EM 1110-2-1100 (Part III)
30 Apr 02
(1) Properties important in dredging.
(a) A hydraulic dredge needs to entrain sediment from the bottom and pump it through a pipe. The
entrainment and the pumping are both affected by the properties of the sediment to be dredged. The subject
is briefly treated in the following paragraphs, but more details on dredging practice can be obtained from
books by Turner (1984) and Huston (1970). Other information is available through the Dredging Research
Program of the U.S. Army Corps of Engineers.
(b) Sediment can be classified for entrainment by a hydraulic dredge as fluid, loose, firm, or hard. Fluid
muds and loose silt or sand can be entrained relatively easily by dragheads. Firm sand, stiff clay, and
organically bound sediment may require a cutterhead dredge to loosen the sediment. Usually, hard material
such as rock or coral is not suitable for hydraulic dredging unless the material has previously been well
broken.
(c) Sediment can be classified for pumping by a hydraulic dredge as cohesive, noncohesive, or mitigated
(Turner 1984). Cohesive sediments get transported through the pipe as lumps and nodules whereas
noncohesive sediments disperse as a slurry, which is more easily pumped through the pipe. Mitigated
sediments consist mainly of noncohesive sediments with a small amount of clay, which increases the transport
efficiency of the pipe.
(d) The diameter of the pipe and the size of the pump limit the size of the material which can be pumped.
Usually, oversize material is prevented from entering the pipeline of a suction dredge by a grid placed across
the draghead, or the cutterhead reduces material entering the pipeline to adequate size.
(e) Another property of the sediment important in dredging is the degree of its cohesiveness that allows
the sediment to stand in near-vertical banks while being dredged. A dredge works more efficiently if the
material will maintain such a steep "face" during the dredging process. Muds and loose sands that flow like
liquids lack this property.
(f) The above statements apply to those dredging systems that remove material from the bottom through
a pipeline by a pump. Such hydraulic dredges are not always the most feasible dredging system to use
because of space constraints, navigation requirements, dredging depth, sediment properties, or disposal
options. Under some conditions, mechanical dredges, which include a grab bucket operated from a derrick,
or a dipper dredge (power shovel) on a barge (Huston 1970), may be more desirable. When mechanical
dredges are used, looser sediments are usually dredged with a bucket and harder sediments with a dipper.
(2) Properties important in environmental questions.
(a) Recently, environmental problems associated with the handling and deposition of sediment have
received increased attention. These concerns most frequently arise from dredging operations, but can occur
anytime sediment is introduced into the marine environment. The usual issues involve the burial of bottom-
dwelling organisms, the blockage of light to bottom-dwelling and water-column organisms, and the toxicity
of the sediments.
(b) The sediment property of most environmental consequence is size. Turbidity in the water column
depends on the fall velocity of the sediment particles, which is a strong function of the grain size. Turbid
waters can be carried by currents away from the immediate project site, blocking the light to organisms over
a wide area and, as the sediments settle out, blanketing the bottom at a rate faster than the organisms can
accommodate. Fine sediments (silts and clays) get greater scrutiny under environmental regulation because
they produce greater and longer-lasting turbidity, which will impact larger areas of the seafloor than will
III-1-2
Coastal Sediment Properties
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