Figure 3. Yield stresses for some natural muds and kaolinite

ERDC TN-DOER-N7

August 2000

and γ = fluid shear-rate. The two models are equivalent when the Herschel-Bulkley exponent n = 1.

Viscous characteristics of channel muds are discussed by Teeter (1992a). Yield stress data are

shown in Figure 3 for kaolinite clay preparations and two natural sediments. Kaolinite is a model

clay often used in laboratory experiment, but, as can be seen in the Figure 3, its yield stress is much

lower for a given solids content than the natural muds from Rotterdam Harbor, Netherlands, or

Calcasieu Channel, LA. Data in Figure 3, with the exception of that of Huang and Garcia (1998),

were obtained using controlled stress rheometry. The Herschel-Bulkley model allows for shear

thinning and yield stress which have been observed in natural muds.

Figure 3. Yield stresses for some natural muds and kaolinite

As mud flows slowly, a yield surface appears above which there is plug flow (no shear) as in the

laminar velocity profile described earlier. Shear stress at the yield surface is equal to the yield stress

of the material. Shear in the flow occurs between the bottom of the plug-flow zone and the bed.

When the yield surface intersects the bed, the mudflow "freezes." That is, the minimum shear stress

required for flow to occur is equal to the yield stress.