Stiff Soil Layer Research Articles

Analysis of silt behavior induced by water waves

Based on the weak non-elastic porous model, the expressions of pore pressure, effective stress and displacements of soil skeletal frame and pore water have been deduced for a finite depth seabed. The distributions of several physical parameters have been analyzed for three kinds of marine sediment, including pore pressure, effective stress, stress angle, displacement of skeletal frame and pore fluid, and the variations of elastic waves with wave period. According to the experimental results, the resonant phenomena in the silt bed and the mechanism underlying such events have been discussed. It is proposed that the existence of a stiff soil layer inside the silt bed is a necessary condition for resonance to occur, and the possible location of resonance can be forecasted.

Kinematic seismic response and bending of free-head piles in layered soil

The paper studies the kinematic response of free-head piles. Such pile deformation has triggered structural damage in many strong earthquakes. In this Paper dimensionless parametric graphs for pile bending moments are presented which pertain to characteristic two-layer soil profiles. The results are derived by using an existing rigorous dynamic finite-element code, and by implementing a realistic beam-on-dynamic-Winkler-foundation formulation specifically developed for the kinematic response of piles in layered soil. The Winkler model is shown to reproduce quantitatively even detailed trends observed in the finite-element results; a simple analytical expression is thereby developed for estimating the Winkler stiffness in terms of the local soil Young's modulus and key dimensionless pile/ soil parameters. The study concludes that even relatively flexible piles may not exactly experience the wavy and abruptly changing ground deformation of the free field. The critical region of pile distress due to such kinematic loading is shown to be at or near the interface between alternating soft and stiff soil layers. The magnitude of the bending moment at such critical interface locations depends mainly on the stiffness contrast of the two layers through which the pile penetrates, the excitation frequency and the relative rigidity of the pile. A constraining cap may exert an important effect on such kinematic deformations. L'article étudie la réponse cinématique de pieux à tete-libre. La déformation de ces pieux a en effet occasionné des endommagements structuraux lors de nombreux tremblements de terre de forte amplitude. L'article propose des graphiques à paramètres adimensionnels permettant de calculer le moment fléchissant des pieux et de caractériser des sols bicouches. Les résultats sont calculés à l'aide d'un code dynamique d'éléments finis, préexistant et rigoureux, et d'une formulation réaliste de "Beam-on-Dynamic-Winkler-Foundation" spécialement mise en oeuvre pour la réponse cinématique des pieux dans des sols stratifiés. Le modèle de Winkler permet de reproduire les tendances, même très détaillées, des résultats obtenus par éléments finis. Une expression analytique simple est alors développée. Elle permet d'estimer la raideur de Winkler en terme de module de Young local du sol et de paramètres clés adimensionnel pieu/sol. L'étude montre que meme les pieux relativement flexibles ne supportent pas totalement une déformation rapide et odulante du terrain. La zone critique des pieux sinistrés résultant d'un tel chargement cinématique se trouve au niveau ou à proximité de l'interface sol souple/sol rigide. L'amplitude du moment fléchissant au niveau de ces interfaces critiques dépend principalement du contraste de raideur existant entre les deux couches traversées par le pieu, de la fréquence d'excitation et de la rigidité relative du pieu. Un capuchon de mise sous contrainte pourrait avoir un effet important sur de telles déformations cinéma-tiques.

Effects of Multiple Modes on Rayleigh Wave Dispersion Characteristics

The effects of multiple modes on Rayleigh wave dispersion are discussed to reduce the ambiguity of uniqueness of shear wave velocity (Vs) profiles estimated by the surface wave method. Based on a review of previous studies, dispersion curves of multiple‐mode Rayleigh waves induced by harmonic vertical point loading are derived for both vertical and horizontal particle motions. Also presented is the variation with frequency of the amplitude ratio between horizontal and vertical particle motions. Numerical studies indicate that a stiff soil layer overlying a softer soil layer induces a higher mode or multiple modes, leading to an inversely dispersive characteristic. Consideration of the effects of higher modes is strongly recommended in the inverse process when the observed data show an inversely dispersive trend. The ambiguity of uniqueness of the inverted soil profiles may be reduced by using either the dispersion data of horizontal motion or the amplitude ratio of particle motions in addition to the disp...

The dynamic response of gravity platforms

AbstractGravity platforms, together with associated regions of foundation and fluid, are idealized using the finite element method. Radiation of wave energy away from the platform region is modelled by appropriate damping applied to the boundary of the foundation mesh. Response results, due to sinusoidally applied forces, are calculated for seven platform‐foundation configurations. Of particular interest are those results for various assumed foundation conditions, including the presence of stiff soil layers. The response of these platforms due to North Sea storm conditions is also estimated. It is concluded that a simple static analysis of the platform‐foundation system does not necessarily give an accurate prediction of the response under extreme loading conditions. It is proposed that each prototype platform must be assessed on its individual merits, particular attention being given to prevailing geotechnical conditions.