Search

International audience; Since a few years, in France, the development and construction of offshore wind farms in different sites of the country is investigated. The wind turbines will be installed in soft carbonate rock formations (calcarenite, limestone), the mechanical properties of which can vary in a significant way according to the location of the site. Once installed, the bored piles will be sealed with grout in the hosting rock. It is well known that the mechanical behaviour of piles is mainly governed by the behaviour of the interface at the contact between the structure and the hosting rock. The study of the mechanical behaviour of these interfaces, being the weakest points in terms of mechanical resistance, is of great importance for the improvement of the design methods of such infrastructures. The shear response of the rock/grout interface is studied with laboratory experiments in the 3SR lab (Grenoble). The interface's mechanical properties are characterised by a series of shear tests under Constant Normal Stiffness (CNS), these conditions being the most representative of the in-situ conditions. Interface samples of a roughness representative of the in-situ drilling traces, are tested under different levels of applied normal stiffness. The shear response is studied for both monotonic and cyclic shear paths, while all tests are performed under wet conditions. The failure mechanisms are explored, taking into account the contrasting mechanical properties of the two materials composing the interface, as well as, the evolution of the geometrical profile of the interface. The correlation between roughness and the mechanical response of the interface is investigated and the importance of an existing roughness is discussed.

International audience; The degradation of the effective normal stress in soil surrounding the pile caused by undrained cyclic loading affects the shaft capacity of the pile and can lead to structural instability. In order to investigate this phenomenon, a series of constant volume monotonic and cyclic simple shear tests on Fontainebleau sand has been conducted. Based on the experimental results, the shear stress at phase transformation state is first determined for different initial void ratios and initial normal effective stresses. Then, the number of cycles to liquefaction is estimated as a function of both the cyclic and the average shear stresses normalised by the shear stress at phase transformation. An empirical equation to evaluate the normal stress degradation is formulated and the procedure of parameter identification is presented. The performance of the suggested formulation has been analysed, based on simple shear test results on Fraser River sand and Quiou sand, and triaxial tests on Karlsruhe sand, and it has been validated by a series of additional tests on Fontainebleau sand. All the results indicate that the proposed formulation is able to estimate with good accuracy the effective normal stress degradation in sand subjected to undrained cyclic shearing.

International audience; In geotechnical and particularly in dam engineering, observations in the field and, in some cases, dramatic failures of structures have often incited research and increased knowledge. For instance, the first rockfill dams were elevated without any prior knowledge of the mechanical capacities of the rockfill mass. As natural and local resources are usually preferred for economic and environmental issues, their non-optimal properties can lead to potential severe damage to the hydraulic structure. Similarly, a basic principle of construction of rockfill dams or embankments consists in sluicing rockfill after deposition and before compaction: this recommendation has been adopted for a long time. However, the beneficial effects of such a practice have been understood only in light of recent studies on unsaturated soil and multi-scale analyses.

In order to better understand the origin of ageing on driven micropiles in sand, a series of field pull-out tests on small diameter driven piles were carried out in Dunkirk. The micropiles were about 51 mm in diameter, 8 mm wall thickness and 2 m embedment. The micropiles were made of different steel types (mild steel or stainless steel) with different shaft roughness. Sand characteristics, field penetrometer tests and pile tests are summarized. Tension tests were performed at different intervals of time after micropiles installation in attempt to investigate the influence of: (i) physiochemical effects, by testing both stainless and mild steel piles; (ii) the early stages of ageing. A significant increase of capacity was observed in the case of mild steel micropiles, regardless of the shaft roughness, while no ageing effects were observed on stainless steel micropiles. Some micropiles have been pre-loaded in compression at different energy levels. These results suggest the great influence of physiochemical effects (corrosion) for small diameter piles driven in sand. In the case of a prior compressive plastification, the loss of the benefit of ageing is obvious.

International audience; To improve our understanding and to quantify the behaviour of a jacket foundation for offshore wind turbines, several tests have been conducted using a geotechnical centrifuge at 100g. The monotonic tension and compression tests carried out on single small-scale model piles (prototype dimensions: 1·8 m in diameter and an embedment depth of 40 m in dense sand) jacked at 1g and 100g show the influence of the set-up method on the axial ultimate capacity and on the tip and shaft resistances. The pile axial capacity in compression is improved, whereas the normalised displacement of the pile is bigger in tension when piles are jacked at 1g. Vertical cyclic loads on single piles jacked at 1g (pure tension tests or two-way tests) and horizontal cyclic loads on a four-pile jacket structure are applied to examine the behaviour under more realistic loading paths. The piles of the jacket structure are considered and studied as singles piles. Finally, the stability diagrams for the single and the jacket piles are discussed to visualise the significant difference in performance of a single pile against the comparable pile in the jacket.

