Options if Required Volume is Too Large

However, the reduction in cap thickness due to consolidation should be

considered from the standpoint of advection of pore water. Consolidation will

reduce the void ratio and thus will force pore water further out into the cap.

Effect on Volume Due to Change in Void Ratio

The volume of material to be dredged for the cap must be calculated to deter-

mine if potential sources of capping material, say from an available maintenance

dredging project, will be adequate. The potential changes in volume due to

dredging and placement must be considered. The required volume of capping

material (in situ in the channel) can be calculated as follows:

ei)

(eo

(H6)

Vci

ei)

where

Vci = volume of cap material in situ in channel

Vc = volume of cap material initially placed

eo = average void ratio of cap material initially placed

ei = average void ratio of cap material in situ in channel

For projects in which the capping material is hydraulically placed, the value

of eo can be determined in the same way as that used in design of confined

disposal facilities (USACE 1987, EM 1110-2-5027). For mechanically dredged

sediments, an approach to determine the minimum cap volume required is to

assume no difference in eo and ei (i.e., Vci = Vc). It is recommended that those

with experience dredging a particular project (USACE District Operations

Division staff, dredging contractors, etc.) be contacted for suggestions on bulk-

ing factors. SAIC (1995) reports that the assumption of no difference in eo and ei

is reasonable.

Options if Required Volume is Too Large

The information from the prior section along with the information in Chap-

ter 6 (main text) on expected contaminated mound footprint should be used to

compute required cap volume. If the estimated cap volume is too large, either

because insufficient cap material is available or the cost is too high, the follow-

ing options are available. As noted earlier, the most obvious is to reduce the

volume of contaminated material. A second option may be to delay dredging

until additional cap material becomes available, perhaps combining several small

H11

Appendix H Level-Bottom Capping Projects