A Treatise On The Principles And Practice Of Harbour Engineering

132 HARBOUR ENGINEERING. displaced in a manner which deserves attention. They were found leaning against adjoining blocks, as if they had been acted upon simultaneously by two forces, one vertical and the other lateral. Throughout the undamaged section of mole, the protection blocks were set back about a yard. One bollard, struck by a portion of falling shelter wall, was sheared flush with the quay level. Several blocks of concrete, each weighing about 40 tons and having a volume of over 600 cubic feet, were driven a distance of 165 feet. This movement, of course, could not have been accomplished by any single stroke, but must have been the cumu- lative effect of repeated blows. Stolen ai Bilbao.—Equally remarkable is the account of the damage wrought by a storm on the last day of the year 1894 at the port of Bilbao. On that evening the action of the waves became so violent that the whole mass of protecting blocks covering the breakwater was completely carried away. These blocks had each a volume of 39| cubic yards and a weight of over 60 tons ; they had been laid with the greatest care in contact with one another, forming an apron to the superstructure 26 feet by 16 feet, and consisting of two rows in width and depth alike. The toe of the super- structure being then unprotected, the latter work was soon undermined and demolished. The most striking feat of the storm, however, was the removal of a large monolithic mass of 1046 cubic yards volume and 1700 tons weight placed at the extremity of the breakwater: it was carried a distance of 105 feet into the interior of the harbour. These instances suffice to exbibit the vagaries which attend a demonstra- tion of wave power by nature in her more violent moods. We pass on now to an application of these facts to breakwater design. Classification of Breakwaters.—Practically all breakwaters fall within the limits of two types, the respective characteristics of which are (1) the heap, or mound, and (2) the wall. The former of these is a heterogeneous assemblage of natural rubble, or undressed stone, in pieces of varying size, supplemented in many cases by artificial blocks of bulk larger than can be conveniently quarried in the natural State, the whole being deposited pell-mell, without any regard to bond or bedding. The latter involves in whole, or mainly, the construction, in a regular and systematic manner, of a masonry or concrete wall, with vertical, or nearly vertical, faces. Subsidiary classes form a series of gradations between these two distinctive types, so that strict lines of demarcation are not always easy to draw. The combination of wall and mound in varying proportions constitutes indeed, by far, the bulk of instances in modern practice. Sometimes the mound pre- dominates and is simply capped by a slight superstructure of regular masonry, as at Algiers and Oran; in other cases, it is reduced to a minimum,