Evaluation of the Shear Modulus in Models for Shallow-Foundation Dynamics within the Elastic Domain

The aim of this presentation is to examine the relationship of the equivalent homogeneous shear moduli used in impedance models with stresses under the footing, through the use of scaled models in the centrifuge and an impact loading. The analysis of time and frequency vertical responses of footings reveals that reflections on the boundaries are negligible. The frequency response of a series of circular and square footings is shown to be rather easily-fitted with Wolf’s models for foundations on an infinite half-space with reasonably consistent parameters for masses, damping and shear moduli. The damping is nearly constant, yet significantly lower than in a prototype scale with real soil. The mass is fitted with a greater level of scatter. The correlation of shear modulus to the square root of the minimum mean stress appears to be better than that to the square root of the uniform stress under the footing

Identification of Different Seismic Waves Generated by Foundation Vibration in the Centrifuge: Travel Time, Spectral and Numerical Investigations

For the analysis of footings under dynamic loading scaled modeling in the centrifuge assumes that the soil behaves like at prototype scale. This paper demonstrates that for a container filled with dry sand, wave velocities can be described by a model based on the relation between the shear modulus and the depth dependent stress level proposed by Iwasaki and Tatsuoka. A preliminary estimation of the shear wave velocities and of the Poisson’s ratio confirms by dynamical measurements the currently use value of 0.25. A FEM modeling also helps to strengthen the validity of the model proposed, providing another insight in the propagation of waves in a soil with a velocity gradient

Interaction Site-Ville : Approches expérimentales et numériques

À l’échelle d’une ville, les structures de surface telles que les bâtiments peuvent modifier le mouvement sismique en 'champ libre' et agir comme des sources sismiques secondaires. Des observations ont en particulier été réalisées sur des données réelles. Elles montrent que cet effet peut être significatif. La conséquence directe de cette 'interaction site-ville' est la pollution du mouvement sismique en milieu urbain par un champ d’onde secondaire. Des modélisations en centrifugeuse et numériques tendent à confirmer que ce phénomène n’est pas anecdotique. En particulier, ces résultats montrent qu’entre deux bâtiments proches des interactions existent, modifiant le mouvement du sol mais aussi la réponse des structures impliquées. À l’échelle d’une ville, ce phénomène sera d’autant plus marqué lorsqu’un fort couplage existe entre la réponse du sol et la réponse du milieu urbain

Caracteristiques physiques et mecaniques locales du bois dans la zone des noeuds

INIST T 76198 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc

乾燥砂地盤における沈下挙動に対する拡張型相似則の適用性

遠心場において拡張型相似則の検証を行う．着目したのは乾燥砂地盤の振動による沈下量である．同相似則の適用性の確認は、測定された各物理量の実物スケール換算値が全実験ケースで一致することをもってなされる。模型地盤はフォンテーヌブロー砂による水平成層乾燥砂地盤(Dr=40%)であり，加速度計10台とレーザー式変位計1台および沈下計3台を設置した。実験は模型縮尺を1/100とし、25gと50gの遠心場における地盤の挙動を計測した．実験ケースは，全5ケースである。各ケースにおいて，正弦波加振を10回行った．また，加振前と10回の加振後に貫入抵抗の計測を行った．入力加速度については，Arias Intensityを計算し，プロトタイプスケールでほぼ一様な入力がなされていることを確認した．地表面の沈下量について，加振第1回目の実物換算値は全ケースで約900mmとなり，同相似則の適用性が確認できた．To verify the generalized scaling law, dynamic centrifuge tests under two different centrifugal accelerations of 25 g and 50 g are conducted. The model ground constitutes of a flat dry sand layer. With the scaling law, a prototype ground is scaled down to 1/100. A sinusoidal input acceleration of frequency 1.0 Hz, maximum amplitude 0.5 g, and duration 14 sec in prototype scale is applied to the model ground. Each model is exposed to the identical input motion sequentially 10 times. In total nine accelerometers are installed in the model. Surface settlements are measured by laser displacement transducers. Settlements at three different depths (300, 200 and 50 mm - model scale - from the surface) are measured by settlement gauges. Measured settlements after the initial shake in prototype scale show agreements between the two models when the intensity of shaking is nearly identical.遠心場において拡張型相似則の検証を行う．着目したのは乾燥砂地盤の振動による沈下量である．同相似則の適用性の確認は、測定された各物理量の実物スケール換算値が全実験ケースで一致することをもってなされる。模型地盤はフォンテーヌブロー砂による水平成層乾燥砂地盤(Dr=40%)であり，加速度計10台とレーザー式変位計1台および沈下計3台を設置した。実験は模型縮尺を1/100とし、25gと50gの遠心場における地盤の挙動を計測した．実験ケースは，全5ケースである。各ケースにおいて，正弦波加振を10回行った．また，加振前と10回の加振後に貫入抵抗の計測を行った．入力加速度については，Arias Intensityを計算し，プロトタイプスケールでほぼ一様な入力がなされていることを確認した．地表面の沈下量について，加振第1回目の実物換算値は全ケースで約900mmとなり，同相似則の適用性が確認できた．To verify the generalized scaling law, dynamic centrifuge tests under two different centrifugal accelerations of 25 g and 50 g are conducted. The model ground constitutes of a flat dry sand layer. With the scaling law, a prototype ground is scaled down to 1/100. A sinusoidal input acceleration of frequency 1.0 Hz, maximum amplitude 0.5 g, and duration 14 sec in prototype scale is applied to the model ground. Each model is exposed to the identical input motion sequentially 10 times. In total nine accelerometers are installed in the model. Surface settlements are measured by laser displacement transducers. Settlements at three different depths (300, 200 and 50 mm - model scale - from the surface) are measured by settlement gauges. Measured settlements after the initial shake in prototype scale show agreements between the two models when the intensity of shaking is nearly identical

