dredging

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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.

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.

Dredging

 

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

dredging

[′drej·iŋ]
(engineering)
Removing solid matter from the bottom of a water area.
References in periodicals archive ?
He was discovered about four hours later, when another scallop dredger, noticing the Ronan Orla appeared to be in trouble, came alongside and boarded the vessel, finding that Mr Jones had died.
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Vessels in the latest record included the Britoil 64, a tug owned by Britoil Offshore Services, which came for routine drydock hull treatment, crane shaft overhaul and top overhauling of port and starboard engines; the Smit Martinique, a tug owned by Smit Lamnalco having routine drydocking jobs, starboard engine overhaul, and seal renewal of lower gear box; the Al Moayyed 26 and 24, tugs owned by Almoayyed Marine Services in for routine drydocking work; the UDC Blue, a hopper dredger owned by United Dredging Company; and Arabiyah 5 a jack-up rig operated by Saudi Aramco.
They included the Britoil 64, a tug owned by Britoil Offshore Services in Asry for routine drydock hull treatment, crane shaft overhaul and top overhauling of port and starboard engines; the Smit Martinique, a tug owned by Smit Lamnalco having routine drydocking jobs, starboard engine overhaul, and seal renewal of lower gear box; the Al Moayyed 26 and 24, tugs owned by Almoayyed Marine Services in for routine drydocking work; the UDC Blue, a hopper dredger owned by United Dredging Company; and Arabiyah 5 a jack-up rig operated by Saudi Aramco.
A source within the authority stated that dredgers used by the Suez Canal Port Authority were of poor quality and caused the process to take longer than necessary.
The air-dropped "GC" mine, containing 1,500 pounds of high explosives, dated from the Second World War and was brought to the surface by a dredger 6 miles north of Sheerness, England.
The 40 metre Dredger II, which was coming from Ajman, hit the breakwater of Hamriyah port, following which the waves pushed it on top of the breakwater, an official said.
BOAT owners in two marinas got a boost as a new dredger was named and launched yesterday - paving the way for more efficient clearing of silt and safeguarding the region's economy.
Middle East Dredging Company (Medco), a joint venture of United Development Company, Qatar Holding and Belgian DEME, has acquired a heavy duty suction dredger.
Qatar-based Middle East Dredging Company (Medco), a leader in capital dredging, port development and land reclamation, recently acquired a new heavy duty suction dredger in Singapore Shipyard.