Effect of Placement Operation on Required Cap Volume

Figure H3. Cap slope length calculation

Effect of Placement Operation

on Required Cap Volume

A number of operational factors should be considered in computing required

cap volume. These factors include the "full" cap thickness versus "average" cap

thickness, the required cap thickness over the apron, and how far beyond the

contaminated boundary the cap should be placed. The following paragraphs

discuss each of these factors in turn. In general, cap volume to contaminated

sediment volume ratios of 1:2 to 1:5 have been used for capping projects. While

the following paragraphs describe how to compute specific cap volume require-

ment, some generalizations can be made. Higher cap to contaminated material

ratios will be found for projects that use thin mounds, those consisting of main-

tenance material that is fine grained with low shear strength, where barges

placing contaminated material will not be required to stop, and sites with deeper

water. Also, for smaller volume contaminated sediment projects, the apron will

tend to occupy an increasingly large percentage of the total area, greatly increas-

ing required cap volume to contaminated sediment volume ratio (particularly if

the full cap thickness is required over the entire apron). Lower cap to contami-

nated sediment volumes can be expected for thicker mounds, those consisting of

material with high shear strength, mounds placed in shallow water, where barges

come to a complete halt or are moving at low speeds (less than 1/2 to 1 knot).

Achieving full cap thickness over the entire contaminated mound footprint is

nearly impossible to accomplish without placing a considerable amount of addi-

tional material over that required for a level cap. This is because underwater

placement is difficult to precisely control. Depending on the method of cap

placement, the cap surface will have greater or lesser amounts of surface relief.

For caps that are "sprinkled," this degree of surface relief will probably be less

for sprinkled caps than for bottom-dumped caps.

One issue that must be resolved for cap design is whether or not the entire

cap area requires the "full cap thickness." While a cap with a constant thickness

Appendix H Level-Bottom Capping Projects