Equipment and Placement Techniques

The equipment and placement technique are selected and rate of appli-

cation of capping material is controlled to avoid displacement or mixing

with the previously placed contaminated material to the extent possible.

Placement of capping material at equal or lesser density than the contami-

nated material or use of placement methods to spread thin layers to gradu-

ally build up the cap thickness generally meets this requirement. However,

sand caps have been successfully placed over fine-grained contaminated

material. Since capping materials are not contaminated, water column dis-

persion of capping material is not usually of concern (except for loss

when slowly placing a sand cap); the use of submerged discharge for cap-

ping placement need only be considered from the standpoint of placement

control.

Equipment and Placement Techniques

The equipment and placement techniques described in the following

paragraphs apply to the contaminated dredged material to be capped as

well as to the capping material, depending on the project conditions. Re-

gardless of the equipment and placement techniques considered, the com-

patibility of contaminated material placement and capping operations

must be determined considering the material characteristics and site

conditions (Palermo 1991a,c).

Surface discharge using conventional equipment

Dredged material released at the water 's surface using conventional

equipment tends to descend rapidly to the bottom as a dense jet with mini-

mal short-term losses to the overlying water column (Bokuniewicz et al.

1978; Truitt 1986a). Thus, the use of conventional equipment can be con-

sidered for placement of both contaminated and capping material if the

bottom spread and water column dispersion resulting from such a dis-

charge are acceptable.

The surface release of mechanically dredged material from barges re-

sults in a faster descent, tighter mound, and less water column dispersion

as compared with surface discharge of hydraulically dredged material

from a pipeline. Placement characteristics resulting from surface release

of hydraulically dredged material from a hopper dredge fall between the

characteristics resulting from surface release of hydraulically dredged ma-

terial from barges and from surface discharge of hydraulically dredged ma-

terial from a pipeline--that is, the descent is slower than the former but

faster than the latter; the mound is looser than the former but tighter than

the latter; and more water column dispersion results from the former than

from the latter.

Field experiences with LBC operations in Long Island Sound and the

New York Bight as described in Chapter 10 have shown that mechanically

dredged silt and clay released from barges tend to remain in clumps during

descent and form nonflowing discrete mounds on the bottom that can be ef-

fectively capped. Such mounds have been capped with both mechanically

Chapter 5 Equipment and Placement Techniques