dredging

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.

The 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.

The 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 Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

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