A Treatise On The Principles And Practice Of Dock Engineering

GRAVING DOCK AT BREMERHAVEN. 497 secured in the same way. The bilge-blocks, for supporting the bottom of a ship on both sides of the keel, consist of strong pitchpine timbers, arranged scaffold fashion, resting upon cradles, which are drawn under the ship and adjusted by wire ropes, passing through rollers. The cradles move on special slides bordered with iron, and are spaced 27 feet apart. The graving dock is constructed parallel to and adjoining a large sea-lock, which, generally speaking, was built under the same conditions as the dock. The experience, gained during the former undertaking, proved very useful in carrying out the second without any serious inter- ruption. The work proceeded in the following manner :— Preliminary operations consisted in digging out trenches to a depth of 3 feet 3 inches below zero or about 14 feet 9 inches below ground level, by means of a land dredger, and the excavated material (soft clay) was carried off in tipping waggons and utilised for raising the ground all over the harbour site. In the trenches thus formed, 12-inch sheet piling was driven, averaging 55 feet in depth, to enclose the dock foundation proper. A second row of piles was driven at the same time to serve as anchorage to the sheet piling. When this had been done, water was admitted into the trenches from the harbour, and excavation was continued down to the bottom of the foundation by means of a floating bucket-dredger. But as the latter was incapable of working at a depth of 58| feet, and as grabs did not work satisfactorily, the trenches had to be closed again by dams and pumped out for the concluding portion of this work. After the trenches had been carried down to the required depth, water was admitted to them a second time, and the bottom layer of concrète, composed of 1 part of lime, 1 of trass, and 1 of sand to 4 parts coarse river gravel from the Weser, deposited in skips. The whole of the foundations were completed in 15 weeks at the rate of 800 cubic yards a day, the maximum output being 930 cubic yards in twenty hours. The average thickness of the foundation was 19 feet 6 inches. The layer of concrete was left undisturbed for a period of three months, after which it was pumped dry and levelled to an even surface, being further strengthened with strong iron bands, to prevent its breaking up. The building of the walls was then proceeded with. They were mainly constructed in concrete, with a hard clinker facing and granité copings, quoins, and bedstones. No leakage occurred through the concrete founda- tion, but a strong flow through a gap in the sheet piling was conducted into the pump well without giving further difficulty. The pumping plant consists of two 49-inch centrifugal pumps for emptying the dock, and two 10-inch centrifugal pumps for dealing with the leakage water. The former set are driven by special, direct-coupled, triple-expansion engines. Each pump can lift on an average 150 cubic feet a second, and as the dock holds 2,700,000 cubic feet, it can be emptied in 2^ hours. The drainage pumps are driven by 30 H.P. compound engines. Only one drainage pump is needed, as a rule, and that intermittently. 32