A Treatise On The Principles And Practice Of Dock Engineering

CONCRETE MASS WORK. 275 singly or in combination : — SMass work. Bag work. Block work. Stone,. . JDressed masonry. (Kubble mound. due to the inertia of a solid mass and the résistance offered by the inherent strength of a scientifically framed structure. The first case is exemplified by piers constructed in huge blocks of masonry and concrete, and in the second by trussed open work piers of timber, iron, or steel. Nature, it is to be noted, opposes the violent onset of stormy seas with huge boulders and rocky headlands, and accordingly such natural features constitute an obvious type of massive construction. Framed structures, on the other hand, represent the result of human thoughtand adaptation. Theoretically, both principles would seeni to be equally effective, but in practice it will be realised that the joints in framed structures are a source of weakness, owing to their tendency to loosen under vibration; and further, that there is the very important factor of deterioration and decay, which gives a decided advantage to the employaient of a practically indestructible material, such as stone or concrete, over less durable substances, such as timber, iron, and steel. In the latter cases, there must be a constant expenditure on main- tenance and repair. Where there is an important littoral current, which it is undesirable to divert in any way, the use of columnar piers becomes a necessity. The current then passes through the openings without perceptible obstruction. Construction of Jettics.—Jetties, wharfs,'and piers, considered as forming a single class, may be constructed on any of the following systems, either !Fascine work. Open framework. ,Crib work. Iron and Steel, . / Columnar structures. (Close iramework. Composite, , . / Monier system [ Henne bique system. It will be useful to deal with the salient features of each of these various systems seriatim. Concrete Mass Work consists in the deposition of a large bulk of fluid concrete within an enclosure, formed either by a boundary of sheet piling or by temporary retaining moulds, which latter are removed when the concrete is sufficiently set. The method is not, generally speaking, satisfactorily adapted to subaqueous construction, as, apart from the awkwardness of setting wooden moulds under water, it is difficult to prevent excessive dilution and washing away of the cement particles, whereby the strength of the concrete is seriously impaired. Accordingly, the method is mainly restricted to situations in which it can be carried out in the open—that is to say, either above low-water line or, when below that level, by tidework and within the shelter afforded by cofferdams. Notwithstanding this, there are undoubtedly instances in which fluid concrete has been successfully deposited under water, but the local conditions in such cases have been peculiarly favourable. One of the main elements of success is perfectly quiescent nater. Where the water level fluctuâtes rapidly and erratically, as in an