Geotechnical Design Considerations for Contained Aquatic Disposal

ERDC TN-DOER-N5

July 2000

Geotechnical Design Considerations

for Contained Aquatic Disposal

PURPOSE: This technical note provides geotechnical design guidance for contained aquatic

disposal (CAD) sites. The geotechnical behavior of the recently deposited dredged material with

respect to consolidation, bearing capacity, and stability are major factors that are considered. The

presence or absence of a cap may be considered in each analysis procedure.

BACKGROUND: As disposal of contaminated sediments becomes more constrained (e.g., regu-

lated and costly), U.S. Army Corps of Engineers District offices look to various and innovative

disposal options for this material. One of the more promising is CAD. This form of dredged material

disposal involves controlled placement of dredged material into a subaqueous site with some form

of lateral confinement. The lateral confinement may be provided by a bottom depression or by

subaqueous berms. The contaminated material is then capped in most instances with clean sediment

to physically separate it from the overlying environment.

CAD is usually used in situations where the properties of the contaminated material and/or bottom

conditions (e.g., slopes) require positive lateral control measures during placement. Use of CAD

can also reduce the required quantity of capping material; thus the costs for disposal may be lowered.

Options might include the use of an existing natural depression, pre-excavation of a disposal pit, or

construction of one or more submerged dikes for confinement (Palermo et al. 1998b).

The majority of CAD projects use excavated borrow pits or cells to contain the contaminated

dredged material. The excavated depth of these cells ranges from a few feet to 9-15 m (30-50 ft),

or more. The horizontal dimensions of the cells vary considerably, but some of the larger cells have

approached 457 m (1,500 ft) wide by 1,524 m (5,000 ft) long. The cells at many of the larger sites

are filled segmentally; that is a smaller portion (an end or a corner) is filled with contaminated

sediments and capped. Other sections of the cell are used later for CAD development. These initial

deposits in the CAD may result in formation of mounds or sloped deposits that may overlap each

other or cover the entire bottom of the CAD cell. With such a sequential plan of filling combined

with regulatory requirements for placing and maintaining a viable cap of given thickness on top of

the contaminated material, it is essential that geotechnical issues be addressed in the design of these

CAD projects. In other situations, very soft, sometimes hydraulically dredged and placed material

is to be capped. In these instances, the ability of the dredged material to sustain a viable cap on its

surface, i.e., bearing capacity, must be evaluated. On some recent CAD projects, problems have

been encountered with keeping the cap above the very soft contaminated sediments.

Although many of the chemical, biological, hydraulic, and operational requirements associated with

CAD sites have been fairly well defined, much of the geotechnical behavior has been ignored.

However, several recent CAD projects have focused attention on geotechnical issues. The 1997

closure of the U.S. Army Engineer District, New York, Mud Dump Site, with its associated

maximized final use coupled with intense monitoring efforts and an expedited capping schedule,