A Treatise On The Principles And Practice Of Harbour Engineering

BREAKWATER CONSTRUCTION. 165 all that is strictly needful is to erect the side-moulds within which the concrète is to be deposited. At first sight this would appear to be a simple operation, but the difficulties of setting temporary wooden monids under water are anything but negligible. Where piling is practicable, a series of uprights may be driven at regular intervals, fitted with grooves within which panels of sheeting may be slid down, and raised again as the work proceeds. At the junction of the planking with the ground, a broad strip of canvas can be laid, forming a liniug to the adjacent surfaces of each, for a width of 18 inches or Cross Section. Sectional Elevation. Fig. 134. —Concrète and Stone Foundation Work. 2 feet. The vertical portion will be backed to the planking, and the horizontal portion weighted to the ground with stone. For mass concrete on a rocky bed, where guide piles are impracticable, the plau to adopt would be to lay external facing blocks and to deposit mass concrete in the space inclosed. The deposition of concrete under water is an operation requiring the utmost care for its satisfactory accomplishment, the danger being that the cement may be washed out of the aggregate. It is useless, therefore, to Benchin g Fig. 135. entertain the idea of tipping, as carried out in ordinary work above water. For the special circumstauces of subaqueous foundations, the concrete must be conveyed in a skip with a bottom flap or flaps, or in a bag with a double mouth, that at the lower end being temporarily bound with a looped rope, capable of being released by a tripping rope. The skip, or bag, is lowered right to the bottom, or as near thereto as is consistent with discharge, and the contents are allowed to flow quietly into place, with as little manipulation as possible. It will be evident that concrete for submarine work should be rich in cement, say 4 to 1.