Long-Term Cap Stability

Long-Term Cap Stability

Considerations in Long-Term Stability

When contaminated material is isolated from the environment through

a dredged material capping operation, it is essential that not only the preci-

sion and thoroughness of initial cap placement be considered but also the

long-term integrity, or stability, of the capped deposit be evaluated on a

regular basis. A critical element in successful performance of a cap is

preservation of an adequate thickness of this clean material to control flux

of contaminants and isolate the contaminated sediments from benthic or-

ganisms. In evaluating long-term cap stability, factors that must be ad-

dressed include the following:

a. Possible consolidation (of capping material, contaminated sediment,

and foundation material) for effect on long-term site capacity, dif-

ferentiation from erosion, and quantification of contaminated pore

water volume expelled.

b. Potential for erosion (considering the wave and current conditions

at the disposal site and dredged material particle size and cohesion).

If erosion or consolidation causes the cap to be too thin to effectively

isolate the contaminated material from the surrounding environment, then

remedial actions will be required to reestablish cap integrity. This chapter

presents detailed procedures to evaluate long-term physical stability of

subaqueous dredged material caps, considering consolidation and erosion

processes. These processes are discussed in the following paragraphs,

along with recommended techniques and computer models available for

analysis.

A critical step in cap design is to use the information from Chapter 7 in

determining a design cap thickness (or a trial thickness for detailed evalu-

ations such as decribed in this chapter). Selecting a design cap thickness

is a function of an acceptable level of risk. Assessment of consolidation

is mathematically straightforward, while the very stochastic nature of ero-

sion makes it much more complicated to predict. Definitive guidance on

cap stability is difficult because the level of acceptable risk will likely vary

from location to location. Further discussion of risk-related cap design

topics are found at the end of this chapter.

Chapter 8 Long-Term Cap Stability