Hopper dredge pump-down

Figure 11.

Conveyor unloading barge with tremie (from Togashi 1983)

Hopper dredge pump-down

Some hopper dredges have pump-out capability by which material from

the hoppers is discharged like a conventional hydraulic pipeline dredge. In

addition, some have further modifications that allow pumps to be reversed

so that material is pumped down through the dredge's extended dragarms.

Because of the expansion at the draghead, the result is similar to using a

diffuser section. Pump-out depth is limited, however, to the maximum

dredging depth, typically about 60-70 ft.

Use of geosynthetic fabric containers (GFCs)

Geosynthetic fabric containers (GFCs) are containers made from geo-

synthetic fabric that line barges. Contaminated dredged material is placed

in the GFCs (either mechanically or hydraulically), which are then sewn

closed prior to placing the GFC at the disposal site. The GFC acts as a fil-

ter cloth, allowing the water to escape but retaining almost all the fine

(silt and clay) particles. Containing contaminated sediments in GFCs for

subsequent placement from split-hull barges offers the potential to elimi-

nate the wide, thin apron normally associated with conventional bottom

dumping of fine-grained sediments, thus substantially reducing the vol-

ume of cap material required and reducing the potential for contaminated

sediments to extend beyond the site boundary. GFCs also have the poten-

tial to eliminate water quality problems at the disposal site by essentially

eliminating loss of fine sediment particulates and associated contaminants

to the water column.

As of 1996, GFCs have been used on only two USACE projects. The

first was construction of training dikes in the lower Mississippi River

(Duarte, Joseph, and Satterlee 1995), and the second was placement of

sandy sediment with heavy metal contaminants in a CAD site in Los Ange-

les Harbor (Mesa 1995). At present, costs of using GFCs are much higher

than for conventional bottom placement due to costs of materials, in-

creased dredge cycle times, increased labor requirements associated with

installation of the GFCs in the barge, and possible reductions in dredge

Chapter 5 Equipment and Placement Techniques