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 well 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 bottom 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 2 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 receptacle 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, Coles, 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. The 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. GRAB, OR GRAPPLE, DREDGERS. ΙΟΙ In excavating, the bucket is closed by the chain, which continues taut during lifting. When the chain is slackened the roller falls and allows the grippers to engage. Then, on hoisting, the grab is pulled open. In the double chain system of the Priestman type (figs. 64 and 65) the outer corners of the bucket are connected, by hinged arms, to a horizontal bar, or cross piece, which is capable of vertical movement in the central groove of the frame. One chain from the jib-head is attached to this bar, and any tension in it causes the bucket to open; the other chain, from the jib-head, is wound round a drum on the pivot, the unwinding of which, with the assistance of two subsidiary chains connected to the horizontal bar previously mentioned, pulls the latter down and causes the bucket to close. Figs. 62 and 63.-Section and Elevation of Peters' Grab. The single chain system has the advantage of being affixable to an ordinary crane, while the double chain system involves the provision of a special crane, but it has the following important points in its favour:— 1. It has fewer working parts, and those of less delicate adjustment. 2. The opening and closing of the bucket can be effected at any point in the lift, whereas, with the one exception of the Peters machine, a single chain grab has always to be lifted to the height of the suspending piece before discharge can be made. If the latter should close upon some immovable object below water, it could not be opened again without lowering the suspending piece, or without the aid of a diver. A false or empty lift has to be completed as well as a full one. 3. The strain upon a single chain from some unseen obstacle might cause a fracture, with the consequent loss of the bucket. With a double chain the risk of fracture is diminished, and loss of the bucket could only occur in the event of both chains giving way. 4. A double chain grab can discharge its load as gradually as may be Figs. 64 and 65.-Priestman Grab. DAMS. 103 considered desirable, whereas the action of a single chain grab is instantaneous. A grab dredger with a bucket capacity of 1 ton may be assumed capable, from actual trial, of raising from 50 to 60 tons of mud per hour, when working in from 15 to 20 feet of water. Of clay, very little more than one-half this amount can be reckoned upon. The grab is an excellent tool and invaluable in confined situations, but it is scarcely suitable for general adoption in works on a large scale. It is not an economical instrument for the removal of stiff clay; its best performances are in regard to mud and soft earth. It cannot be counted upon to work with the same regularity and evenness as the ladder dredger; in fact, its tendency is to pit the surface of the ground with a series of hollows and depressions. But, in spite of these drawbacks, it has demonstrated its utility to such an extent that it is looked upon as an essential accompaniment of most dock and harbour undertakings. Cost of Dredging. The conditions prevailing in regard to dredging are of so variable a nature that no information respecting the cost, at any locality, is of much use elsewhere. Further than this, the available statistics are far from uniform, and there is considerable diversity of extent in the operations included. It can only be said broadly that, within ordinary limits, dredging is effected at some price between a penny and half a crown a cubic yard, distributed, roughly, somewhat as follows:- Suction dredging, 1d. to 6d. These figures do not apply to rock-dredging, the cost of which exceeds the higher limit, often very considerably. AUXILIARY APPLIANCES. Dams.--In dock construction, a dam is a temporary contrivance for the exclusion of water from a site during the progress of the undertaking. It is accordingly composed of material susceptible of easy removal, either in bulk or in parts. Timber and clay form two of the most prominent substances for the purpose. Stone and concrete are occasionally used, under restrictions to be noted later. Iron is rarely employed, and then only with a view to its ultimate incorporation in the permanent work. In spite of its temporary character, a dam should be substantially made. The damage and delay, to say nothing of possible loss of life, resulting from the failure of any part of it, during a critical period, would far more than counterbalance any economy in construction. Too much stress cannot be laid upon this point. It is infinitely better to err on the side of excessive strength than to run the risk of disaster through an insufficient margin of stability. |