A Treatise On The Principles And Practice Of Harbour Engineering

PILING. 61 Thus a solid pier inevitably deflects the course of a littoral current, thereby diverting navigable channels into unknown directions, and bringing about physical results which it is not possible to forecast with any certainty. A columnar pier, on the other hand, offers very trifling obstruction to current flow, and practically leaves the coastal régime unaltered. On these grounds, it has been deemed politic, at Zeebrugge for instance, to construct in open- work the portion of a projecting mole which immediately adjoins the shore, while that portion which lies beyond the range of the littoral current, or which is not likely to offer any injurious opposition to the motion of the sea, is built in the solid. Framework, as adapted to maritime situations, consists of two distinct parts: the supporting columns or piles and the superstructural trussing. With the first of these we propose to deal in this chapter. On the second it will be necessary to touch but lightly, as the principles upon which it is based are common to all branches of engineering work, and in no sense can it be con- sidered as a special feature of harbour engineering operations. Piling ’is the term applied to all columnar members driven vertically, or nearly so, into the ground to form a foundation for constructional purposes. It includes two varieties : first, sheeting* piles, which are employed to inclose or confine an area, and secondly, bearing1 piles, which act as isolated supports. Sheeting piles are often much wider than they are thick, and are set with their edges in close contact, so as to form a continuous wall or partition. In order to achieve this result, they are driven in bays of moderate length, between leading or guide piles, to which horizontal walings are affixed. Bearing piles are more equilateral in cross section, and are driven quite separately, or in clusters. Sheeting piles are provided with a knife edge at their lower extremities ; bearing piles have either pointed or butt ends. The materials from which piles are made are extremely varied, and include timber, iron and steel, concrete and ferro-concrete. Timber piles are, perhaps, those which have been most extensively used up to the present time. They have been adapted to purposes both of a temporary and of a permanent nature. For the former class of work, they are still in universal demand, but for the latter class they are now only utilised when considérations of economy outweigh all others. For jetties and piers destined to wear and rough usage, the durability of material, composed of metal and mineral which is practically indestructible, gives it an enormous advantage over perishable fibre; but for temporary work, such as gantry staging, cofferdams and the like, the cheapness and adaptability of timber confer upon it qualities relatively superior. The character of the timber employed in harbour work depends upon the probable or estimated duration of its services. When utilised for permanent structures, only the best, hardest, and soundest timbers are admissible. In other cases, softer and less durable wood will suffice, provided it be kept under constant supervision and renewed whenever necessary.