Physical Properties and Engineering Behavior

ERDC TN-DOER-D1

August 2004

Physical Properties and Engineering Behavior. The most common physical property

needed for dredged material characterization is the grain-size distribution, based on either the

weight (which is the standardized method) or volume measurement. The material classification is

then selected using the grain-size distribution and the Atterberg limit criteria. Other material

properties include water content, specific gravity, organic content, bulk density, percent solids,

and void ratio. Engineering behavior properties include shear strength, consolidation behavior,

hydraulic conductivity (permeability), critical erosion stress, and viscosity.

Undisturbed native soil and rock materials exhibit mechanical properties that are influenced by

the material structure. Intergranular bonding and physico-chemical effects influence the materi-

al's behavior (Mitchell 1993). As the water content in the material matrix is increased during

remolding and the solid particles become more separated, the material behaves more like a slurry

or suspension, and the intergranular bonding forces decrease. As the bonding and cohesive forces

decrease, the resulting shear strength decreases. If fine-grained particles (silt and clay) constitute

more than about 35 percent of the total matrix solids, the slurry will behave as a viscous material

(Spigolon 1993). Fibrous materials, high organic content, and gas bubbles are known to affect

the shear strength behavior of high-water-content soils (Klein and Sarsby 2000, Edil and Wang

2000). Examining the rheological behavior of soil slurries is not a traditional geotechnical topic,

and standardized laboratory tests for determining the engineering properties of soil slurries have

yet to be developed.

One objective for determining the engineering properties of soil slurry during nearshore dredging

operations is to establish the navigation channel bottom location (horizon) when clayey sus-

pended sediments are surficial to the channel bottom (Teeter 1992). The slurry threshold (yield)

shear stress (or the amount of stress imposed on the material to initiate its movement) is deter-

mined as a function of slurry density and viscosity for a given sediment, and the nautical horizon

is chosen based on established procedures (Dasch and Wurpts 2001). DeMeyer and Mahlerbe

(1987) determined that the threshold (yield) shear stress of most clayey slurries is less than 2 psf

(10 Pa), with in situ bulk densities of 72 to 84 pcf (1.15 to 1.35 kg/l). As the sediment consoli-

dates and develops a higher density and threshold (yield) shear stress beyond its rheological

behavior range, the geotechnical concept of effective stress (Terzaghi 1953) has been shown to

govern and describe the material's physical and mechanical behavior.

Another reason for determining the properties of soil slurries is to quantify slurry strength during

nearshore or contaminated dredged material placement operations. For example, when placing a

sand cap over contaminated dredged materials, the engineering behavior of the dredged material

must be predicted or known in order to achieve successful cap placement. If the soft dredged

material has insufficient shear strength to resist the imposed cap stress, failure will occur and the

purpose of the cap will be defeated. As the soft dredged material is allowed to consolidate from a

slurry state into a non-zero effective stress state, its ability to resist imposed stresses will signifi-

cantly increase. This phenomenon of strength increase was observed at a subaqueous capping

site by Myre et al. (2000), where the soft dredged material was allowed to consolidate for about

5 months before it achieved a shear strength of about 20 psf (1 kPa), which allowed subsequent

successful cap placement.

Measuring the Physical Properties and Engineering Behavior of Dredged Materi-

als. When fine-grained dredged material is remolded from a soft and highly compressible soil