Mound Geometry for Level-Bottom Capping

8 ft and a movement of material outward of about 1,000 ft. This section

of mound was placed on an ambient slope of up to 1.45 deg, which likely

contributed to the adjustment and the outward movement. In a second case, a

portion of the same mound with an elevation exceeding 10 ft experienced

an apparent slope adjustment after capping began. Losses in elevation of

3 to 4 ft occurred as a result of the adjustment, though the significant out-

ward movement seen on the upcapped section did not occur. This section

of the mound was placed on a nearly flat slope. The above illustrates the

need to consider the potential for slope adjustments in mounds over 6 to 8 ft

tall. Analysis of slope stability for taller mounds, particularly those placed

on slopes, is recommended (Moritz 1997).

Mound Geometry for Level-Bottom Capping

Evaluation of contaminated material mound geometry for an LBC project

requires a series of steps:

a. Determine volume of material to be disposed. The first step in a

capping project is to compute the volume of contaminated material

to be dredged. An accurate estimate of the volume of contaminated

material to be dredged should be a fairly straightforward process.

Normally computer programs that compare authorized channel di-

mensions with existing bathymetry determine the volume of material

to be dredged, with a combination of core, subbottom profiler, and

sediment chemistry and bioassay/bioaccumulation testing done to

determine the volume of contaminated sediments. The designer

should consider including possible overdepth in the volume calcu-

lation. Normal clamshell allowed overdepth is about 2 ft. Some

of the "environmental" clamshells claim lower overdepths 6 in. to

1 ft. Very high-quality instrumentation in addition to a special

bucket is needed to achieve the lower overdepth values.

b. Bulking. Some bulking of the sediments during the dredging process

may be factored into computing the volume required for capping.

For mechanically dredged sediments, bulking of 10 to 20 percent

(Herbich 1992) is reasonable. For materials dredged by hopper, a

large volume of excess water is initially stored in the hopper, but

the volume of water may be reduced prior to material placement

by overflow. Following placement by hopper, a large portion of

the excess water is almost immediately expelled from the material

as it settles to the bottom.

In most instances capping will involve mechanical dredging of

maintenance material with relatively low densities. These materials

can experience fairly rapid consolidation. Most contaminated

dredged projects will require several weeks or longer to conduct

dredging. Thus, by the time capping is ready to begin, some con-

solidation will have taken place such that the volume to be capped

may be nearly the in situ volume. Without site-specific data, a

net bulking volume (including the apron) of 10 to 20 percent is

reasonable.

Chapter 6 Sediment Dispersion and Mound Development and Site Geometry During Placement