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dredging,process of excavating materials underwater. It is used to deepen waterways, harbors, and docks and for mining alluvial mineral deposits, including tin, gold, and diamonds.
The Dutch at an early period cleared their canals of silt with a pole to which was attached a bag held open by a steel ring. The apparatus, operated from the side of a stationary barge, was dragged along the bottom and then emptied into the barge.
Modern dredging equipment may be divided into four main classes. The
grab dredge is used where the amount of excavation is relatively small. It consists of one or more grab buckets, operated by cranes mounted on a vessel or barge or sometimes on the shore. Each bucket has jaws that are hinged together. The bucket is lowered to the bottom with its jaws open and pointing down. When it sinks into the material to be dredged, its jaws close. The material can then be lifted to the surface and discharged into a hopper for removal to a disposal area. The
dipper dredge, also known as the boom-and-dipper assembly, is similar in appearance to a land power shovel. It is used extensively in canal construction and was employed in the cutting of the Panama Canal.
ladder-bucket dredge, a more elaborate type, is generally mounted on a self-propelling vessel built with a longitudinal well in the center, open to the water beneath for a considerable length. Mounted and hinged over the well is a long steel frame, which may be raised or lowered at will; it is equipped with a long string of buckets passing over sprockets at each end. The buckets, operating through the well, scoop up material from the bottom and discharge it into a chute that projects over the vessel's side to a hopper barge moored alongside or into a receiving hopper in the dredge itself.
suction dredge, or hydraulic dredge, an entirely different type, is used principally where material such as sand or mud is to be removed. It consists of a flexible pipe connected at one end to a powerful centrifugal pump. At the other, open end there is usually a device designed to break up the material to be dredged. The open end of the pipe is lowered to the bottom, where the material to be dredged is mixed with water, pumped up, and then discharged into hopper barges. There the heavy material settles, and the surplus water is allowed to overflow.
Material from these dredges is sometimes pumped through pipes for long distances and used to build up low-lying ground. Hopper barges made to carry away and sink the material brought up by dredges are of a special type. In the space where the material is carried are hinged doors, or flaps, held closed by chains and opening downward. Around the space are watertight compartments to give the barge buoyancy. When the dredge is above the disposal area, the bottom doors are released and the material discharged; the doors are then closed again by winches.
the aggregate of operations performed by a dredge for extraction of submerged mineral deposits. In the dredging of placers, a distinction is made according to the system used: with a single face (single longitudinal and single transverse), with adjoining faces (adjoining longitudinal and adjoining transverse), and with residual pillars (longitudinal and transverse). Systems consisting of various combinations of the main types are widely used. In dredging, the methods of excavation differ according to the sequence and procedure of extraction of rock in the vertical and horizontal planes of the face. The layer method of excavation is the most widespread; it makes possible the control of the filling of scoops depending on the nature of the rock and its content of valuable components and the separate extraction of barren rock and productive sands. The length of the extracting season for dredging depends on the climatic conditions of the deposit area. It generally ranges from 150-160 days in the extreme northeast to 340-350 days in the Urals and Western Siberia.
Multiscoop dredges are used to a limited extent for working placers on the ocean floor. They differ from their continental prototype in terms of the distinctive design features of their maneuvering, transportation, and dumping equipment. During operations in the open sea, the dredge is usually navigated and maneuvered along the face by means of a cable-anchor device. The availability of sufficient dumping capacity (since the surface of the deposit is located significantly below the water level) makes possible the gravity-flow transportation and free disposal of washing wastes in worked-out areas. Cable-bucket dredges equipped with one or more (as many as four to six) grapples or draglines are considered to be promising for dredging placer deposits on the ocean floor. A great deal of attention is being devoted to the possibilities of using pumping and hydraulic and pneumatic suction dredges in working placers. Pumping dredges work placers most successfully at depths of 9-27 m; however, they are also built for operations at depths up to 80 m. As of 1971, work was being conducted on the construction of a suction dredge with deep-well pumps that would make economically feasible the working of ocean-floor deposits lying in deep-water areas (1,200 m and deeper).
Efficient working of heavy gravel in conditions of deep-water excavation is implemented by dredges equipped with airlift working members. The use of an ejector working member, which makes possible a further increase in the efficiency of working rock through the addition of hydraulic attachments for loosening, thus significantly intensifying the process of excavation, is promising under such conditions.
V. G. LESHKOV