1 September 1997
e. Project dimensions. Project dimensions may
d. Mechanical Dredging. Mechanical dredges
include such items as length, width, depth, and
include bucket, bucket-ladder, clamshell, and dragline
dredges. Transportation of the dredged material is
made by additional plant, tug and barge, or scow.
f. Site restrictions. This may include such items
Mechanical dredges are classified by bucket size and
as waterway usage, vessel traffic, as well as time,
are best adapted for dredging fine-grained material.
weather, noise, and environmental restraints. Many
They are the most efficient dredge for working near
areas are subject to restricted dredging seasons. To
bridges, docks, wharfs, piers, or breakwater
minimize environmental impacts creating scheduling
conflicts, higher costs may occur and must be
considered and documented in the cost estimate.
e. Specialty Dredging. Some dredging projects
have unusual conditions or unique project
requirements that cannot readily utilize standard
15-5. Selection of Equipment
dredge plant. There are a variety of specialized dredge
plant which must be considered under these
a. An economical dredge shall be selected.
Dredge type and size depend mainly on availability,
job duration, type of material, exposure to the
elements, disposal area restraints, environmental
restraints, and production requirements. The narrative
a. In calculating production rates for dredging,
will include the rationale used by the cost engineer for
effective time is commonly used. Lost time due to
major repairs and alteration, cessation, and collisions
is not used in dredging time calculations. Whenever
b. Pipeline Dredging. The term "pipeline" refers
possible, the production rate used in CEDEP should
to the discharge method through on-board pumps.
be based on historical data. When historical data is
The common types are cutterhead, suction, and
not available, the sequence described in this chapter
dustpan dredges which refer to the intake method.
shall be used.
Pipeline dredges are sized by the inside diameter of the
dredge discharge flange, and they are effective in
b. Pipeline Dredging. Production is determined
dredging densely packed materials. Although they are
by the pumping rate and the effective time.
best suited for low-traffic areas and sheltered
waterways such as rivers, bays, harbors, and canals,
(1) Pumping rate is affected by items such as
some pipeline dredges are equipped to operate in calm
water depth, density of material, distance discharged,
to moderate seas offshore. Pipeline dredges lend
themselves well to shore disposal operations.
disposal area restraints, environmental restraints, and
dredge configuration, such as spud carriage, ladder
c. Hopper Dredging. Hopper dredges are sized
pump, degassers, and hoffa valve.
or classed by their hopper capacity. However, a
particular size dredge is actually limited by its weight
(2) The effective time is affected by items
carrying capacity and the environmental restrictions of
such as weather, handling pipeline, moving swing
the project. Hopper dredges operate in cycles, and
wires, minor operating repairs, vessel traffic,
they normally cover the length of the total dredging
repositioning the dredge, and surveys.
area, deepening it gradually. They are the most
efficient type dredge for excavating loose,
c. Hopper Dredging. Hopper dredge production
unconsolidated material and are used mainly in
is best evaluated in terms of its cycle components and
exposed harbors and shipping channels where traffic
the effective time.
and operating conditions rule out the use of stationary
(1) The hopper dredge cycle consists of
excavation time, transport time, and disposal time.