Effect of Placement Operation on Required Cap Volume (Cont.)

To calculate required cap volume, it is recommended that the "full cap

thickness" volume (i.e., a level cap at full thickness) be computed over the main

mound and inner flanks. Up to an additional 10-20 percent of cap material

should be identified as possibly being required and should be available.

The required cap thickness over a few centimeters-thick mound apron can

become an important issue when one considers the volume (and cost) of cap

material required to cover mound aprons. Table H3 compares volumes and

dimensions from the Port Elizabeth/Newark project (which required a 1-m cap

over the entire contaminated mound) and two generic cap projects based on the

mounds shown in Figures H1 (0.9-m cap over the entire mound) and H2 (0.9-m

cap over the main mound and a 0.3-m cap over the apron). Volume calculations

show that over half (55.6 percent) of the 1,870,000 m3 (2,446,000 yd3 ) of mate-

rial placed at the Port Elizabeth/Newark mound covered the contaminated

mound apron, which contained about 12 percent of the contaminated material

volume. Table H3 also shows that in the generic mounds shown in Figures H1

and H2 (identical contaminated mound shapes), the total volume of cap material

material required is reduced by nearly 60 percent, from 847,200 m3

(1,108,100 yd3) to 347,800 m3 (454,900 yd3) when the required cap thickness

over the apron is reduced from 1 to 0.3 m. The volume required to cover the

contaminated apron reduces from 16.4 to 4.3 percent of total cap volume. The

dredging and cap placement over the wide area covered by the apron will, for

most projects, significantly increase the project costs. In rare instances where an

abundance of cap material is being dredged as part of an authorized dredging

project, the cap material can be considered "free." However, the capping project

must still cover the additional cost of precisely placing the cap.

For low levels of contaminants, bioturbation-induced mixing of the cap-

contaminated material and native sediment may be sufficient to reduce the

resulting level of contamination to an acceptable level. McFarland (in prepara-

tion)1 describes procedures that can be used to determine the effects of reduced

cap thicknesses over the apron based on bioaccumulation studies. For the sedi-

ments used on the Port Newark/Elizabeth 1993 project, McFarland (in prepara-

tion) found that a cap thickness to apron thickness ratio of 2:1 was sufficient to

reduce bioaccumulation of the contaminant of concern (dioxin) to acceptable

levels. The apron thickness for the Port Newark/Elizabeth mound ranged from

1 to 10 cm with a 5 cm averge thickness. Thus using McFarland's results, a cap

thickness over the apron of 10 to 20 cm would have been sufficient. Most of the

capped mounds created as part of the New England Division's capping program

have cap thicknesses over the apron of 20 to 50 cm.

Another issue impacting the amount of cap required is how far beyond the

known contaminated mound boundary to place cap material. Because the edge

of the cap will normally be located with a sediment profiling camera, the edge of

the contaminated material will normally be defined to a precision of about 50 m.

Therefore, it seems reasonable to place cap material such that the cap material

References cited in this appendix are listed in the References at the end of the main test.

Appendix H Level-Bottom Capping Projects