Laboratory experiments on cohesive soil bed fluidization by water waves

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-dependentbehavior 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 bedthickness was 16 cm with density ranging from 1170 to 1380 kg/m 3, and water above was 16 to 20 cmdeep. Waves of design height ranging from 2 to 8 cm and a nominal frequency of 1 Hz were run fordurations up to 2970 min. Part I of this report describes experiments meant to examine the rate at whichthe bed became fluidized, and its relation to the rate of wave energy dissipation. Part II gives results onin-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 developedin a companion study was employed to calculate the rate of wave energy dissipation. The dependence ofthe 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 lossof 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 andapparently approached zero with time. This trend coincided with the approach of the rate of energydissipation to a constant value. In general it was also observed that, for a given wave frequency, the largerthe wave height the faster the rate of fluidization and thicker the fluid mud layer formed. On the otherhand, increasing the time of bed consolidation prior to wave action decreased the fluidization rate due togreater bed rigidity. Upon cessation of wave action structural recovery followed.Dynamic rigidity was measured by specially designed, in situ shearometers placed in the bed atappropriate elevations to determine the time-dependence of the storage and loss moduli, G' and G", ofthe viscoelastic clay mixture under 1 Hz waves. As the inter-particle bonds of the space-filling, bedmaterial matrix weakened, the shear propagation velocity decreased measurably. Consequently, G'decreased and G" increased as a transition from dynamically more elastic to more viscous responseoccurred. These preliminary experiments have demonstrated the validity of the particular rheometrictechnique used, and the critical need for synchronous, in-situ measurements of pore pressures and modulicharacterizing bed rheology in studies on mud fluidization.This study was supported by WES contract DACW39-90-K-0010.(This document contains 151 pages.)

Bibliographic Details

Main Authors:	Feng, Jingzhi, Mehta, Ashish J., Williams, David J. A., Williams, P. Rhodri
Format:	monograph biblioteca
Language:	English
Published:	University of Florida, Coastal and Oceanographic Engineering Department 1992
Subjects:	Engineering, Cohesive sediments, Resuspension, Energy dissipation, Rheology, Fluidization, Rheometry, Fluid mud, Water waves, Pore pressures,
Online Access:	http://hdl.handle.net/1834/18393
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