Equipment and Placement Techniques

Equipment and Placement

Techniques

Equipment and techniques applicable to placement of contaminated ma-

terial to be capped and clean material used for capping include conven-

tional discharge from barges, hopper dredges, and pipelines; diffusers and

tremie approaches for submerged discharge; and spreading techniques for

cap placement (Palermo 1991c, 1994). This chapter describes basic dredg-

ing, transportation, and placement processes as they relate to capping and

considerations in selecting equipment and placement technique for both

contaminated and capping materials. Considerations for scheduling for

placement of the cap, navigation and positioning needs, placement options

and tolerances, and inspection and compliance are also discussed.

Flow and Mounding Versus Dredging Method

The behavior of materials upon placement (especially their tendency to

mound or to flow) and the ability to cap a deposit of contaminated material

depend on several factors, including the method of dredging, the method

of placement, material characteristics (cohesive/noncohesive), and site

conditions such as water depth or current velocities (Headquarters, U.S.

Army Corps of Engineers 1983).

The dredging process may be subdivided into two categories: mechani-

cal and hydraulic dredging. During mechanical dredging, the sediments

are physically lifted from the bottom by a mechanical process such as a

bucket or clamshell. Mechanically dredged material is typically placed

into and transported to the disposal area in barges (also commonly known

as dump scows). Barges either have hoppers with doors through which

material is released to the bottom or they can be split-hull, allowing the

entire barge to open and release material to the bottom. Mechanically

dredged material placed in this manner is ideally suited for creating

subaqueous mounds because the dredged material stays close to the in situ

density throughout the dredging process. This relatively constant density

lends to effective mound construction because less water is entrained in

the material, stripping during descent is minimized, and material spread

on the bottom is reduced (Sanderson and McKnight 1986).

Chapter 5 Equipment and Placement Techniques