Evaluation of Consolidation

Evaluation of Consolidation

For LBC projects, dredged material typically forms a mound of mate-

rial on the bottom of the water body. If a clean sediment is placed to iso-

late the contaminated material from the surrounding environment, the

capping material increases the size of the existing mound and also places

a surcharge load on the underlying dredged material and further increases

the surcharge load on the foundation soil. Because the contaminated sedi-

ments are usually fine grained and have a relatively high moisture content,

they are often susceptible to large amounts of consolidation. For CAD

projects, the materials are layered but are subject to the same consolida-

tion processes.

Assessing consolidation potential of capped dredged material mounds

or deposits requires consideration of the consolidation potential of three

elements: the cap, the contaminated dredged material, and the native or

substrate sediments (foundation soils). The contaminated dredged material

(which is usually fine-grained, cohesive material) likely will undergo con-

solidation resulting both from its own self-weight and from the surcharge

load of the capping material. If the capping material is fine grained (e.g.,

silt or clay), it will also be susceptible to consolidation. Coarse-grained

capping material (e.g., sand or gravel) would not normally be expected to

consolidate. The final element to be considered is consolidation potential

of the foundation soils. If these soils are fine-grained materials suscepti-

ble to consolidation, the loading applied by the contaminated and capping

material will probably be sufficient to cause consolidation.

Quantifying consolidation is necessary for three reasons. First,

changes in elevation due to consolidation must be delineated from those

due to erosion. Decreases in the elevation of the mound or deposit surface

caused by erosion of the cap may require remedial actions to replenish and

restore the cap to its required thickness. If consolidation of constituent

materials accounts for the change in elevation, then no cap replenishment

is necessary, particularly if cap thickness design accounted for, a priori,

potential cap consolidation. Thus it is imperative that consolidation be

distinguished from erosion. Second, consolidation should be considered

when determining long-term site capacity. As a mound consolidates and

decreases in elevation, additional volume becomes available between the

mound surface and the plane of maximum acceptable mound elevation;

this volume can be used for storage of additional dredged material. The

increases in the storage capacity of subaqueous disposal sites due to con-

solidation are especially important when these sites will be used to store

large quantities of material from several dredging operations occurring

over a number of years. Thus the ultimate holding capacity of repeated-use

sites will be significantly increased if consolidation is considered. Third,

a consolidation analysis will provide data needed to evaluate the potential

movement of pore water from the contaminated sediment upwared into the

cap, and this is necessary in evaluating the potential for long-term flux of

Chapter 8 Long-Term Cap Stability