In all but a few situations the excavation is undertaken by a specialist floating plant, known as a dredger. Dredging is carried out in many different locations and for many different purposes, but the main objectives are usually to recover material of value or use, or to create a greater depth of water. Dredging can be done to recover materials of commercial value; these may be high value minerals or sediments such as sand and gravel that are used by the construction industry. Dredging is a four-part process: loosening the material, bringing the material to the surface , transportation and disposal. The extract can be disposed of locally or transported by barge or in a liquid suspension in pipelines. Disposal can be to infill sites, or the material can be used constructively to replenish eroded sand that has been lost to coastal erosion, or constructively create sea-walls, building land or whole new landforms such as viable islands in coral atolls. Ancient authors refer to harbour dredging. The seven arms of the Nile were channelled and wharfs built at the time of the pyramids , there was extensive harbour building in the eastern Mediterranean from 1000 BC and the disturbed sediment layers gives evidence of dredging. The remains of three dredging boats have been unearthed; they were abandoned at the bottom of the harbour during the first and second centuries AD. The Banu Musa Brothers during the Muslim Golden Age in while working at the Bayt-Al-Hikmah in Baghdad, designed an original invention in their book named ‘book of ingenious devices’ A grab machine that that does not appear in any earlier Greek works. The grab they described was used to extract objects from underwater, and recover objects from the beds of streams. During the renaissance Leonardo da Vinci drew a design for a drag dredger. Dredging machines have been used during the construction of the Suez Canal from the late 1800s to present day expansions and maintenance. The completion of the Panama Canal in 1914, the most expensive U.S. engineering project at the time, relied extensively on dredging. Operate by sucking through a long tube like some vacuum cleaners but on a larger scale. A plain suction dredger has no tool at the end of the suction pipe to disturb the material. A trailing suction hopper dredger trails its suction pipe when working. The pipe, which is fitted with a dredge drag head, loads the dredge spoil into one or more hoppers in the vessel. When the hoppers are full, the TSHD sails to a disposal area and either dumps the material through doors in the hull or pumps the material out of the hoppers. Main design specs for the Cristobal Colon and the Leiv Eriksson are: 46,000 cubic metre hopper and a design dredging depth of 155 m. Next largest is HAM 318 with its 37,293 cubic metre hopper and a maximum dredging depth of 101 m. A cutter-suction dredger's suction tube has a cutting mechanism at the suction inlet.
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