Laboratory experiments on cohesive soil bed fluidization by water waves (M.S. Engineering Thesis)

Feng, Jingzhi (1992) Laboratory experiments on cohesive soil bed fluidization by water waves (M.S. Engineering Thesis). Gainesville, FL, University of Florida, Coastal and Oceanographic Engineering Department, (UFL/COEL, 92/005)

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The mechanism by which fluid mud is formed by water wave motion over coastal and estuarine cohesive soil beds is of evident interest in understanding and interpreting the microfabric of flow-deposited fine sediments in shallow waters, and hence the erodibility of muddy beds due to hydrodynamic forcing. This study investigated water wave-induced fluidization of cohesive soil beds composed of a 50/50 (by weight) mixture of a commercial attapulgite and a kaolinite in a laboratory flume. 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 previously developed hydrodynamic wave-mud interaction model of the two-layered water-mud system was employed to study the nature and the degree of wave dissipation, in terms of energy dissipation rate, during the bed fluidization process. By evaluating the mud rheological properties separately, a mud viscosity model was developed, which was then used in conjunction with the wave-mud interaction model to obtain an effective sheared thickness of the bed resulting from wave action. This thickness, considered to be a representative of the fluidized mud thickness, was compared with the latter obtained from pressure measurements. Also, through this wave-mud model the relationship between the rate of fluidization and the rate of wave energy dissipation during fluidization was examined. In general, for a given wave frequency, a larger wave fluidized the bed at a faster rate and to a greater depth than a smaller one. Furthermore, increased bed consolidation time decreased the rate of fluidization due to increased mud rigidity. The rate of bed fluidization was typically greater at the beginning of wave action and decreased with time. Eventually this rate approached zero, while in some cases the wave energy dissipation rate approached a constant value, which increased with wave height. As the fluidization rate approached zero, there appeared to occur an equilibrium value of the bed elevation, and hence a fluid mud thickness, for a given wave condition. During the fluidization process the bed apparently lost its structural integrity by loss of the effective stress through a build-up of the excess pore water pressure. After wave action ceased, the bed structure exhibited recovery by dissipation of the excess pore water pressure. Further studies will be required in which the hydrodynamic model must be improved via a more realistic description of mud rheology and relaxation of the shallow water assumption, and better pressure data must be obtained than in the present study. Nevertheless, this investigation has been instructive in demonstrating relationships between the degree of mud fluidization, wave energy dissipation and bed consolidation time, and thus offers insight into an important mechanism by which coastal and estuarine muds are eroded by wave action. (Document has125 pages.)

Item Type: Monograph or Serial Issue
Title: Laboratory experiments on cohesive soil bed fluidization by water waves (M.S. Engineering Thesis)
Personal Creator/Author:
Feng, Jingzhi
Series Name: UFL/COEL
Number: 92/005
Date: 1992
Publisher: University of Florida, Coastal and Oceanographic Engineering Department
Place of Publication: Gainesville, FL
Additional Information: Thesis, M.S., Engineering
Uncontrolled Keywords: Muds; Rheology; Waves
Subjects: Oceanography
Earth Sciences
Item ID: 497
Depositing User: Stephanie Haas
Date Deposited: 19 Jan 2008 00:33
Last Modified: 29 Sep 2011 21:58

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