Penns Grove CDF Slurry Wall

ERDC TN-DOER-C18

August 2000

Once the wall was completed, wells were pumped on the outside of the wall to recover chloride

plumes down gradient from the wall. Piezometer data indicated that the groundwater flow was

reversed, and the pumping drained several small ponds. Pumping was therefore discontinued when

the majority of the plume was recovered (Personal communication, Les Wyatt, Wilmington District,

1999, Chief, Design Section, Engineering Branch, Technical Services Division).

Penns Grove CDF Slurry Wall. A slurry wall was constructed at the Penns Grove CDF located

adjacent to the Delaware River near Penns Grove, NJ (USACE Philadelphia District 1981). This

136-ha CDF was sited on an extensive sand and gravel foundation. Private contractors mined sand

and gravel for resource recovery and creation of additional disposal capacity within the CDF prior

to any placement of dredged material under a lease agreement with the Philadelphia District. This

operation removed large portions of the surficial silts and clays, exposing the underlying aquifer to

potential infiltration of leachate from the CDF. The mining operation was terminated when leachate

effects were identified as a potential problem.

A cutoff slurry wall was subsequently constructed outside the disposal area

dikes and exterior drainage trench as shown in Figure 26. The trench had a

minimum width of 1 m (3 ft) and was cut to depths of 9 to 18 m (30 to 60 ft)

and keyed into an impervious clay layer underlying the site. A mixture of 70

parts sand, 30 parts silt, and 2 parts bentonite was used. The total length of

trench was approximately 3,400 m (11,000 ft), extending generally around two

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sides of the CDF. Silt layers are intact along the remaining two sides that

would substantially reduce infiltration. Monitoring wells were also installed.

Figure 26

This site has not yet been used for dredged material placement.

Monroe Harbor CDF Dike Mattress. A grout mattress was used at a CDF located at Sterling

State Park to seal prepared limestone dikes. The goal was to make the CDF watertight because of

concern for groundwater contamination due to PCB concentrations in the dredged material. The

original design included a bentonite seal on the bottom and side slopes of the CDF. The bottom

and most of the slope were to be under water because of the proximity to Lake Erie. The bentonite

was removed from the design during a value engineering review due to the difficulty expected in

placing the material under water. A grout mattress was substituted for the side slope, and additional

clay was left on the bottom. The mattress was put in place, but it tended to float due to Lake Erie

flowing into the CDF. To stop that from occurring, a clay seal was constructed on top of the mattress

to hold it in place. The clay sealed the mattress and dike successfully. As with other CDFs with

prepared limestone dikes, once dredged material is placed against the dike, it becomes watertight.

The mattress's primary benefit is to prevent erosion on the side slopes until dredged material can

be placed against the slopes (Personal communication, Doug Zande, Detroit District, 1999, Chief,

Operations Technical Support Office).

Waukegan Harbor (Outboard Marine Corporation Superfund Site) Dike Cutoff. PCB-

contaminated sediment from Waukegan Harbor was dredged hydraulically and placed in contain-

ment cells constructed in a boat slip and other nearshore areas. Design of these cells included a

double sheetpile cutoff wall to isolate the slip from the harbor and a clay slurry wall that was

anchored to the underlying clay till (see Figure 27). The slip held approximately 23,000 cu m of

contaminated sediment (USEPA GLNPO 1998).