MATERIAL PROPERTY DETERMINATION

ERDC TN-DOER-N5

July 2000

stability analysis. Thus the accuracy of the analyses will be limited until actual conditions within

CAD deposits can be more accurately determined.

Only very limited geotechnical evaluations have been considered in past CAD and other capping

projects. In virtually all of the past projects, the design was empirical, i.e., prior field experience

showed that it worked, but actual geotechnical design calculations were not made. Development

and application of appropriate geotechnical design guidance should result in better designs.

MATERIAL PROPERTY DETERMINATION: Before any geotechnical analyses and evalu-

ations can be conducted, the physical properties that control engineering behavior must be deter-

mined for the soil mass. To do this requires that representative samples of the dredged material be

collected and tested and the results be used in geotechnical analyses to predict engineering behavior.

Sample Collection. Sample collection must be conducted in a manner that will ensure that the

material tested is representative of the dredged material that has been or will be placed in the CAD

cell/site. Since sediments will be disturbed (at least to some extent) during the dredging process, it

will not be necessary to obtain truly undisturbed samples for laboratory classification and consoli-

dation testing; however, for shear strength determination, undisturbed samples are required. If

sampling is to be completed prior to dredging, use of a coring device that can penetrate the sediment

may be required if the depth, consistency, or variability of the deposit prevents adequate, repre-

sentative sample collection with a grab sampler. If the depth to be dredged is small and the sediments

have been recently deposited and have been frequently mixed by ship traffic, it will be sufficient to

collect grab samples using some kind of sampler or mechanical dredge, e.g., Petersen dredge. If

sampling is to be accomplished during dredging, samples may be taken periodically in 0.02-cu-m

(5-gal) buckets (or other suitable containers) throughout the dredging process. If mechanical

dredging is used, samples for laboratory testing can simply be recovered from the dredged material

as it is placed in the barge for transport. More specific guidance regarding sample collection and

preservation is given in the Green Book (U.S. Enviromental Protection Agency (USEPA)/U.S.

Army Corps of Enginers (USACE) 1991).

For classification and consolidation testing, the collected samples may be combined to form one

homogeneous composite sample unless there is additional information regarding material type,

placement operations, etc., that would preclude compositing. The idea here is to have a laboratory

sample that is the most representative possible of the sediment in the CAD deposit. Therefore, it is

necessary to have some decisions about material representative of the sediment made by individuals

familiar with the specific project.

Material Characterization Testing. After samples are collected and composited, they must be

tested by a competent geotechnical testing laboratory. The following tests should be performed to

properly characterize the material as a first step in predicting its engineering behavior:

Natural water content, solids concentration, and/or density.

Grain size distribution.

Plasticity indices, i.e., Atterberg limits.