In driving through sand and sandy gravel, very excellent assistance has been derived from the use of the water-jet. A pipe led down the side of the pile to be driven, transmits water under pressure to the ground in advance of the pile, and main tains the former in a state of fluidity until the required depth has been obtained. Immediately after the withdrawal of the pipe, the sand consolidates firmly round the pile and there is no further tendency to sinkage even under load. Piles treated in this manner rarely have pointed ends, as a butt end affords greater bearing area without appreciably increasing the difficulty of driving. Indeed, the perpendicularity of a butt-ended pile is more easily maintained. The limit of driving varies so strikingly according to local requirements that no precise figure can be assigned to it. Obviously, a pile may support a light load with ease where a heavier one would cause sinkage. With a ram of one ton weight falling through 10 feet, the pile may justifiably be considered adequately driven when eight or ten blows fail to produce a depression of more than | of an inch. This will indicate the attainment of thoroughly firm ground, and any further attempts at driving will only tend to shatter the pile. Pile-ends become “broomed” or splintered under an excessive amount of impact. Apparently easy driving, after a check, may be due to this cause, and there is no means of ascertaining the faet except by withdrawing the pile. Sustaining Power.—Piles, if completely embedded and driven to the limits stated above, may be loaded safely to the extent of half a ton per square inch of the area of the pile section. Those in soft, muddy ground,