PILE. A piece of timber, or of iron (either cast or wrought), driven into the ground for the purpose of acting in the manner of a column, to transmit the weight of a superincumbent structure to an inferior resisting stratum, when the surface stratum is of so com pressible a nature as to be unable to support the weight intended to be put upon it, and is so thick that it cannot be removed. Piles may be used either in water, or on land ; and they may be used either isola tedly, or in close rows, retaining strictly the name of piles, in the first case, or those of gauge piles, whole piling, or sheet piling, in the latter. The gauge piles are those driven to mark out the position the close piling is intended to enclose ; the whole piling is a close enclosure of whole baulks of timber ; and the sheet piling is a similar enclosure formed of half baulks. When cast-iron piling is used, the gauge piles are made with grooves on the side, into which plates with stiffen ing ribs are inserted ; whereas in the case of wood piling, the close piling is made, in the best description of works, with grooves and tongues, and is kept in its position by being driven between whaling' pieces, or horizontal ties.
The descriptions of wood used for piles, in England at least, are beech, elm, larch, Baltic fir, rock elm, live oak, green heart, and the sabacu from Guiana. Of these woods the beech decays rapidly when exposed to alternations of dryness and of humidity, but it resists well when constantly immersed ; elm resists changes of condition more successfully ; and fir, when free from sap, is still more durable ; the American hard woods, such as the rock elm and livo oak, are preferable to the soft woods when tho piles have to be driven into hard gravel, and they appear to be very durable ; and it may be observed that the green heart and the sabacu have been used to a great extent on account of their supposed immunity from the attacks of the teredo. It is, however, now ascertained that those destructive creatures will attack the green heart with nearly as much readiness as they will attack any other kind of wood ; and inasmuch as the lymerylon attacks the green heart on land, there can be little d priori reason for supposing that the limnoria should avoid it in the water. In fact, the result of all the enquiries or experiments which have been made with respect to the value of the processes for the preservation of piles, or to the un assisted resistance of woods in such positions, has hitherto been to show that nothing but the careful injection of creosote is able to prolong, even temporarily, their durability. As the boring worms,
however, do not attack wood which is entirely buried in the ground ; and as the durability of the wood itself is infinitely greater when it is protected from the action of external atmospheric influences by being buried; the common sense of the use of piling appears to be to carry the solid masonry or concrete foundations below the ground line, so as to ensure that the piles shall always be covered.
Wooden piles are generally pointed and shod with iron at the lower end, and they are temporarily hooped at the top, in order to prevent their heads from being split or beaten up during the operation of driving. Occasionally piles are driven with their broad ends down wards, when foundations are required to be executed in soft mud of indefinite thickness, as in the case of the port of L'Orient. It is con sidered that in such cases, the piles only aet to resist compression by the friction developed upon their surfaces, and that their resistance is only equivalent to about 45 lbs. per foot superficial of those surfaces ; but in addition to this small amount of resistance, piles, such as arc thus described, are exposed to the great inconvenience of being likely to be deflected from the perpendicular by any subsequent compression of the subsoil. If, however, such a mode of driving piles be resorted to, and indeed in almost all cases of piled foundations, it is advisable to commence with the outer rows, and to drive inwards, or towards the centre of the mass, in order to compress the enclosed ground as much as possible under the seat of the intended foundations. Of late years, the introduction of the Mitchell's screw piles has greatly facilitated the execution of works of this description, in homogeneous and soft foundations ; but they do not act in any way to compress the ground, and are of more use for the purpose of fixing piles intended to resist lateral strains, than for driving piles intended to resist vertical weights.