Wave-induced Effective Stresses Research Articles

Transient wave-induced pore-water-pressure and soil responses in a shallow unsaturated poroelastic seabed

An analytical solution is developed for studying transient water wave-induced responses inside an unsaturated poroelastic seabed of finite thickness. The soil skeleton and the pore fluid are compressible and the constitutive relationship of the soil skeleton is described by Hooke's law. Assuming that the horizontal length scale of wave motion is much larger than the seabed thickness, the leading-order analytical solutions for the seabed responses, including pore fluid pressure and soil skeleton motion, are obtained. The present solutions are suitable for general transient wave loading and for the shear modulus of the soil skeleton being of the same order of magnitude as the effective bulk modulus of elasticity of the pore fluid. The present theory is first validated by checking the solutions with the experimental data for the pore pressure induced by periodic-wave loading. The present analytical solutions are then used to investigate the seabed responses under transient waves, including linear periodic wave, a solitary wave and a bore. The effects of the wave-induced effective stresses on the bed failure potential are further analysed. The results show that the shear failure potential and its duration are highly dependent on the soil properties, such as saturation degree, shear modulus and permeability. Sensitivity analyses are presented.

An experimental study on non-linear progressive wave-induced dynamic stresses in seabed

In this study, the dynamic stresses within the seabed induced by non-linear progressive waves were explored through a series of hydraulic model tests on a movable bed within a wave flume. By comparing Stokes’ 2nd-order wave theory with the theory of wave-induced dynamic stresses within the seabed as proposed by Yamamoto et al. [1978. On the response of a poro-elastic bed to water waves. Journal of Fluid Mechanics 87 (1), 193–206.] and Hsu and Jeng [1994. Wane-induced soil response in an unsaturated anisotropic seabed of finite thickness. International Journal for Numerical and Analytical Methods in Geomechanics 18, 785–807], the experimental results show that the pressure on the seabed surface, the pore water pressure within the seabed as well as the vertical and the horizontal stresses are all smaller than their theoretical values. If we were to obtain the characteristics of seabed soil, the analytical solution of Hsu and Jeng [1994. Wane-induced soil response in an unsaturated anisotropic seabed of finite thickness. International Journal for Numerical and Analytical Methods in Geomechanics 18, 785–807] might agree to the simulation of the wave-induced effective stresses and shear stress in the sandy seabed. A different phase shift exists among all the three soil stresses. Their influences on the three dynamic stresses within seabed soil are important for seabed stability, and can be used in the verification of numerical models. In the whole, the non-linear progressive waves and the naturally deposited seabed are found to have a strong interaction, and the behavior of the induced dynamic stresses within the seabed is very complicated, and should be investigated integrally.

Effects of dynamic soil behavior and wave non-linearity on the wave-induced pore pressure and effective stresses in porous seabed

Most previous investigations for the wave-induced soil response have only considered the quasi-static soil behavior under linear wave loading. However, it is expected that the dynamic soil behavior and wave non-linearity will play an important role in the evaluation of wave-induced seabed response. In this paper, we include dynamic soil behavior and wave non-linearity into new analytical models. Based on the analytical solution derived, the effects of wave non-linearity on the wave-induced seabed response with dynamic soil behavior are examined. Numerical results demonstrate the significant effects of wave non-linearity and dynamic soil behavior on the wave-induced effective stresses. The applicable range of dynamic and quasi-static approximations is also clarified for engineering practice.

Wave-Induced Seabed Instability: Difference Between Liquefaction and Shear Failure

The mechanism of wave-induced instability in a permeable seabed have been studied for more than two decades. The distinction between shear failure and liquefaction, however, has not been clearly defined. This paper presents a fundamental study on the differences in two failure modes for a fully saturated seabed of both finite and infinite thickness. The wave-induced effective stresses and pore pressure, obtained from an analytical solution of Biot’s pore-elastic consolidation theory, were employed to examine the failure modes under a two-dimensional plane strain condition. A case study is presented to examine the failure modes with respect to several parameters, such as excess pore pressure, seepage flow, seepage force, failure areas and stress path in the seabed. The conclusions obtained from this study were as follows; (1) the thickness of a permeable seabed affects the pore pressure and effective stress response to ocean waves and the failure mode of the seabed, (2) either a liquefaction or shear failure, or both, occur in the seabed, even in the saturated seabed, (3) the Mohr-Coulomb’s failure criterion, when combined with elastic stresses, can not be employed to estimate the liquefaction failure in the seabed, (4) the liquefaction can be evaluated by a criterion in terms of the excess pore pressure, (5) The liquefied zone in the seabed is significantly different from the shear failure zone. The shape beneath the seabed surface for the former is almost identical to the contour where the upward seepage flow is concentrated.

