12 December 2001
Grab samples. Samples for maintenance dredging are often not
necessary as review of historical records reveals sediment characteristics.
If samples are necessary for maintenance projects they are usually grab
sampled taken from the bed surface.
Subsurface investigation. New-work dredging requires subsurface
investigation. Recoverable cores are recommended where consolidated
sediments may be encountered.
Probing or sonar profiling. If rock pinnacles or debris are detected by
grab or core samples, extensive probing or sonar profiling of the dredging
area should be accomplished to locate and quantify rock and debris.
Sediment testing. To evaluate dredging-plant requirements and disposal
procedure, cohesionless samples should undergo mechanical sieve
analysis. A chemical analysis is necessary for cohesive sediments.
Bioassays may be necessary for cohesive sediments, depending on
results of chemical analysis and proposed disposal action. If the project
involves dredging of new sediments, a principal element of interest may
be the density (or consistency) of material and, for cohesive sediments,
the shear strength.
UNEXPLODED ORDNANCE. Currently the Navy has embarked on a
number of efforts to locate and dispose of unexploded ordnance (UXO). Where there is
a past history indicating potential existence of such ordnance, it must first be located so
that care can be exercised to avoid injury to individuals or property damage.
MAGNETIC SILENCING FACILITIES. In order to deal with magnetic
silencing facilities (for degaussing and deperming), the facility should be designed with
future dredging requirements in mind. If the facilities are already in place, a dredging
method that will not impact the facilities must be chosen.
DREDGING EQUIPMENT. A comprehensive summary of descriptions
and optimum uses for various types of dredging equipment is found in Chapter 7 of The
Handbook of Dredging (Herbich, 1992) and in Chapter 7 of Dredging: A Handbook for
Engineers (Bray et al., 1997).
Mechanical Dredges. Mechanical dredges dislodge and raise sediment
by mechanical means. Mechanical-dredging methods are generally used in protected
waters, but because the equipment is relatively mobile, some mechanical dredging may
be accomplished in open water during short-term, calm-water conditions. Mechanically
dredged sediments may be disposed alongside the dredge at a dumpsite or may be
transferred to scows which transport the sediments to a dump site. The production rate
by means of mechanical dredging is relatively low. Mechanical dredges produce a
more irregular bottom than hydraulic dredges.