Evaluate spread, mounding and site geometry (10,11)

selected. Guidance on dredged material cap design therefore focuses on

the thickness of the cap as the major design criterion.

The determination of the required cap thickness is dependent on the

physical and chemical properties of the contaminated and capping sedi-

ments, the potential for bioturbation of the cap by aquatic organisms, the

potential for consolidation and the resultant expulsion of pore water from

the contaminated sediment, and the potential for consolidation and erosion

of the cap material. The minimum required cap thickness is considered

the thickness required for physical isolation plus any thickness needed for

control of contaminant flux. The integrity of the cap from the standpoint

of physical changes in cap thickness and long-term migration of contami-

nants through the cap should also be considered. The potential for a physi-

cal reduction in cap thickness due to the effects of consolidation and

erosion (12,13) can be evaluated once the overall size and configuration

of the capped mound is determined. A precise calculation of the erosion

thickness component requires consideration of mound shape, mound

height, and water depth. Since these parameters also depend on the total

capping thickness, some iterative calculations may be required. The de-

sign cap thickness is the required cap thickness for isolation plus that re-

quired for consolidation and erosion and operational considerations.

Guidance on cap design is found in Chapter 7, and details on specific test-

ing and evaluation procedures and models to support cap design are found

in Chapters 6 and 8 and Appendixes B, C, E, F, G, and H.

Evaluate spread, mounding and site geometry (10,11)

For LBC sites, the mound geometry, including contaminated material

mound and cap, will influence the design of the cap and volume of cap-

ping material required. The smaller the footprint of the contaminated

material as placed, the less volume of capping material is required to

achieve a given cap thickness. The spread and development of the con-

taminated material mound is dependent on the physical characteristics of

the material (grain size and cohesion) and the placement technique used

(hydraulic placement results in greater spread than mechanical place-

ment). Assuming that the material from multiple barge loads or pipeline

can be accurately placed at a single point, mound side slope and the total

volume placed dictate the mound spread. The formation of a thin layer or

apron surrounding the central mound must also be considered in defining

the footprint to be capped for LBC.

For CAD projects, in which lateral containment prevents spreading and

apron formation, the footprint will be determined by the site geometry.

However, the volume occupied by the sediments will govern the capacity

of the CAD site and must be considered as a factor in site design. If the

mound geometry or CAD site geometry is unacceptable, an alternative

site (3), alternative capping sediment (4), or alternative placement tech-

niques (5,6) can be considered. Guidance on mound spread and develop-

ment and site geometry is found in Chapter 6 and Appendixes E and H.

Chapter 2 Design/Management Sequence for Capping