| A plane strain analysis based on Biot's theory of consolidation is utilized to investigate the pore-water pressures and displacements induced by steady-state linear planar waves beneath offshore gravity structures placed on poroelastic seabed of finite thickness. The response is characterized by three controlling parameters, i.e. poro-elasticity factor, fluid compressibility factor, and Poisson's Ratio.; A parametric study, using a range of the controlling parameters encountered in practice, indicates that the response is governed primarily by the poro-elasticity factor and to a lesser extent by Poisson's Ratio (fluid compressibility factor has negligible influence for saturated soils). The assumption of limiting cases of "drained" and "undrained" behavior can be made most rationally in terms of the poro-elasticity factor and the effective drainage path. For the offshore gravity structures (typically more than 80 m in diameter), the foundation response approaches essentially the undrained condition. However, the use of the simplified uncoupled analysis for the response in the undrained case is shown to be inconsistent with the boundary conditions. For such structures, the wave-induced moment and horizontal force exerted by the structure on the foundation contribute most to the overall response while the influence of sea water pressure penetration through the sea bottom is, in general, negligible. For smaller structures, however, the latter influence dominates the response for soils as fine as silty sand. The maximum shear strain amplitudes evaluated imply that the soil response remains primarily in the linear elastic range, even for severe storm loadings.; An experimental procedure was developed for the measurement of a composite poro-elastic property which combines all the pertinent soil properties. Two test configurations are found suitable depending on the hydraulic conductivity of the soil considered. The results of tests on nine specimens reconstituted from three soil samples conform with the theoretical estimates and indicate that the developed procedure is viable. The evaluated Poisson's Ratio depends on the cyclic frequency and stress within the range of test variables employed. |
