A Treatise On The Principles And Practice Of Dock Engineering

2o8 DOCK ENGINEERING. piles are cut off to the desired height and fitted with iron collars to support the cross beanis, 9 to 12 inches square, which, in turn, carry the flooring, 4 inches thick. Two consecutive lengths of platform are prepared in this way, and then the caisson is berthed over the interval between theni, and the decking made continuons. In one week, 14 men working within the pneuraatic chamber can completely prepare a length of more than 22 yards of platform. Upon the foundation thus constructed, the wall is built to its full height with concrete blocks, 8 feet long, 3 feet high, and as wide as the wall is thick, having a facing of basalt. Brickwork has been tried, with unsatis- factory results. The fascine work is made good between the beams to the underside of the decking, and an additional mattrass is sunk behind the timber work so as to present an upper surface level with the planking. Finally, a mattrass is laid partly upon the platform and partly upon the fascine work behind, and the whole is filled with sand. A lineal yard of wall constructed in this manner costs at the present day 900 florins, of which one-third may be assigned to the fascine mattrass work. Fig. 113 shows a cross section of the completed wall. Construction with Monolithic Blocks.—By this system, which consists in building the submerged portion of a quay wall in a series of massive blocks, the use of cofferdams is avoided, and also that of diving bells, except in so far as the latter are found necessary for providing a suitable and level foundation for the blocks. The blocks themselves may be set by means of a floating crane or sheers, and accurately adjusted with the assistance of a diver, who may also, under favourable circumstances, be able to prepare the site for their reception. Perhaps the most notable instance of the adoption of this method is to be found at Dublin, where the quay walls have a monolithic base course, 27 feet in height, reaching to 3 feet above equinoctial low water. The width of each block at the base is 21 feet 4 inches, forming the entire thickness of the wall ; the face length is 12 feet, and the total contents are nearly 5,000 cubic feet of masonry, weighing 350 tons. Adjacent blocks are connected by means of dowels, formed by filling with concrete long vertical grooves, 3 feet square in plan, one-half of which is arranged in the side of each block. The following particulars relate to the quay wall of a tidal basin built, in 1871,* under the direction of Dr. Stoney, F.R.S., the engineer to the Port Trust. The necessary préparation and levelling of site were effected by the agency of a diving bell, covering an area of 400 square feet, and furnished with a shaft, 3 feet in diameter, rising from its roof above the surface level of the water, where it was connected to an air-lock for the passage of men * Stoney on “ The Construction of Harbour and Marine Works with Artificial Blocks of Large Size,” Min. Proc. Inst. C.E., vol. xxxvii.