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

Abstract

The mechanism by which fluid mud is formed by water wave motion over coastal andestuarine cohesive soil beds is of evident interest in understanding and interpreting themicrofabric of flow-deposited fine sediments in shallow waters, and hence the erodibilityof muddy beds due to hydrodynamic forcing. This study investigated water wave-inducedfluidization of cohesive soil beds composed of a 50/50 (by weight) mixture of a commercialattapulgite and a kaolinite in a laboratory flume. Temporal and spatial changes of theeffective stress were measured during the course of wave action, and from these changesthe bed fluidization rate was calculated. A previously developed hydrodynamic wave-mudinteraction model of the two-layered water-mud system was employed to study the natureand the degree of wave dissipation, in terms of energy dissipation rate, during the bed fluidizationprocess. By evaluating the mud rheological properties separately, a mud viscositymodel was developed, which was then used in conjunction with the wave-mud interactionmodel to obtain an effective sheared thickness of the bed resulting from wave action. Thisthickness, considered to be a representative of the fluidized mud thickness, was comparedwith the latter obtained from pressure measurements. Also, through this wave-mud modelthe relationship between the rate of fluidization and the rate of wave energy dissipationduring fluidization was examined. In general, for a given wave frequency, a larger wave fluidized the bed at a faster rateand to a greater depth than a smaller one. Furthermore, increased bed consolidation timedecreased the rate of fluidization due to increased mud rigidity. The rate of bed fluidizationwas typically greater at the beginning of wave action and decreased with time. Eventuallythis rate approached zero, while in some cases the wave energy dissipation rate approached aconstant 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 mudthickness, for a given wave condition. During the fluidization process the bed apparentlylost its structural integrity by loss of the effective stress through a build-up of the excesspore water pressure. After wave action ceased, the bed structure exhibited recovery bydissipation of the excess pore water pressure.Further studies will be required in which the hydrodynamic model must be improved viaa 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, thisinvestigation has been instructive in demonstrating relationships between the degree of mudfluidization, wave energy dissipation and bed consolidation time, and thus offers insight intoan important mechanism by which coastal and estuarine muds are eroded by wave action. (Document has125 pages.)