Arnold Seibel, age 13, of Wilmington, Del, for his question:
HAVE DREDGES BEEN USED FOR A LONG TIME?
Dredges are used when it is necessary to excavate or dig under water. Dredges have been used to clear waterways for hundreds of years. Long ago the Dutch dredged their canals by dragging a weighted leather bag between two barges. And the Venetians cleared their channels of sand bars hundreds of years ago by stirring up the sand so that the current would carry it away.
It wasn't until 1796 that a dredge driven by steam power was invented. It was some time after that date that large dredges were commonly used.
In 1863, when the Suez Canal was being constructed, the value of using dredges was definitely proved. As long as all the work on the canal was done by hand it went slowly. As soon as dredges were installed, the work speeded up and only three quarters as many men were needed.
Since that time, dredges have been used in the construction of all important canals, such as the Manchester Ship Canal and the Panama Canal. They have also been used in any smaller canal large enough to float a dredge.
Dredges have also been used for the reclamation of land from water, for deepening docks, slips, basins, harbors and rivers. In some types of mining, dredges have proved useful.
The type of dredge most commonly used is the dipper dredge. It differs from a power shovel only in that it is mounted on a barge instead of a wheeled truck.
At the front of the barge is a derrick. The derrick is supported at the top by cables running to a strong mast. At the bottom it is pivoted onto the deck of the barge so that it can be swung around in a semicircle.
Running between the arms of this derrick is a long boom which can be lowered into the water. At the end of this boom is the dipper or scoop. It is a large steel box which is open at the top and has hinged doors on the bottom. The dipper is let down to the bottom and moved so it scoops up a mass of mud and sand.
The dipper dredge then raises the steel box to the surface, or over the spoil bank of the side of a canal. The derrick is swung around until the dipper is directly over one of the waiting barges or over the hopper in the hold of the dredge vessel. The door in the bottom of the dipper is opened by pulling a cable and the load is emptied.
Usually the barge is held steady by wooden spuds, or beams, driven into the muddy bottom.
Dipper barges are very powerful. They can remove tree stumps and can evacuate in soft rock as well as mud or clay. For this reason they are often used in reclaiming swamps.
Grab or grapple dredges may be used in deeper water than dipper dredges. The buckets are attached to a long cable instead of to a stiff boom.
The ladder dredge has a whole series of buckets attached to an endless moving chain.
Suction dredges, also called hydraulic dredges, work on an entirely different principle from the others. In these, the mud and sand are pumped up to the surface instead of being dug up.