Interaction inertielle et interaction cinématique sol - pieu

Le présent travail vise d'abord à étudier expérimentalement les interactions inertielle et cinématique sol-pieu dans un sol non-cohérent à l'aide de modèles réduits centrifugés, à construire ensuite une plate forme de modélisation numérique dynamique non-linéaire de la réponse temporelle du système sol-pieu et à proposer une première génération des modèles d'élément d'interface de type Winkler calés sur les résultats expérimentaux. Les essais sous impact en tête fournissent des données sur l'interaction inertielle pure, les essais sous séisme sans chevêtre, puis avec chevêtre permettent d'observer respectivement l'interaction cinématique pure et l'interaction couplée inertielle et cinématique. Les boucles P - réaction du sol Y déplacement du pieu aux différentes profondeurs sont établies et analysées dans les trois situations. La modélisation numérique de type Winkler dynamique non-linéaire 1D implémente l algorithme de Newton-Raphson et l algorithme de Hilbert-Hugues-Taylor. Des modèles d'éléments d'interface sont choisis dans la littérature parmi ceux fondés sur des paramètres physiques. La plate-forme numérique permet de proposer un calage des paramètres de ces éléments sur les résultats expérimentaux pour reproduire les trois types d interaction. La séparation des interactions inertielle et cinématique a été observée expérimentalement pour la première fois dans le présent travailThe present work is first an experimental study of the inertial and the kinematic soil pile interaction of a single pile in a non-coherent soil through reduced scale model tests in the centrifuge. Then a numerical modelisation is built to compute the nonlinear dynamic response of the soil-pile system in the time domain. The experimental results make it possible to build a first generation of Winkler interface elements numerical models. Impacts experiments on a single pile with a pile cap provide an insight in the purely inertial interaction. Tests in the earthquake simulator embarked in the centrifuge give also experimental data about the purely kinematic interaction (without pile cap) and the combination of the two forms of interaction (with a pile cap). P-y loops (Soil reaction Pile displacement) are established for the three different experimental conditions and reveal important differences. The numerical model is that of the 1D Winkler beam on nonlinear interface punctual elements. It implements the modified Newton algorithm and the Hilbert-Hugues-Taylor algorithm. The different interface elements based on physical parameters are chosen from the literature. They are fitted on the experimental data to reproduce the three forms of interactions. A separation of the inertial type interaction and the kinematic type has been experimentally observed for the first time in the present workNANTES-BU Sciences (441092104) / SudocNANTES-Ecole Centrale (441092306) / SudocSudocFranceF

Site-city interaction. In : Assessing and managing earthquake risk

Structural and geotechnical engineers have been aware for a long time of soil-structure interaction (SSI) phenomena, which modify the seismic response of massive or tall buildings erected on soft soils. On their side, seismologists have known for a long time that it is not a good idea to install seismological stations close to trees. During the last decades, it has also become clear how large the effects of surface heterogeneities, commonly called 'site effects' (SE, concerning soft soils as well as topographic features), can be. On this basis, it is legitimate to wonder whether a large building on a soft soil can contaminate the ground motion in its immediate vicinity (phenomenon hereafter abbreviated as 'CGMB', Contamination of Ground Motion by Buildings). Going one step further, we may ask about the overall effect of such contamination in a densely urbanized area. The aim of this chapter is to present an overview of the results that substantiate the plausibility of this kind of 'global' interaction between all the buildings of a city and its subsoil, which we call hereafter 'site-city interaction' (SCI)

Site-city interaction. In : Assessing and managing earthquake risk

Structural and geotechnical engineers have been aware for a long time of soil-structure interaction (SSI) phenomena, which modify the seismic response of massive or tall buildings erected on soft soils. On their side, seismologists have known for a long time that it is not a good idea to install seismological stations close to trees. During the last decades, it has also become clear how large the effects of surface heterogeneities, commonly called 'site effects' (SE, concerning soft soils as well as topographic features), can be. On this basis, it is legitimate to wonder whether a large building on a soft soil can contaminate the ground motion in its immediate vicinity (phenomenon hereafter abbreviated as 'CGMB', Contamination of Ground Motion by Buildings). Going one step further, we may ask about the overall effect of such contamination in a densely urbanized area. The aim of this chapter is to present an overview of the results that substantiate the plausibility of this kind of 'global' interaction between all the buildings of a city and its subsoil, which we call hereafter 'site-city interaction' (SCI)

Seismic hazard in urban environments : Can man modify the hazard ?

It is most generally admitted that seimic ground motion is the convolution of source, path and site effects. The question addressed here is whether a fourth term, called "Site-City Interaction" (SCI), should be added for densely urbanized areas, corresponding to feedback effects from the shaken buildings in the soil. After a short overview of various, consistent observations, several specific experimental and numerical results are briefly summarized, that clearly evidence the wave radiation from isolated buildings and the possibilty of interactions between close buildings as well. The seismic response of a simple "city model" represented by a group of buildings is then addressed through a series of numerical simulations, which allow to determine under which conditions such SCI effects may be significant. These findings are consistent with a very simple "rule of thumb" approach based on the comparison between kinematic energies in soils and buildings, pointing to the significance of SCI effects for dense cities where building and soil periods coincide. The last section thus discusses the practical consequences of such SCI effects, and some possible directions to get unambiguous experimental evidence in real cities