International audience; Grain crushing is in part responsible for soil plasticity and dissipates most of the input energy at high stresses. This energy dissipation, as well as the energy dissipated by friction, occurs through heat exchanges. Real-time observations of temperature fields on the surface of a breaking specimen are performed by the infrared thermography technique to determine the heat generated at the crack tip when a crack is propagating. This research was motivated by recent trends in soil mechanics where constitutive models have been developed based on thermodynamic potentials including energy dissipation. A high-speed infrared camera was used to monitor the cracking of multiple specimens of rock and mortar in a diametral compression configuration. In the case of rock specimens, cracking was accompanied by a sudden rise in temperature. No temperature changes were observed in the case of mortar specimens. The maximum temperature reached and the heat dissipation profile attained depend on the nature of the rock specimen.

International audience; A novel approach has been developed to predict the mechanical behavior of very coarse granular materials with a constitutive model, which considers both grain breakage and size effect. The behavior of granular assemblies is significantly influenced by particle breakage. A critical-state double-yield-surface model incorporating the change in the critical-state line and in elastic stiffness caused by grain breakage during loading has been adopted. The amount of grain breakage was estimated by extending the size effect theory on individual grains to granular assemblies. The results from earlier studies on granular materials with parallel gradations have been usefully exploited to calibrate and to validate the model. Comparisons between experiments and simulations suggest that this approach can predict the mechanical behavior of very coarse granular materials from test results performed on a finer fraction with a homothetic gradation.

The main objective of this study is to understand the effects of the coordination number on the behavior of a single grain, before and during fracture. A new apparatus is designed, with the purpose of conducting multipoint crushing tests, and to investigate the effects of the contacts number, type and position on the failure of a particle. Some of the tests were monitored using digital image correlation (DIC), in order to get an in-depth view into the mechanics of fracture. The fragmentation of the grains was studied, and the cracks were shown to follow specific paths. The failure forces of multiple contact configurations were compared to show the effect of the number and position of contact forces on the primary and secondary cracks. It was shown that the position and magnitude of the contact forces plays a significant role in the fragmentation of the grain.

International audience; It has been widely shown that particle crushing increases the compressibility of granular materials. For a particular crushable material and given test conditions, an empirical relation can be established between the breakage ratio and the plastic work. Along these lines, constitutive models have been developed based on the effect of grading evolution during crushing. In parallel, due to corrosive attacks of the humid environment at the tip of microcracks within solid grains, the mechanical behavior of crushable granular materials depends also on the water content: the higher the material humidity, the higher the particle crushing. However, the experimental data on the relation between loading–wetting conditions and the breakage ratio are still quite scarce. In this paper, we present experimental results on crushable sand to study the effect of flooding under isotropic, oedometric, and triaxial stress paths. The main objective of this study is to obtain a consistent framework for the effect of water based on the breakage ratio. Our results have shown that, for a given initial density and stress path, the dry material after flooding reaches the equivalent behavior of the initially wetted material in terms of compression curve, particle crushing, and creep compressibility index, regardless of the point of flooding. Moreover, the relation between the breakage ratio and the final void ratio is unique and depends neither on the stress path, the water content, the point of flooding, nor the loading condition (time of creep or relaxation), but exclusively on the initial density and on intrinsic parameters. These findings could improve the prediction of the effect of water and time on the mechanical response of crushable granular materials through constitutive models based on grading evolution.; Il est reconnu que la rupture des grains augmente la compressibilité des matériaux granulaires. Pour des conditions d'essai données, une relation empirique peut être établie entre le taux de rupture des grains et le travail plastique. Ainsi, des modèles constitutifs ont été développés sur la base de l'évolution de la granulométrie liée a ` cette rupture des grains. Parallèle-ment, du fait des attaques corrosives de l'environnement humide en pointe des micro-fissures dans les grains solides, le comportement mécanique des matériaux granulaires broyables dépend également de la teneur en eau: plus l'humidité du matériau est élevée, plus la rupture semble favorisée. Toutefois, les données expérimentales sur la relation entre les conditions de chargement, les conditions hydriques et le taux de rupture sont encore très rares. Dans cet article, on présente des résultats expérimentaux sur un sable affecté par la rupture des grains pour étudier l'effet de l'imbibition sur des chemins de contraintes isotropes, eudiométriques, et triaxiales. L'objectif principal de cette étude est d'obtenir un schéma de comportement cohérent décrivant l'effet de l'eau sur le taux de rupture des grains. Les résultats montrent que, pour une densité initiale et un chemin de contraintes donné, après l'imbibition d'un échantillon sec, il suit un comportement identique à celui d'un échantillon saturé dès le début du chargement en termes de compressibilité, ruptures des grains et d'indice de compressibilité au fluage, quel que soit le point d'imbibition. Par ailleurs, la relation entre le taux de rupture et l'indice des vides final est unique et ne dépend pas du chemin de contraintes, ni de la teneur en eau, ni du point initial de l'imbibition, ni des conditions de chargement (temps de fluage ou de relaxation), mais uniquement de la densité initiale et de paramètres intrinsèques. Ces résultats pourraient améliorer la prédiction de l'effet de l'eau et du temps sur la réponse mécanique des matériaux granulaires broyables à l'aide des modèles constitutifs basés sur la granulométrie.

