Consolidation testing

Many soft fine-grained materials may undergo on the order of 50-percent

vertical strain during the consolidation process. Therefore, the objective

of consolidation analysis is to determine the amount and rate of consolida-

tion that the mound and/or foundation soils will undergo as a result of

self-weight consolidation and/or surcharge loading. One-dimensional (1-D)

consolidation analysis is normally used in geotechnical engineering. In a

1-D analysis, pore water is expelled vertically (upward and/or downward)

from soil layers; no horizontal flow or strain is allowed. Few 2-D or 3-D

analyses are ever performed, and these are usually conducted on research

projects. Because of the configuration of subaqueous sediment mounds

(relatively flat slopes and thin lifts), a 1-D analysis of mound consolida-

tion should provide adequate results for either design or analysis of these

mounds. However, in the future, development and use of 2-D or 3-D

consolidation models would permit more accurate prediction of the actual

direction and magnitude of flows and movements.

Fine-grained dredged sediments, especially those placed by pipeline or

hopper dredge, are initially soft and have a high water content, with an as-

sociated high compressibility. Potential changes in height (strains) due to

consolidation are large; therefore, a finite strain approach that accounts

for the large strains should be used to evaluate consolidation (Rollings

1994; Poindexter 1989).

Consolidation testing

Laboratory consolidation test data are necessary for an evaluation of

consolidation; however, standard procedures for consolidation tests

(USACE 1970) may not be applicable for testing of soft sediment samples.

A modified version of the standard oedometer consolidation test (USACE

1987) and a self-weight consolidation test (Cargill 1985) have been devel-

oped that provide data for the wide range of void ratios that may be en-

countered in the context of dredged material placement operations.

Additional details on consolidation testing are given in Appendix I.

Consolidation models

The complexity and number of calculations required to predict consoli-

dation of deposits using large strain consolidation theory require use of a

computerized solution technique. The theory of finite strain consolidation

(Gibson, England, and Hussey 1967) has been incorporated into several

generations of computer models for analyzing consolidation of capped

sediment mounds (Cargill 1985; Poindexter-Rollings 1990; Stark, in prepa-

ration). To run any of these models, consolidation test data from self-

weight consolidation tests and/or standard oedometer tests (USACE 1970;

USACE 1987) are required (See Appendix I).

Initial work on consolidation of dredged material was done with the

computer model PCDDF (Primary Consolidation and Desiccation of

Dredged Fill) (Cargill 1985), which was later modified and released as

PCDDF89 (Stark 1991); these programs were developed specifically for

analysis of confined upland disposal sites. Subsequent work on

Chapter 8 Long-Term Cap Stability