A Treatise On The Principles And Practice Of Dock Engineering

100 DOCK ENGINEERING. The apparatus, consisting of a single bucket at the end of a long arm, is mounted upon a barge in any suitable position, working, for instance, either through a weil in the centre, or from one end. After being lowered the bucket makes a curved upward cut, the contents being discharged into a hopper through the bottoni of the bucket, which is hinged. The machine is capable of executing cuts at any required level, down to a depth of about 35 feet. Like the ladder dredger, it is not suitable for use in an exposed seaway, but it has done very effective service in sheltered positions, and when operating under favourable conditions, its capabilities may be gauged by the performances of its prototype, the steam navvy. A machine employed in the construction of a canal connecting the rivers St. Lawrence and Grasse, with a bucket capacity of 21, cubic yards and excavating to a depth of 20 feet below the surface of the water, removed 138,000 cubic yards of indurated material in a period of 183 working days of 10 hours each, at an average cost of 4d. per cubic yard, including attendance, upkeep, and renewals, both for itself and the attendant barges and tug.* Grab, or Grapple, Dredgers, known also as Clam-shell dredgers in the United States (the country of their origin), are essentially segmental scoops, generally two quadrants, which rotate about a central pivot, and which, on meeting in the closed position, form a semi-cylindrical réceptacle or bucket. On the same principle, grabs have been constructed with spherical sides in two or three parts. This latter type is principally adapted to excavation for cylinder and circular well foundations. Either apparatus is manipulated in connection with a crane. The grab dredger is based on two distinct systems—the single chain and the double chain. The former system is exemplified in the patents of Wild, Ooles, Peters, Cooper and Holdsworth, and others ; the latter in the Priestman and the Kingston dredgers. The Wild grab has a single chain, leading from the jib-head of the crane, fitted with a catch in the form of a half ball, or hemisphere, with the flat surface uppermost. Such a form permits the downward passage of the catch between two small tumblers, but prevents its rising again, and the grab from closing, until the bottom is reached, when the chain’ becomes slack and the tumblers are opened by the weight of a sliding sleeve. 1 he grab can then be closed and drawn up until it reaches a point where a ring in the lifting gear engages two steel hooks, from which the grab is suspended whilst being discharged. The hooks are withdrawn by a simple contrivance when the grab is slightly lifted. The action of the Peters machine (figs. 62 and 63) depends upon the gripping of the lifting chain, prior to the opening process, by a pair of steel arms, which are actuated and controlled by a roller, bearing against the chain, and a governing rod, attached to the upper edge of the bucket. Bogart on “Dredging Machines in Recent Excavations in Large Magnitude ” Ninth Int. Navigation Cong., Dusseldorf, 1902.