A Treatise On The Principles And Practice Of Dock Engineering

MASS WORK. 277 All three of the foregoing systems may be, and have been, used in combination, such as, for instance, a construction of block work below low-water level, resting upon a bag-work foundation course, and having a superstructure of mass concrete. The south breakwater at Aberdeen was carried out in this manner, and as the statement of expenditure affords a comparison of the cost of the several methods, it is appended here : —* Cubic Yards. Expenditure. Cost per Cubic Yard. Bag work in foundations, .... Block work, including blocks inserted in fluid 3,202 £4,045 25/3 concrete, 22,851 18,175 15/11 Mass concrete in frames, 23,356 18,868 16/2 A better appreciation of the relative cost will be gained by a brief statement of the precise conditions obtaining in each case.f Bag Work.—The bags were deposited by iron skips, the greater part by two skips each holding 5^ tons of concrete, their inside dimensions being 6 feet by 4 feet by 3^ feet deep. In the last year, a skip of 16 tons capacity was used, its dimensions being 9 feet by 6 feet by 6 feet. The bottoms of the skips opened on hinges, the hook which held them being released by a trigger. In the larger skip the closing of the doors, after the bag was deposited, was assisted by counterbalance weights. The bag, of the same shape as the skip but rather larger, was fitted into it and temporarily lashed at the top so as to line the skip. It was then filled with liquid concrete (1 cement, 21 sand, 31 gravel), the temporary lashings removed, and the mouth of the bag sewn up. The skip, with its contents, was lowered by a crane to the divers, and moved about, in obedience to their signals, until close over the required position, when the trigger was pulled by a rope from above, and the bag discharged. Block Work.—The blocks were all 4 feet high and usually 6 feet wide. At first, they were of sizes varying in weight from 7^ to 18 tons; latterly, the small blocks were mostly used for incorporation among the fluid concrete or mass work, and the larger, from 10^ to 24 tons weight, for block building. The blocks were cast in wooden moulds in the usual manner, the proportions of the concrete being 1 cement to 4 of sand and 5 of gravel, with large pieces of broken stone imbedded. They were staked by cranes in the block-yard to harden, and then taken down an incline, on waggons, to the staging cranes, by means of which they were lowered to and set by the divers. Mass Work.—A framework of posts was erected round the site of the building, excepting at the ends of the completed work, which formed one This statement does not inelude items for preparatory works, plant, staging, &c. + Cay on “The South Breakwater, Aberdeen,” Min. Proc. Inst. C.E., vol. xxxix.