Duwamish River Demonstration

around and place it on an exact location at a specific heading. For the

split-hull hopper dredges, problems associated with cleanup were due to

the fact that once the hull is split, disposal of material continues until the

hopper is empty, i.e., the spilt hull cannot be closed until the hopper is

empty. Thus during cleanup, considerable amounts of sand end up being

placed on areas adjacent to the cleanup locations that already have suffi-

cient thickness.

After completing the project, the hopper dredges were found to have

problems with sealing of the hoppers, possibly as a result of structural de-

formations due to long hours of sailing with the hull cracked.

Duwamish River Demonstration

The first CAD project in Puget Sound in the northwestern United States

was in the Duwamish Waterway (Figure 34) as reported by Sumeri (1989).

A shoal that limited navigation through the waterway was found to con-

tain contaminated sediments that eliminated the possibility of unconfined

open-water disposal. Thus, the Seattle District initiated a demonstration

project to dispose of 840 m 3 of contaminated material in a subaqueous de-

pression in the West Waterway and to cap it with 3,220 m3 of clean main-

tenance dredged material from the upper Duwamish River (Sumeri 1984).

The fine-grained contaminated sediment exited the bottom-dump barge as

a slurry and descended rapidly to the bottom as a cohesive mass (convec-

tive descent). Three barges using survey positioning systems were used to

place the sand cap by "sprinkling" sand at an average rate of 21 m3/min

from incrementally opened split-hull barges. The resulting average cap

thickness was 61 cm. The sprinkling procedure using conventional equip-

ment minimized displacement of the contaminated sediment and hastened

the consolidation process. Since the capping material was released

slowly, it tended to settle to the bottom as individual grains and not as a

contiguous mass. Vibracore sediment samples taken up to 5 years follow-

ing capping showed the interface between the contaminated and cap sedi-

ments was sharp throughout the entire monitoring program. Measured

contaminant concentrations were either absent or present in low concentra-

tions in the cap material.

One Tree Island Marina

A CAD project involving direct mechanical placement of material was

conducted in 1987 for the expansion of the One Tree Island Marina at

Olympia, WA (Figure 34). The operation involved dredging of 2,980 m3

of contaminated material by clamshell with disposal in a deep conical pit

dredged on the project site and capping with 2,980 m3 of clean material.

The dredging operation was conducted in somewhat crowded conditions

with the project dimensions of 48.8 by 91.5 m situated between two other

marinas (Figure 35). First, the contaminated layer overlying the location

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Chapter 10 Case Studies