Discussion: A simplified finite element analysis of wave-induced effective stresses and pore pressures in permeable sea beds

Discussion: A simplified finite element analysis of wave-induced effective stresses and pore pressures in permeable sea beds

Simplified finite element analysis of wave-induced effective stresses and pore pressures in permeable sea beds : Gatmiri, B GeotechniqueV40, N1, March 1990, P15–30

Simplified finite element analysis of wave-induced effective stresses and pore pressures in permeable sea beds : Gatmiri, B GeotechniqueV40, N1, March 1990, P15–30

A simplified finite element analysis of wave-induced effective stresses and pore pressures in permeable sea beds

The wave-induced pore pressures and effective stresses in a saturated submarine sediment are treated by means of the finite element method. By using the general theory of behaviour of the saturated poro-elastic media presented by Biot (1972, 1973), field equations of motion are established. By introducing a valid approximation for very slow phenomena, the full formulation is simplified and the well-known equations of consolidation are obtained. Stability conditions and accuracy of the process of solution are discussed. A verification of the written finite element program is made by a comparative study with the infinite depth solution given by Yamamoto (1978) and Madsen (1978). The effects of bed thickness, permeability and soil stiffness on the wave-induced pore pressures, effective normal stresses, shear stresses, horizontal and vertical displacement at the mud line and with depth are investigated by various examples. The response of a heterogeneous sea bed to a harmonic wave loading is compared with a homogeneous case. As an example of application, the vertical distribution of wave-induced pore pressures and effective stresses within the submarine sediments limited by a sloping surface are considered. La méthode par éléments finis est employée pour traiter les pressions interstitielles induites par les vagues et les contraintes effectives dans un sédiment saturé sous-marin. Les équations de mouvement dans le champ sont établis à l'aide de la théorie générale présentée par Biot (1972, 1973) pour le comportement des matières poroélastiques saturées. La formulation complète est simplifiée et les équations connues de consolidation sont obtenues par l'introduction d'une approximation valable pour des phénomènes très lents. L'article discute les conditions de stabilité et la précision de la méthode de solution. Le programme écrit en elements finis est vérifié par moyen d'une comparaison avec la solution à profondeur infinie donnée par Yamamoto (1978) et Madsen (1978). Des examples sont donnés pour étudier les effets de l'epaisseur de la couche, de ia perméabilité et de la rigidité du sol sur les pressions interstitielles induites par les vagues, les contraintes normales effectives, les contraintes de cisaillement et le déplacement horizontal et vertical à la surface du fonds marin et avec la profondeur. La réponse d'un fonds marin hétérogène au chargement harmonique par les vagues est comparée avec un cas homogène. L'article présente comme exemple pratique d'application la distribution verticale des pressions interstitielles induites par les vagues et des contraintes effectives à l'intérieur de sédiments sous-marins limités par une surface en pente.

Wave-induced pore pressures and effective stresses in inhomogeneous seabed foundations

Stability analyses of homogeneous and inhomogeneous seabed foundations under attack by storm waves are made by calculating the wave-induced effective stresses. Wave-induced effective stress analysis of homogeneous seabed is made using the theory previously developed by the author which is based on the poro-elastic theory of Biot. Effective stresses in inhomogeneous seabeds induced by waves are calculated by approximating aa inhomogeneous bed by many layers of homogeneous soils each of which has different geotechnical soil properties. A good agreement is obtained between the theory and the pore pressure data from in situ field measurements. For a given wavelength, it is found that there exists a most unstable thickness of homogeneous seabed when the thickness is one-fifth of the wave length. As a realistic example of an inhomogeneous bed, the effective stresses in a typical seabed formation at the Mississippi Delta area of the Gulf of Mexico under the attack of design storm waves are calculated. The numerical results indicate that the storm waves induce a continuous submarine landslide which extends as deep as 9 m from the mud line. Numerical calculations also indicate that such landslides and liquefaction of seabeds can be prevented by placing a layer of concrete blocks or rubble on top of the seabeds.

Wave-induced pore pressures and effective stresses in a porous bed

A general theory for the pore pressures and effective stresses induced in a porous bed by ocean waves is developed. The pore fluid as well as the soil skeleton are considered compressible and the flow in the porous bed is assumed governed by Darcy's Law for an anisotropic medium. The general solution methods for a homogeneous bed of arbitrary thickness as well as for a horizontally layered bed are presented. Explicit solutions for pore pressures and effective stresses are given for an infinite homogeneous bed. The nature of these solutions' dependency on relative compressibility of pore fluid and soil skeleton, soil permeability and hydraulic anisotropy is examined in detail and illustrated by example calculations. It is found that an extremely simple form of the general solution is appropriate in most cases when the bed consists of soils finer than medium sand. The geotechnical implications of the wave-induced effective stresses are discussed in some detail. It is found that storm waves may induce stresses in cohesionless soils as well as in normally consolidated clays approaching or exceeding failure conditions. Une théorie général est développée pour les pressions interstitielle et les contraintes effectives produites dans une couche poreuse par les vagues de l'océan. Le fluide du pore ainsi que le squelette du sol sont considérés comme compressibles et l'écoulement dans le lit poreux est gouverné par la Loi de Darcy pour un milieu anisotrope. Les méthodes de solution générale sont présentées pour un lit homogène d'une épaisseur arbitraire ainsi que pour un lit à couches horizontales. Des solutions explicites pour des pressions interstitielles et des contraintes effectives sont données pour un lit homogène infini. La nature de la relation de ces solutions avec la compressibilité relative du fluide du pore et du squelette du sol, la perméabilité du sol et l'anisotropie hydraulique est examinée en détail et illustrée par des exemples de calculs. On constate qu'une forme extrêmement simple de la solution générale est appropriée dans la plupart des cas où le lit est constitué par un sol plus fin qu'un sable moyen. Les conséquences géotechniques des contraintes effectives produites par les vagues sont présentées en détail. On constate que les vagues de tempête peuvent produire des contraintes dans des sols non cohérents aussi bien que dans des argiles normalement consolidées approchant ou dépassant les conditions de rupture.