A Treatise On The Principles And Practice Of Dock Engineering

CONSTRUCTION WITH MONOLITHIC BLOCKS. 209 and materials. Operations were carried on at a maximum depth of 44 feet. With two gangs of six men, each working alternately in 4-hour shifts, at a cutting 4 feet deep, in stiff clay, the préparation of the foundation for one block occupied about 62 hours. The masonry of which the blocks were composed consisted of a bulk of irregularly bonded rubble, in pieces not exceeding 2 tons weight, set in cement, with a facing of calp limestone, squared and jointed, the mortar being composed of 4 parts sand to 1 of Portland cement. The blocks were built in wooden frames at a wharf some distance away, and, when ready for depositing, were lifted by a pair of floating sheers. Por the purposes of lifting, four wrought-iron suspension bars, 5 inches diameter, having T-shaped extremities, passed through vertical rectangular holes in each block, at the foot of which were circular* cast-iron washers, 2 feet 2 inches diameter (figs. 144 and 145), to distribute the pressure. By turning them through an angle of 90° the bars could be engaged or released. Fig. 144. —Plan of Cast-iron Washer. Fig. 145. Conveyance was usually made with the block submerged to half its height, thus relieving the strain on the lifting tackle by some 80 to 100 tons. Arrived at the site, no difficulty was found in bringing the block rapidly into its assigned position. Ranging was performed while it was about 3 or 4 inches off the ground, by means of short timber uprights wedged into the dowel grooves at each side of the block. These were brought into line against a horizontal balk, extending from and attached to the blocks already set. Close contact of adjoining blocks was achieved by the use of a small tackle, the average joint in a length of 300 feet being only 1 inch. The dowel grooves were then filled with concrete and the operation was coneluded. The upper portion of the wall was built by tide work to a height of 18 feet 10 inches above equinoctial low water, giving a total height to the * This, however, is a later improvement ; the earliest type of washer was girder- shaped. 14