Dredging << DREHJ ihng >> is the work of clearing out the bottom of rivers, harbors, and other bodies of water so that ships can use them. The machines that do the work are called dredges. They work somewhat as a power shovel does on land. Dredges usually are run by steam or diesel engines.
The dipper dredge has a large scoop shovel, or dipper, that is shaped like a box, which hangs on a chain from a long steel beam. The steel beam, or derrick, is attached to a strong mast. The mast can swing the beam and the dipper in a wide semicircle. The chain can be wound and unwound to raise and lower the dipper, and the derrick also can be raised and lowered.
When the dredging begins, the dipper is lowered to the bottom of the river or harbor. The derrick arm is swung in a semicircle to drag the dipper across the bottom so that it scoops up dirt and mud. Then the dipper is raised above the water and swung above a barge nearby. The bottom of the dipper has a door that is pulled open by a long cord to dump the dirt into the barge. Then the dipper is again lowered beneath the water to dig more mud.
The first steam dredge was used in England in 1796. It consisted of a 4-horsepower (3-kilowatt) steam engine mounted on a dredger that dug with long-handled ladles. In the early 1800’s, steam-driven bucket and chain dredges came into common use. Diesel engines were first installed on dredges in the United States in the early 1920’s.
The hydraulic dredge is particularly efficient for moving beach or river sand. A suction pipe carries the sand and water to a pump. A discharge pipe leads from the pump to a barge or to a disposal area. Earth deposited by this process for dams, dikes, or building sites is called hydraulic fill.
See also Mining (Surface mining methods).
