12 December 2001
Types of Mechanical Dredges.
Clamshell, grab, or bucket dredge. This system consists of a crane, or
derrick, mounted on a floating barge, with a clamshell, orange peel, or
dragline bucket used to pick up sediment and transfer it to an adjacent
scow or barge. This dredge may be a specially built machine or may
consist of land equipment on a suitable floating platform. This form of
dredging can remove loose, unconsolidated sediments ranging in size
from silts and clays to blasted rock. The dredge can be used in moderate-
swell conditions. The system is not exceedingly efficient but has the
advantage of high mobility. This mobility enables dredging at the base of
bulkheads, piers, and fender piles without damaging these structures or
the dredge equipment.
Ladder, or bucket-ladder, dredge. This dredge consists of a floating
dredge that has a continuous chain of buckets on a frame which is called
a ladder. Each of the buckets possesses a cutting edge for digging into
the sediment. The ladder is lowered to the bed so that the buckets can
reach and cut sediments to be dredged. The buckets dump the dredged
sediment by gravity at the opposite end of the ladder onto a conveyor
system or an adjacent open barge. The barge may then transport the
material to the disposal site. This dredging system is effective in hardpan
and cemented sediments, but is ineffective in firm rock. The system
cannot be used in swell conditions. This system is not often used in the
Dipper-barge dredge. This dredge consists of a backhoe mounted on a
barge equipped with a trapdoor shovel. Sediment is removed from the
bed and deposited alongside the dredge, in another barge, in the water, or
onshore. Where the sediment is deposited depends on the length of
backhoe reach. Spuds, which penetrate the bottom, are usually used to
keep the barge from moving during a dredging activity. This dredging
method is effective for hardpan and cemented sediments, as well as for
firm rock that has been blasted. The effectiveness of this type of dredging
system is limited in moderate-swell conditions.
Hydraulic Dredges. Hydraulic dredges lift sediment from the bottom and
transport it by means of a centrifugal pump. Hydraulic dredges can be used in either
open or protected waters, depending on the type of dredge. The dredged material is
transported in a slurry and is generally discharged by a pipeline in the hull of the
dredge; the slurry is discharged alongside the dredge, or it may be pumped ashore.
The rate of production depends on sediment type, depth of cut, and dredge size and
power; it generally exceeds that of mechanical dredges.
Types of Hydraulic Dredges.