A Treatise On The Principles And Practice Of Harbour Engineering

CHAPTER VII. BREAKWATER CONSTRUCTION. Mound Construction—Barge System: Loading, Aligniug, Discharging—Staging System : Erection and Maintenance, Trackways—Low Level System—Wall Construction: Staging and End-on Systems —Functions of Titans, Mammoths, and Goliaths— -Caisson System—Foundations: Nature and Characteristics—Settlement—Wall Foundations— Piers—Piling—Limiting Loads—Surface Treatment—Levelling—Benching—Deposition of Concrète under Water—Bagwork—Block Making—Bond—Sloping Bond—Grouting —Minor Brealrwaters—Crib and Box Work—Fascine Work— Examples of Breakwater Construction from Tynemouth, Alderney, Zeebrugge, Cette, Bilbao, Bizerta, and Dover. Methods of breakwater construction are naturally as diverse as the local conditions which govern them, yet they fall, without undue constraint, under the same heads as those enumerated in our classification of the principles of breakwater design. Thus, we have the spécial methods appertaining to the formation of the mound and to the building of the wall. We will subdivide our observations accordingly. Mound Construction.—For the purposes of a mound, no preliminary dredging operations are necessary. The material for the mound may be deposited upon the sea-bottom direct, for, from the very nature of things, it will spread itself sufficiently to distribute its weight within the limits of support, or it will sink until it reaches some firmer substratum by which the settlement becomes arrested. Nevertheless, it should be pointed out that dredging has not infrequently been resorted to when the surface of the sea floor is mud of a particularly impalpable character, and likely to prove treacherous. At Trieste, for instance, in consequence of certain mishaps, it was found necessary to remove a proportion of the softer mud. The rubble work did not subsequently sink so deeply as before, yet the settlement con- tinued still to be considerable, amounting to 9 or 10 feet in depth. We shall have occasion later on to discuss more fully the question of settle- ment in foundations. Meanwhile, we are concerned solely with methods of construction. Rubble may be deposited in one or other of three ways. These are: — (1) By tipping or discharging from barges, scows, or other vessels afloat. (2) By discharging from travelling gantries or from cranes, running on temporary overhead staging. (3) By discharging from wagons passing over roads laid at or about the level of the top of the mound. The wagons are tipped in advance of the 149