A Treatise On The Principles And Practice Of Harbour Engineering

70 HARBOUR ENGINEERING. applicable also to piles which are whole and perfect, as a preservative. On the Pacific coast a wrapping of jute burlap, in combination with a préparation of paraffin, powdered limestone, and kaolin, is reported to have achieved successful results. From the foregoing details, it is obvious that the use of timber piles, though oonvenient, is attended by a nuinber of serious disadvantages. There can never be any complète sense of security in reference to the part they play iu permanent structures, and the increasing scarcity of logs of a suitable size, together with the difficulty of obtaiuing them at a moderate cost, has led to the introduction of piles composed of metal entirely or of metal and con- crète combined. Metal piles.—Metal piles are ordinarily either of wrought iron or steel. The pointed or driving end is frequently cast, but, generally speaking, cast iron is of too brittle a nature for use in the shank of a pile, unless special précautions be taken in driving, or the ordinary method of impulsion by a falling weight be replaced by some other system. Thus, with screw ends, cast iron tubes or pipes are often used instead of timber logs (which are equally available), the means of forcing into the ground being rotation round the vertical axis. This constitutes, however, a method of treatment so distinct and exceptional that it may be regarded as not affecting the general question. For the sake of dismissing it from further consideration, it is convenient to introduce here a few explanatory words concerning the system of screw piles. The screw end consists of a broad blade, forming, in most cases, little more than a single turn or a turn and a quarter. It has the property, Fio. 62. —Screw Pile Bases. therefore, of furnishing a base of much greater area than that afforded by the ordinary pile, and on this account is useful for foundation work in compressible strata, where it is desirable to spread the load over as large an area as possible. Moreover, there is an absence of vibration in the process of driving, which is a distinct advantage. The piles are driven by means of a capstan head or a drum of large diameter temporarily bolted on to the shank, and raised from time to time as the rate of driving requires. In the former case, capstan or lever bars are used; in the latter, a winch, to which is led a wire rope wound round the drum, supplies the motive power. In primitive and isolated cases, animal labour has been utilised. Steel or wrought iron piles partake of all the recognised forms emanating from mauufacturers’ rolling-mills. Channel and joist sections are most common. Such piles, though available for solitary positions, are more generally found in close association, as sheet piling. When this is the case, a certain, and by no means negligible, amount of mutual interdependence and support is afforded by binding intimately together the adjacent edges of the