Variability in Geotechnical Properties of Sediments and Dredged Materials

ERDC TN-DOER-D2

September 2004

Variability in Geotechnical Properties

of Sediments and Dredged Materials

PURPOSE: This technical note provides an overview of selected uncertainties involved in

estimating or characterizing pre-dredged sediment and dredged material geotechnical properties.

Variations in geotechnical properties of dredged materials and sediments are expected to be

similar to those found in typical geotechnical materials, but this technical note addresses some of

the unique aspects of identifying and evaluating the variability of selected material properties

before, during, and after dredging operations.

BACKGROUND: Characterizing dredged material sediment properties is a basic requirement

for any proposed dredging operation, and the basic geotechnical parameters need to be measured

or predicted in many cases. The most basic parameter is the material's physical property classifi-

cation based on the grain size distribution of gravel, sand, silt, and clay. Other physical proper-

ties include water content, density, specific gravity, and percent solids. Engineering behavior

properties such as shear strength and consolidation are also measured or predicted. Technical

Note DOER-N13 (www.wes.army.mil/el/dots/doer/) includes a list of the most common

geotechnical properties used in characterizing dredged materials.

The dredging process often involves radical manipulation of in situ sediments. Undisturbed (or

previously disturbed) soil and rock deposits are ripped apart, agitated, pressurized, remolded,

transported, discharged, and/or re-deposited using various dredging techniques and equipment.

Dredging may be accomplished by hydraulic suction or mechanical means (or a combination of

techniques), and various types of dredging platforms are available for each technique (Depart-

ment of the Army 1983). Once the material is dredged, it is delivered by various techniques and

equipment to its final placement (or disposal) site above or below water.

Disturbed or remolded geologic materials exhibit different engineering properties when com-

pared to their undisturbed state (Scott 1963), and most dredged materials undergo significant

remolding during transition from their in situ to final disposal states. By the time most in situ

fine-grained cohesive materials have been dredged, transported, and redeposited, their original

(pre-dredged) geotechnical properties may no longer be valid. Coarse-grained sediments in

maintenance dredging may revert to their original properties, but even they may undergo changes

in grain size distribution depending on the placement operation. The degree to which the proper-

ties have been changed depends on the techniques and equipment utilized during the dredging

and placement processes as well as the changes imposed by the surrounding environment.

This technical note describes some underlying reasons for geotechnical property variability,

including equipment and environment effects, in order to acquire a better understanding of the

relationships between selected geotechnical properties and dredging processes. General trends in

property variability were measured by obtaining and testing several dredged materials and by