Consolidation Analysis

ERDC TN-DOER-N5

July 2000

The term failure used in this technical note simply describes the results of an equilibrium analysis

for bearing capacity or slope stability. The displacement, sliding, or sloughing of the slopes of a

dredged material mound is not necessarily detrimental provided the material is not resuspended or

left with an inadequate cap thickness. Instead, it simply represents the material adjusting itself to

a configuration of static equilibrium. In this sense, the analysis needs to be kept in perspective.

Consolidation Analysis. Many soft, fine-grained dredged materials may undergo vertical strain

on the order of 50 percent during the consolidation process. Therefore the objective of consolidation

analysis is to determine the amount and rate of consolidation that the CAD deposit will undergo as

a result of self-weight consolidation and/or surcharge loading. Because of the large strains involved

in dredged materials deposits, the general Terzaghi consolidation theory is totally inadequate to

model the problem. Instead a finite or large strain consolidation theory provides the proper

analytical approach.

The most general and least restrictive of the many one-dimensional primary consolidation formu-

lations was developed by Gibson, England, and Hussey (1967) and has been expanded and applied

to various types of dredged material and soft soil analyses (e.g., Gibson, Schiffman, and Cargill

1981; Cargill 1985; Townsend 1987; Poindexter 1988; Townsend and McVay 1990). These models

include consideration of primary consolidation with the option to include desiccation crust forma-

tion, as applicable. Secondary compression (which can be significant in these thick, soft deposits)

is not explicitly considered but is implicitly included in the desiccation model. Attempts have been

made to include secondary compression directly in the PSDDF model (Stark, in preparation), but

this portion of the program is not currently efficacious. (It does not adequately model real dredged

material behavior.) However, this model does include capability for modeling a sand or fine-grained

cap and multiple dredged material layers. The most recent version of the PSDDF finite strain

consolidation model, including some typical input data, is included in the desktop personal computer

version of the Corps of Engineers Automated Dredging and Disposal Area Management System

(ADDAMS), an assortment of dredging-related computer programs that can assist in management

of dredging and disposal operations (Schroeder and Palermo 1990).

In evaluating consolidation, both the time-rate and the ultimate magnitude of consolidation should

be determined separately for the contaminated sediment, the capping material, and the foundation

layers, as appropriate. Then for any given time of interest, the individual settlement values for the

various layers are summed to provide an estimate of the total amount of consolidation settlement

to be expected at that particular time. This information can be used in conjunction with field

monitoring data in the ongoing assessment of cap integrity. Any change in thickness of the capping

material is of primary concern from an environmental perspective. However, the total amount of

consolidation settlement, or decrease in elevation, of the cap surface over time is necessary to

delineate between mound height changes caused by erosion and those accounted for by consolida-

tion of constituent materials. Figure 3 illustrates surface elevation changes over time for various

hypothetical CAD deposits with a range of initial void ratios.

CONCLUSIONS: Predictions of both consolidation behavior and shear strength/stability can be

made prior to construction of a capped dredged material mound. However, the quality of the

prediction will be only as good as the input data or assumptions used in lieu of site-specific data.