DREDGE

large floating device for underwater excavation. Dredging has four principal objectives: (1) to develop and maintain greater depths than naturally exist for canals, rivers, and harbours; (2) to obtain fill to raise the level of lowlands and thus create new land areas and improve drainage and sanitation; (3) to construct dams, dikes, and other control works for streams and seashore; and (4) to recover subaqueous deposits or marine life having commercial value. Dredges are classed as mechanical and hydraulic. Many special types in both classes, and combinations of the two, have been devised. All types of dredges may have living quarters on board. Though dredges have been constructed to remove many kinds of deposits, the bulk of material removed has consisted of sand and mud. A dipper dredge is essentially a power shovel mounted on a barge for marine use. Distinctive features are the bucket and its arm, the boom that supports and guides the arm and is mounted to work around a wide arc, and the mechanism that gives excavating movement to the bucket. A grab, or clamshell, dredge lowers, closes, and raises a single bucket by means of flexible cables. In operation the bucket is dropped to the bottom, where it bites because of its weight and the action of the bucket-closing mechanism. A grab dredge can work at virtually unlimited depths. A ladder dredge employs a continuous chain of buckets rotating around a rigid adjustable frame called a ladder. When the ladder is lowered to the bottom at a slant, the empty buckets descend along the underside to the bottom, where they dig into the mud; the loaded buckets return along the ladder's upper side and dump at the top. The scraper dredge, also called a dragline, handles material with a scoop suspended from a swinging boom. The scoop is drawn forward by a line attached to the front, while a second line attached to the rear holds the scoop at the proper angle to slice the earth away as the device is pulled along. A hydraulic dredge makes use of a centrifugal pump. In the pump casing, an impeller expels by centrifugal action a mixture of solids, water, and gases. As a partial vacuum is created within the pump, atmospheric pressure on the outside water surface and the weight of the water itself (hydrostatic pressure) both act to force water and suspended solids from the bottom through the suction pipe into the pump. The materials emerging from the pump are conveyed into barges or through another pipe to the shore. Long stakes, called spuds, are frequently used to pinion a dredge to the bottom.