International audience; The aim of this paper is to investigate the effect of soil grading on the stress-strain and the critical-state behavior of granular materials from idealized spheres to natural soils as well as from discrete to continuous modeling. The three-dimensional discrete-element method has been applied to study the mechanical behavior of idealized granular materials. The results confirm that the critical-state line (CSL) shifts down-ward as grading broadens with an increase of the coefficient of uniformity Cu. An exponential relationship between the critical-state parameters and the coefficient of uniformity Cu can then be established. Furthermore, experimental investigations on an artificial material (glass beads) and a natural material (Hostun sand) were carried out. The experimental results confirm the grading dependency of the critical-state behavior, and a similar relationship between CSL and Cu, for both glass beads and natural sand, extending previous literature results for high Cu values up to 20. Moreover, the experimental results show that the CSL tends to stabilize when the coefficient of uniformity Cu becomes higher than 10. Based on all test results, a simple elastoplastic model accounting for the influence of the grain size distribution (GSD) on the mechanical behavior of granular materials has been developed within the framework of critical-state soil mechanics. Comparisons between experimental results and simulations show that the model can accurately reproduce the mechanical behavior of granular materials with different GSDs.

International audience; Testing the mechanical response of coarse granular materials requires very large and expensive laboratory equipments. During the 1960s, pioneering experimental programs were carried out on several rockfill dam materials, and those results are still a reference for engineers and researchers. However, only few experimental works have been reported to this day, and due to the scarcity of empirical data, the role of the size effect caused by grain crushing is not well known. To improve understanding of this rarely studied issue and the influence of individual particle strength, this paper analyzes the size effect on rock aggregate crushing strength and its connection with the shear envelope of rockfills. The suitability of the 4-parameter Weibull equation to describe size effects on the crushing strength reported in the literature is discussed. Furthermore, a Weibull statistical analysis was carried out for a wide number of experimental results on rock aggregates, where it has been observed that strength decreases with particle size. In parallel, the results of large triaxial tests on homothetic scaled rockfill samples of 250 and 1,000 mm in diameter reveal that the coarser the material, the higher the amount of grain breakage and the lower the shear strength. The impact of size effects obtained from the experiments is analyzed and discussed in terms of the factor of safety of rockfill slope stability. Furthermore, the results are compared with the only existing theoretical method that links the rock aggregate with the strength of the granular assembly. Good agreement between the empirical results and this theoretical method has been confirmed.

International audience; The paper summarizes the results from undrained triaxial compression tests on Merville clay, a highly overconsolidated clay at Merville in the north of France. The specimens were prepared in four different overconsolidation ratios (OCR=7, 14, 28, 56), and for each OCR, undrained shear tests were performed using three or four axial strain rates varying from 0.26 %/h to 25.89 %/h. The strain-rate dependency of the undrained stress level q/q f , the pore pressure, the effective stress path and the stress ratio q/p' were investigated. The undrained shear strength was found to increase with the strain rate. The shear band formation during loading was observed for all strain rates. For each OCR test series, a linear relationship between the normalized peak deviatoric stress and the strain rate was obtained in the semi-logarithm plane. As OCR increasing, the slope of best-fitting line for the linear relationship increases. Therefore, the strain rate influence on the normalized undrained shear strength is greater for higher OCRs.

