PILE-ENGINE. The machinery used for the purpose of driving piles is rather varied ; but it is only within a very few years that it has assailed anything like • scientific, character, or has admitted the application of any mechanical power other than mere brute strength.
The pile-engines which are still used in countries behind the spirit of the age in the development of the mechanical arts, such as Holland, Germany, Spain, or Italy, are the old fashioned ringing engine, in which a monkey, or a large hammer, is raised by means of a pulley and a rope, the latter being connected with a great number of cords, which are pulled iu unison by an equal number of men. The weight of the monkey of these machines is usually about 10 or 12 cwt., and the fall is only from 4 feet to 4 feet 8 incluse ; and in practice it is found that the dynamical force, thus brought to bear upon the piles, is not sufficient to drive them through grounds of any notable degree of resistance ; the ringing engines, moreover, require so =eh space, and so great a number of moon, as to render their use extremely incon venient when numerous piles have to be driven in a small area.
In England and In Franco the pile.engines generally used consist of a vertical frame of about 24 feet, or at times 30 feet. in height, bearing at the top a pulley, over which passes a chain starting from a crab fixed on the ground, and able to reach a monkey resting, when the machinery is in repose, on the pile head. When the pile is being driven the monkey is raised until some fixed pine open the clutch which supports the monkey, and the Latter then falls on the pile head ; the usual weight of the monkey is from 12 to 16 cwt., and the fall varies from 12 to 18 feet. according to the resistance the pile meets with in its descent ; occasionally the, fall may be extended to 26 feet, and the crabs are often worked by steam, when the number of piles to be driven within a given area is considerable. The principal objection to the use of these engines consists in the danger which must exist of their splitting the piles, either by turning the shoe, or by the develop. meut of cross fissures, from the enormous force of the blow produced by a great weight falling from • great height ; but et the same time the rate of advance they produce is very rapid, and they are econo mically worked.
The various systems for sinking plies by pneumatic pressure have not hitherto succeeded well, when applied to piles of the dimensions ordinarily used, although their results have been satisfactory when largo cylinders have been used. At the present day these systems
have been almost entirely abandoned; and when it is necessary to sink even large hollow cylinders, to a very great depth through light soil, the use of cabisous working with compressed air, in the same manner as was done at the Rochester or the Saltash has been snbetituted for the costly and partially inefficient methods of pile driving by pneumatic pressure. In fact, the latter system consists in creating a vacuum beneath a surface exposed to the air, and the weight of the atmosphere in excess of the weight of the enclosed air is supposed to cause the pile to descend; but it frequently happens, that ill proportion as the internal air is withdrawn, so does the moveable ground beneath it follow into the open space; and by thus compressing the air in the interior it annihilates the difference between the two pressures. Whereas, when the compressed air is used, a fresh quantity is pumped into the interior of the eyliudere until it attains a pressure of about 60 lbs. on the square inch, which is sufficient to prevent any running sand, or semifluid mud, from rising under ordinary hydrostatic pressure at leant. But it must be repeated that either of these systems adapted to sinking caissons rather than piles, although they might originally have been applied to the latter.
The Mitchell's screw-piles are so called, because they are made with the worm of a screw at their extremity, and they are driven by turning them in the ground in precisely the same manner as an ordinary screw 'is driven into wood. It must, however, be evident, that piles so driven do not compress the ground in any appreciable manner ; and that they do not furnish any direct evidence of the amount of resistance they offer to vertical compression. The Mitchell's piles are admirably adapted to resist efforts of tension, as in the case of the dead work, of a floating buoy, or of a hauling lump, or mooring block ; because the whole prism of earth around tho screw must be torn out ,before they can move, and the same condition renders them equally !fitted for the piles of piers driven into tolerably firm and even soils, which piers are exposed to cross seas, but are not exposed to heavy vertical weights. The may descent of the Mitchell's piles, and the absence of vibration whilst they are descending, may, in many practical instance., constitute a peat recommendation for them.