University of Florida, Coastal and Oceanographic Engineering Department
Abstract
Part I. Relationships between the rate of bed fluidization and the rate of wave energy dissipation, by Jingzhi Feng and Ashish J. Mehta and Part II. In-situ rheometry for determining the dynamic response of bed, by David J.A. Williams and P. Rhodri Williams.
A series of preliminary laboratory flume experiments were carried out to examine the time-dependent
behavior of a cohesive soil bed subjected to progressive, monochromatic waves. The bed was an aqueous,
50/50 (by weight) mixture of a kaolinite and an attapulgite placed in a plexiglass trench. The nominal bed
thickness was 16 cm with density ranging from 1170 to 1380 kg/m 3, and water above was 16 to 20 cm
deep. Waves of design height ranging from 2 to 8 cm and a nominal frequency of 1 Hz were run for
durations up to 2970 min. Part I of this report describes experiments meant to examine the rate at which
the bed became fluidized, and its relation to the rate of wave energy dissipation. Part II gives results on
in-situ rheometry used to track the associated changes in bed rigidity.
Temporal and spatial changes of the effective stress were measured during the course of wave action,
and from these changes the bed fluidization rate was calculated. A wave-mud interaction model developed
in a companion study was employed to calculate the rate of wave energy dissipation. The dependence of
the rate of fluidization on the rate of energy dissipation was then explored.
Fluidization, which seemingly proceeded down from the bed surface, occurred as a result of the loss
of structural integrity of the soil matrix through a buildup of the excess pore pressure and the associated loss of effective stress. The rate of fluidization was typically greater at the beginning of wave action and
apparently approached zero with time. This trend coincided with the approach of the rate of energy
dissipation to a constant value. In general it was also observed that, for a given wave frequency, the larger
the wave height the faster the rate of fluidization and thicker the fluid mud layer formed. On the other
hand, increasing the time of bed consolidation prior to wave action decreased the fluidization rate due to
greater bed rigidity. Upon cessation of wave action structural recovery followed.
Dynamic rigidity was measured by specially designed, in situ shearometers placed in the bed at
appropriate elevations to determine the time-dependence of the storage and loss moduli, G' and G", of
the viscoelastic clay mixture under 1 Hz waves. As the inter-particle bonds of the space-filling, bed
material matrix weakened, the shear propagation velocity decreased measurably. Consequently, G'
decreased and G" increased as a transition from dynamically more elastic to more viscous response
occurred. These preliminary experiments have demonstrated the validity of the particular rheometric
technique used, and the critical need for synchronous, in-situ measurements of pore pressures and moduli
characterizing bed rheology in studies on mud fluidization.
This study was supported by WES contract DACW39-90-K-0010.
(This document contains 151 pages.
