dredging-machine

Century Dictionary and Cyclopedia

1. n. An apparatus for lifting mud and silt from the bottoms of rivers, harbors, canals, etc. Some dredging-machines employ a single bivalve or clam-shell scoop; others a series of scoops on an endless chain; others some form of suction apparatus. The earliest form appears to have been a single box-like shovel or spoon, suspended from a crane rigged up on a large flat-boat. It was lowered into the mud, dragged along by means of ropes till filled, and then raised and emptied into the boat. Another early form is the chapelet or chain-pump, which, by means of an endless chain carrying buckets traveling in a trough, lifts mud and water, discharging them at the top into a flat alongside the machine. On this plan are now built some of the largest and most powerful dredging-machines in use. They consist of large, flat-bottomed boats, usually of iron, with a bucket-chain carrying nearly 40 buckets, each with a capacity of about 13 cubic feet. In excavating the Suez canal, the lifting buckets of some of the larger machines had a capacity of 5 cubic feet each, and the delivery was 20 buckets a min ute. For the delivery of the sand or spoil both chutes and traveling buckets were used, the spoil being, in some instances, delivered 230 feet from the dredger. The clam-shell dredger is largely used in the United States, and has the merit of ease of management, the scoop operating in a half-circle about the boat, so that a wide channel can be excavated without moving the boat. The scoop is suspended from a crane at the bow of the boat, and is operated by means of chains controlled by steam-power, two long flexible poles serving as guides for the clam-shell. In the machines employing a suction or exhaust, a tube is lowered into the mud, and the mud and water are raised by means of a revolving disk in the tube, or by the aid of a vacuum or an ejector. A large vessel on the boat, being exhausted of air, is connected with the submerged pipe, when the mud and water readily rise into the receiver. In another form of pneumatic dredger a pipe is lowered into the silt and closed air-tight, and steam is then turned into the upper part of the pipe, driving out the air. Many other forms are used.