Ocean wave-soil-caisson interaction Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2r36v148d

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  • Caissons on permeable seabeds have been designed and constructed for a variety of needs in coastal and offshore engineering. An evaluation of the adequacy of the foundation beneath the structure is required for an economic and safe design. To address this requirement, a two-dimensional analytical model of the foundation response is developed. The caisson is assumed to be placed on a rubblemound bedding layer overlying a soil of finite depth. The soil is considered to be homogeneous, isotropic, and linearly poroelastic. The soil responses, including displacements, stresses and porewater pressure, are modeled by Biot consolidation theory. This theory couples the soil skeleton motion and fluid flow. A boundary layer approximation technique is employed which enables the soil motion and pore pressure in the Biot theory to be solved separately. Linearity allows the wave-soil-caisson system to be decomposed into scattering and radiation problems. In the scattering problem, the caisson is assumed to be fixed and the soil response depends on the wave pressure on the mudline alone. In the radiation problem, the soil and wave forces on the caisson determined in the scattering problem impose caisson motion which in turn forces the soil response. Both problems are solved for the total stress employing classical elasticity. In the vicinity of the mudline, the boundary layer approximation yields one-dimensional Terzaghi consolidation theory to adjust the soil response due to fluid flow. In solving both scattering and radiation problems, a mixed-type boundary condition arises at the mudline. This mathematical complication is simplified by applying a solution technique developed for a contact problem of a rectangular stamp on a thin elastic layer. This thin layer limitation is to ensure negligible shear stress and vertical normal stress along the exposed surface of the seabed. The analytical solution is verified by comparison with a numerical solution developed for an elastic soil of finite depth. Results are in good agreement. In addition, large-scale experiments were conducted at the 0. H. Hinsdale Wave Research Facility at Oregon State University. The wave flume is 342 ft long, 12 ft wide, and 15 ft deep. A 10-ft-high, 8-ft-long and 4-ft-wide caisson was placed on a bedding layer and 1 to 3 ft of underlying soil. The caisson was exposed to 0.68 to 4.40 ft waves with periods of 1.77 to 8.84 sec. Wave pressures were measured on the face of the caisson. Pore pressures were monitored in the bedding layer and soil under the caisson. Displacements of the caisson were also monitored. Comparison indicates that the theory may underestimate the horizontal and vertical displacements, but the predicted pore pressure is in good agreement with the data.
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