International audience; This paper aims at investigating the influence of grading on the undrained behavior of granular materials. Series of undrained triaxial tests were carried out with two different materials, glass balls and Hostun sand. For each material, samples with different gradings and similar relative densities were prepared. Experimental results show that the undrained shear strength decreases when the coefficient of uniformity Cu=d60/d10Cu=d60/d10 increases from 1.1 to 20. The conditions of instability for the selected granular materials were also analyzed, based on the sign of the second-order work during undrained triaxial loading. The results demonstrate a significant influence of the grading: increasing the coefficient of uniformity heightens the potential of static liquefaction and the materials become more unstable. Furthermore, numerical tests using the three-dimensional discrete element method (DEM) were conducted on assemblies of spheres. The DEM inter-particle parameters were derived from the experimental test results on glass balls. The DEM simulations showed similar behaviors compared to experimental results and confirmed the influence of the grain size distribution on the stress–strain relationship and instability phenomena.; Cet article vise à étudier l'influence de l'étalement granulométrique sur le comportement non drainé des matériaux granulaires. Des séries d'essais triaxiaux non drainés ont été réalisés sur deux matériaux différents, des billes de verre et du sable Hostun. Pour chaque matériau, des échantillons de différentes granulométries et de densités relatives similaires ont été préparés. Les résultats expérimentaux montrent que la résistance au cisaillement diminue lorsque le coefficient d'uniformité Cu=d60/d10Cu=d60/d10 augmente de 1,1 à 20. Les conditions d'instabilité pour les matériaux granulaires sélectionnés ont également été analysées, sur la base du signe du travail du second ordre au cours d'un chargement triaxial non drainé. Les résultats montrent une influence significative de l'étalement granulométrique : l'augmentation du coefficient d'uniformité accroît le potentiel de liquéfaction statique et les matériaux deviennent plus instables. En outre, des essais numériques en utilisant la méthode des éléments discrets en trois dimensions (DEM) ont été effectués sur des assemblages de sphères. Les paramètres inter-particules DEM ont été calculés à partir des résultats d'essais expérimentaux sur des billes de verre. Les simulations DEM ont montré des comportements similaires comparés aux résultats expérimentaux et confirment l'influence de l'étalement granulométrique sur la relation contrainte–déformation et les phénomènes d'instabilité.

International audience; Since the pioneering work of Griffith, Linear Elastic Fracture Mechanics has been widely experimentally validated and successfully developed in solid mechanics modeling. However, recent theoretical models applying the energy balance found in Griffith theory specifically for quasi-static confined comminution have until now not been systematically confronted to experiments. In this study, we analyze data of compression tests on crushable sand, where grain breakage has been triggered by flooding the initially dry material at constant stresses. We consider a partition of the dissipation between surface fracture energy and the rearrangement of fragments and grains surrounding crushed particles. Our results show that the role of the surface fracture energy is stressdependent and that its influence becomes less significant at high stresses.

Pick excavatability of controlled low-strength materials was experimentally studied 10 to circumvent the fact that current classification systems or empirical excavatability criteria lack relevance (they do not refer to digging facilities and depend on the mix design and the granular skeleton of the material). Manual excavation is separated into two steps: the impact of the pick and decompaction. This was analysed by a mechanical approach. Energy is mainly consumed during the first step. A laboratory punching test was therefore proposed to evaluate excavatability (a confined cylindrical specimen was loaded by a cylindrical punch). Results were compared between the mechanised and the manual pick excavation tests on field trenches backfilled with air-entrained mortars or very lean concretes. These comparisons confirmed that a limited compressive strength was an indicator of excavatability. A model was proposed to evaluate the excavated volume by punching penetration for a given level of energy and a maximum size of aggregates in the material

This paper aims to investigate the influence of grain size distribution (GSD) on the critical state of Hostun sand. Drained triaxial tests were conducted on saturated specimens with different GSD (Cu=1.4, 5, 10, 20) under different confining pressures (100, 200, 400 kPa). The experimental results highlight that the critical state of the tested sand predominantly depends on its GSD.

