Required Cap Thickness (Cont.)

The thickness for chemical isolation (if required) and/or the thickness

for bioturbation must be maintained to ensure long-term integrity of the

cap. The integrity of the cap from the standpoint of physical changes in

cap thickness and potential for a physical reduction in cap thickness due

to the effects of consolidation and erosion can be evaluated once the over-

all size and configuration of the capped mound or deposit and resulting

water depth over the cap are determined. The design cap thickness for

the various components can then be adjusted by iterative calculations if

needed.

At present, the design of caps composed of clean sediments is based

on a combination of laboratory tests and models of the various processes

involved (contaminant flux, bioturbation, consolidation, and erosion),

field experience, and monitoring data. Since the number of carefully de-

signed, constructed, and monitored capping projects is limited, the design

approach is presently based on the conservative premise that the cap thick-

ness components are additive. No dual function performed by cap compo-

nents is considered. As more data become available on the interaction of

the processes affecting cap effectiveness, this additive design approach

can be refined.

Before the design cap thickness can be determined, the following must

be resolved: (a) the intended functions and design objectives of the cap

must be defined (see Chapter 1); (b) suitable capping material must be iden-

tified (see Chapter 3); (c) a specific site must be identified and charac-

terized (see Chapter 4); (d) equipment and placement techniques must be

selected (see Chapter 5); and (e) overall geometry of the contaminated

mound or deposit must be evaluated (see Chapter 6). The recommended

sequence for determining the design cap thickness is as follows:

a. Assess the bioturbation potential of indigenous benthos and deter-

mine an appropriate cap thickness component for bioturbation.

b. Determine if the capping material is compressible, and if so, evalu-

ate potential consolidation of the cap material after placement. If

contaminated sediments or native underlying sediments are com-

pressible, evaluate potential consolidation of those materials. If re-

quired, add a thickness component to offset consolidation of the

cap.

c. Considering the mound or deposit geometry and site conditions,

conduct a screening evaluation of potential erosion. If there is po-

tential for erosion, conduct a detailed evaluation, considering both

ambient currents and episodic events such as storms. If required,

add a thickness component to offset potential erosion.

d. Evaluate operational considerations and determine restrictions or

additional protective measures (e.g., institutional controls) needed

to ensure cap integrity. If needed, add a thickness component to

offset operational considerations.

e. If a design function of the cap is to control contaminant flux, evalu-

ate the potential for short-term and long-term flux of contaminants

through the cap as necessary. Determine any necessary additional

Chapter 7 Dredged Material Cap Design