NEED FOR GEOTECHNICAL EVALUATIONS (cont.)

ERDC TN-DOER-N5

July 2000

consolidation potential of three elements: the cap, the contaminated dredged material, and the native

or substrate sediments. Quantifying consolidation is necessary to meet environmental, regulatory,

and economic requirements. Quantification of consolidation addresses three issues.

First, changes in elevation due to consolidation must be differentiated from those due to

erosion. Decreases in elevation of the surface of the deposit caused by erosion will likely

require remedial actions to replenish and restore the cap to its required thickness.

Second, consolidation should be considered when determining long-term site capacity. As

the CAD deposit consolidates and decreases in elevation, volume within the facility becomes

available for storage of additional dredged material.

Third, a consolidation analysis will provide data needed to evaluate the potential movement

of pore water from the contaminated sediment upward into the cap, a necessary analysis in

evaluating the potential for long-term flux of contaminants.

While hydraulic dredging may result in a CAD deposit with sand or clay ball deposits immediately

below the discharge pipeline and thus segregated from the majority of the fill, the remainder of the

dredged material is normally relatively homogeneous as a result of mixing during dredging and

disposal. The undefined and unknown macrotexture of mechanically dredged and subaqueously

placed dredged material deposits makes analysis of these deposits extremely difficult. When some

materials are mechanically dredged, almost all of the clump or bite of dredged material contained

within the clamshell bucket may retain its in situ consistency. In other cases (at higher water

contents), the material from each clamshell bucket may flow together to form a relatively homoge-

neous amalgamation of material with a minimum of seams, pockets, or veins of wetter material.

The geometry of not only the deposit, but also the homogeneous material and the location and extent

of heterogeneous pockets and seams, directly affects the behavior of a soil deposit. These factors

are critical for an accurate engineering assessment of soil behavior. For consolidation analyses, this

does not pose too difficult a problem, as the process of consolidation considers the gross behavior

of the mass of the deposit. The only discontinuities or rapid material changes that would have a

significant effect on consolidation would be continuous, free-draining layers (such as continuous

sand layers) which are highly unlikely to occur in typical dredged material deposits. Even if such

layers were present, they would only shorten the time required for consolidation; they would not

change the ultimate consolidation that would occur.

However, matters of shear strength present an entirely different situation. When dealing with shear

strength of geotechnical materials and shear failures in deposits of these materials, it is imperative

to remember that the average strength means nothing regarding deposit stability. While average

values can usually be used for consolidation (unless structures that cannot undergo differential

settlement are going to be built) and for many other physical, chemical, or biological properties or

conditions, average values are completely useless for assessing the strength condition and stability

of deposits. This is because the stability of a soil deposit is controlled by the weakest layer or stratum

in or under the deposit of interest. Thus one very thin weak seam will become the potential failure

plane, and the fate of the deposit depends upon the strength of this one layer. For subaqueous

dredged material deposits, particularly mechanically dredged material, conditions within the deposit

are not and cannot be determined to the level normally desired/required to conduct an exacting slope