International audience; The shear strength of coarse granular materials, widely used in civil works such as rockfill in dam construction, is seldom measured because of severe practical experimental limitations. This paper presents an original method for evaluating the shear strength of such materials, based on size effects in granular materials affected by grain breakage according to fracture mechanics. Through a general size effect relation operating on shear strength envelopes, this method makes it possible to determine the shear strength of a coarse-grained granular material from the measured properties of a finer-grained granular material made of the same mineral. In the paper, the method is explicitly proved, taking into account the statistical distribution of breakage resistances of particles with different sizes, within materials that can be considered as physically similar, particularly having parallel grain size distributions and the same compactness. A wide set of independent experimental results is shown to validate the method consistently.

International audience; A particle breakage has a significant impact on the mechanical behavior of granular materials. In this paper, we present an elasto-plastic model with two yield surfaces to which the influence of particle breakage has been introduced. The main feature of this model is to incorporate the change in the critical state line (CSL) consequent to the grain breakage induced by isotropic and deviatoric stresses during loading. For this purpose we propose a breakage function which connects the evolution of the CSL to the energy consumed. Results from earlier studies on drained and undrained compression and extension triaxial tests were used to calibrate and validate the model. Comparison between earlier results and our simulations indicates that the model can reproduce with good accuracy the mechanical behavior of crushable granular materials and predict the evolution of the grain size distribution during loading.

A model to predict how grain size distribution evolves during one-dimensional consolidation is proposed, based on the probabilistic phenomenon of grain splitting in a confined granular assembly and a population balance equation. Results from micromechanical simulations allow us to describe the variability of force distribution at grain contacts from a macro-mechanical stress state. Particle strength from individual grain crushing tests can be fitted to a Weibull distribution that defines the survival probability of grains as a function of stress. Particle fragmentation process in the granular assembly is simulated by a simple fractal type equation, which integrates crushing and abrasion phenomena using only one fitting parameter. The predictions of the model properly reproduce experimental data under low and high 1D loading.

International audience; Natural soils or coarse granular materials containing large particles such as rockfill or ballast are often an issue for geotechnical projects. However, since it is usually complicated or impossible to set up a big enough experimental apparatus to test these materials, their mechanical behavior remains poorly known.To circumvent this difficulty, we have developed a set of triaxial cells of various sizes to study the experimental behavior of granular soils containing particle sizes of up to 160 mm. This paper presents the first results of a two-part experimental study. The first part consists of triaxial tests on sand samples of different sizes in order to examine specimen scale effects. Pre-peak behavior is not affected by the specimen size, whereas post-peak behavior depends on the test conditions that control the development of strain localizations. In the second part, we present drained triaxial compression tests on large samples of calcareous rockfill materials that have parallel grain size distributions.The results show a nonstandard evolution of the friction angle in relationship to grain size if compared to similar studies on coarse granular materials. Although grain crushing occurs during compression, the strain-stress curves of the different materials with different grain sizes do remain close. We explain this phenomenon by the strength evolution of the individual grains with respect to their sizes

International audience; The elastic behaviour of compacted soils subjected to very small strains (smaller than 10 －5) is essential, because the serviceability of most geotechnical structures depends on soil elastic properties. Small-strain stiŠness of soils was studied in the past years using diŠerent experimental devices (mainly resonant columns). However, the results have been relatively inconsistent. The bender elements technique, now extensively used in soil mechanics, oŠers an e‹cient non destructive alternative, since it is based on the propagation of shear waves. To enrich the common bender elements testing results providing only shear modulus values, an evolution of the bender elements technique, named bender-exten-der element's is used in this paper. This device allows the simultaneous measurement of the shear and compression wave velocities. As a result, the two independent elastic constants are measured for the same sample in order to avoid indirect estimations. Also, for unconˆned sample's the GrindoSonic test is performed: this test is based on the transient vibrational response of the sample to a slight shock. The present study provides new results about the elastic properties of intermediate unsaturated soils made of mixtures of sand and clay. The results for the elastic properties are presented as a function of the suction level. Finally, a model relating the elastic behaviour of these unsaturated soils is proposed.

