Areal coverage of the full cap versus apron cap

Tt = Tb + Te + Tc + To +Ti

where

Tt = total cap thickness, cm

Tb = thickness for bioturbation, cm

Te = thickness for erosion, cm

Tc = thickness for consolidation, cm

To = thickness for operational considerations, cm

Ti = thickness for physical/chemical isolation, cm

Areal coverage of the full cap

versus apron cap

For a capping operation to be successful, the required cap thickness

must be placed over the deposit of contaminated material. Typically, the

edge of the contaminated mound will be detected with an SPC, which can

reliably detect contaminated layers of thickness of 1-2 cm. Within this

context, the contaminated material deposit is considered that which can be

detected. However, it is not possible or necessary to cap every particle of

contaminated material with the full design cap thickness.

For LBC projects, capping operations should be aimed at placing the

full design cap thickness over the central portion of the mound and inner

and outer flanks of the mound as defined in Chapter 6. As contaminated

material is placed to form the mound, material settles to the bottom as the

apron in ever-decreasing thicknesses with increasing distance from the point

of discharge. The capping material is similarly dispersed, especially if the

grain size and placement methods are similar. Therefore, operations aimed

at placing the design thickness over the geometry of the mound that can

be defined by bathymetric surveys will result in somewhat thinner layers

of capping material being placed over the apron, as defined in Chapter 6.

Monitoring techniques are discussed in Chapter 9. Differential

bathymetric surveys can determine the extent of a deposit down to a thick-

ness of approximately 15 to 30 cm, while an SPC can detect sediment

thicknesses from 2 to 20 cm. A combination of these approaches can be

used to define the areal extent of the contaminated material mound and

subsequently the required areal extent of the full capping thickness.

For CAD projects in which the contaminated material is placed as a

layer of uniform thickness within the contained area, the full design cap

thickness should be placed over the entire surface area.

Volume calculations

Once the design cap thickness and required areal extent of the cap are

determined, the required volume of capping material can be estimated.

There is no minimum acceptable ratio of capping to contaminated sediment

Chapter 7 Dredged Material Cap Design