Terminal 3 Seattle CDF Cover

ERDC TN-DOER-C18

August 2000

The enclosed area was subsequently filled with contaminated sediment

dredged from the remaining portion of the bay with a cutterless pipeline

dredge. To avoid volatilization of mercury, a water pond of at least 50 cm was

maintained over the dredged material during the filling operations. After the

completion of dredging, a surface cover of soil was placed over the reclaimed

sediment for confinement. It was intended that the cover be placed under To view figure

larger, click here

overlaying water; however, the sediment surface was too soft to be covered

with clean soil directly with equipment, and measures were needed to stabilize

Figure 22

the sediment to improve trafficability and bearing capacity. This was accom-

pished by first placing a synthetic fiber cloth and a net of ropes by workers in boats maneuvered by

cables (Figure 22). Once the fiber cloth and rope net were placed, the dredged material was

compacted by an 80-cm layer of sand placed hydraulically. A 1-m-thick layer of volcanic ash earth

of low specific gravity (1.1 in the air) was then spread using small bulldozers.

Terminal 3 Seattle CDF Cover. Several nearshore

CDF sites in the Puget Sound, WA, region have been

capped with clean material and converted to container

facilities or parking areas (Palermo et al. 2000; Boat-

man and Hotchkiss 1997; Converse Consultants 1992). To view figure

To view figure

larger, click here

One example is the Terminal 3 site in Seattle which larger, click here

was capped upon completion of the CDF fill. The

specification for the cap material was for select struc-

Figure 23

Figure 24

tural fill to support the pavement section. The material

was likely sand and gravel from a nearby quarry. The contractor placed the initial lift of material

using a conveyor (Figure 23). After several meters (feet) were placed in this manner, the remaining

cap section was pushed into place with dozers (Figure 24).

Sunny Point CDF Slurry Wall. A slurry wall was constructed at an upland

CDF on the U.S. Army facility at Sunny Point, NC (Figure 25). This slurry

wall was required to control saltwater intrusion into a sand aquifer underlying

the site. Water was ponded to within 1.8 to 2.4 m of the dike crest and held at

that level, presumably to maintain good water quality for the effluent dis-

charge. However, this produced a static head, resulting in saltwater intrusion To view figure

larger, click here

into the aquifer, which eventually impacted a number of large and historic

trees on property adjacent to the CDF. This was a nontypical groundwater

Figure 25

intrusion problem, in that the contaminant of concern was chloride (salinity),

which is highly mobile, and does not adsorb to dredged material or foundation soil particles, as do

most contaminants of concern found in sediments.

A cutoff slurry wall was constructed along the land side of the CDF near the dike toe to correct this

problem. The wall was constructed to a depth of approximately 18 m using a long reach backhoe

and a "traveling slurry pond." A grout mixture was prepared in a bermed pond, and the backhoe

excavated sections of the wall within the bermed pond, allowing the grout to fill in the trench as

excavation progressed. Slurry was added and the pond was "shifted" to travel along the slurry wall

alignment until the wall was completed. The completed width of the slurry wall is 0.6 to 0.9 m.