International audience; An elasto-plastic model with two yield surfaces has been developed for the simulation of granular materials with consideration of grain ruptures. Grain breakage is induced by deviatoric as well as isotropic stresses. One yield function is based on a Mohr-Coulomb criterion with a hyperbolic hardening function of the plastic deviatoric strain. A second yield function is introduced to describe the plastic behaviour under compression, in which a hardening function of the plastic volumetric strain is introduced. The main assumption of this model is that, upon loading, the position of the critical state changes as a consequence of grain breakage. The effect of the grain size distribution is introduced in the relationship between the void ratio at critical state and the mean effective stress. Triaxial and oedometer tests have been performed on crushable granular materials. Comparison of experimental results and numerical simulations shows that the new model can reproduce with good accuracy the behaviour of granular materials subjected to grain ruptures during mechanical loading.

Because of the lack of experimental devices for testing granular materials with large particles, the mechanical behavior of rockfill is not very well known. This lack of knowledge can lead to improper design of rockfill dams, and as a consequence, to severe accidents including dam failures. A method has been developed by Frossard (2005) for evaluating the shear strength of rockfill based on the estimation of size effect in granular materials, affected by grain breakage. Comparison between experimental results and numerical predictions shows that this method can estimate with good accuracy the strength of granular materials made of large particles.

Granular materials with large particles have been widely used to construct embankments or rockfills dams. However, due to limitations in the size of experimental set-ups, their behavior remains relatively less investigated than the behavior of sands for instance. A new large triaxial apparatus, with a maximum specimen diameter of 1000mm, has been designed to study the behavior of such coarse granular materials. This device is described in this paper. The results of a triaxial test on calcareous particles with a maximum particle size of 160 mm are presented. Special attention has been paid on grain crushing under shear loading.

International audience; The identification of soil parameters by inverse analysis of two pressuremeter tests is investigated as part of a predictive benchmark dealing with a 3x3m square spread footing, resting on a relatively homogeneous sandy soil layer. The inverse analysis is performed by a FEM-simulation considering a linear elastic perfectly plastic Mohr-Coulomb model and several optimization algorithms. Firstly, deterministic and stochastic mono-objective algorithms are compared. Secondly, in order to take into account simultaneously the results of several experimental tests, a multi-objective problem is considered and solved with a multi-objective genetic algorithm which provides a set of equivalent solutions in terms of Pareto. These solutions have to be treated according to the expertise of the user but also to the importance and the quality of the test results. The sets of parameters obtained by these optimization techniques are then used to calculate the footing response. The numerical results are discussed and compared to those obtained by the benchmark participants.

International audience; The identification of constitutive parameters by inverse analysis of limited experimental information is investigated. In order to ensure the quality of measurements, all the data come from a thick wall cylinder test performed on a saturated sand sample. A numerical FEM-simulation considering a linear elastic perfectly plastic model (Mohr-Coulomb) is carried out in order to determine the pressure – cavity deformation relationship. Two kinds of optimization algorithms, a deterministic and a stochastic one, are presented and implemented in the inverse procedure. The performance of theses algorithms taking for identifying three parameters (Young’s Modulus, friction angle and dilatancy angle) is compared. The two kinds of algorithms provide a quite similar response for the mathematical optimum, but some parameters sets did not have an acceptable physical meaning. Contrary to the deterministic algorithm, the genetic method provides an assembly of “satisfactory” individuals. Some of the parameters sets have acceptable physical meaning and the expert has to choose the best one considering his scientific and technical background.

International audience; Inverse analysis of pressuremeter tests has been widely used to identify the parameters of low permeability soils assuming the modified Cam-Clay model. In such a case, the simultaneous identification of two parameters (for instance, the consolidation pressure $p_{c0}$ and the critical state parameter M) can be achieved from a single pressuremeter curve obtained in fully drained conditions. Additional experimental information is therefore required to determine the others parameters of the constitutive model. This paper focuses especially on the determination of the plastic compressibility $β$ (where $β=λ−κ$) of prime importance in many engineering problems. Based on sensitivity studies, a procedure to identify simultaneously the plastic compressibility and the parameters $M$ and $p'_{c0}$ is proposed.

International audience; A comparison of the behavior of uncemented and grouted sands is presented. Four sands Fontainebleau sand and three types of alluvial deposits of the Seine River were tested. Specimens of grouted sands were prepared in the laboratory by injection of very fine cement or mineral grouts. An initial series of unconfined uniaxial compression tests and tensile tests was performed to highlight the effect of some key factors mainly the cement-to-water ratio of the grout and the relative density of the granular skeleton on the strength of the grouted sands. Subsequent triaxial tests showed that when a soil is impregnated by either a very fine cement grout or a mineral grout, both stiffness secant stiffness or small-strain stiffness and strength of the soil improve. Similar trends were observed for the behavior of both uncemented and grouted sands. The behavior of grouted sands can be roughly reproduced by applying a linear elastic, perfectly plastic model with a nonassociated Mohr–Coulomb yield criterion whose parameters can be easily determined. Finally, preliminary recommendations are proposed relative to improvements ratios of the parameters of this simple constitutive model that is still commonly used in geotechnical engineering.

Monopile is the most common foundation system for offshore wind turbine structures, statistically about 80, as per Wind Europe, 2018 report. A pile can be defined as flexible or rigid depending on the embedded length to diameter ratio (Le/D) and the relative stiffness of pile and soil. The existing codes for pile design are mainly developed for flexible piles, whereas monopile for new offshore wind turbines typically falls in the rigid category. The dominant complex cyclic wind and waves loads on offshore wind turbine structure and consequently on the monopile foundation, act in the lateral direction. The API design procedure, representing the lateral and vertical soil response through uncoupled non-linear springs, is developed for flexible piles and is recognised as conservative for rigid piles. The behaviour shall represent a coupled vertical and lateral soil-structure interaction because a rigid pile presents rotation deformation mode instead of deflection. The deformation mode further demands a different formulation mechanism, including the distributed moments along the pile shaft and shear & moment behaviours at the pile base as an essential part of the soil response investigations.This work presents the numerical models aimed to address the limited understanding of coupling consideration of monopile installed in sand. The cone penetration tests performed in the calibration chamber, data treatment with ICP method and available Fontainebleau sand NE34 properties database in the literature provide the constituent parameters for model definition.First, a PLAXIS 3D finite element model presents the model pile in the calibration chamber configuration with representative boundary conditions and the constitutive behaviour of the sand. The model pile geometry and load magnitudes are the outcomes of a similitude relationship with a representative prototype. A constant mass placed on the pile head represents the vertical load (the dead weight of the wind turbine structure). A simplified lateral point load represents the complex environmental loads, acting at a distance above foundation level, represents the lateral and moment load at mudline. Thus, vertical (V), lateral (H) and moment (M) collectively represent the combined load, investigated in both monotonic and cyclic loading cases. Different combined loading case in the limits of horizontal and vertical load capacities represent the overall behaviour of the model pile. The observation of normal and shear stress changes close to the pile-soil interface at different depths quantify the pile-soil interaction. The response investigation at some strategic stress points in the FE model soil volume provides a basis for soil-stress transducers (SSTs) layout plan in the experimental soil volume. A methodology to formulate the lateral and shear stresses evolution close to the pile surface as representative of the coupled interaction is presented.Second, a local-macro element (LME) model, an assembly of non-linear springs formulated using a Matlab toolbox ATL4S, presents the soil-pile interaction with inherent coupling considerations at different embedment depths. The PLAXIS model outcomes define the basis for a corresponding model scale LME model. The obtained results from both numerical investigations demonstrate the significance of vertical-lateral coupled interaction as a set of hypothesised equations representing the coupled interaction for rigid monopile foundation. A similitude work provides a relation between a prototype scale and the lab-scale monopile model. It further aids in comparison and validation with the available experimental and numerical results in the literature.; Le monopieu est le système de fondations le plus courant pour les structures d'éoliennes en mer, avec une occurrence d'environ 80% d’après le rapport Wind Europe de 2018. Un pieu peut être qualifié de souple ou rigide en fonction de son élancement (Le / D) et de la rigidité relative du pieu et du sol. Les codes existants pour la conception de pieux sont principalement développés pour les pieux flexibles, alors que les monopieux pour les nouvelles éoliennes en mer entrent généralement dans la catégorie des pieux rigides. Les charges dominantes dues au vent et à la houle, de nature cyclique, sur le mât de l’'éolienne et, par conséquent, sur la fondation, agissent dans la direction latérale. La réglementation API, qui représente la réponse du sol sous charges latérales et axiales au moyen de ressorts non linéaires non couplés, est développée pour les pieux flexibles et est reconnue comme étant sécuritaire pour les pieux rigides. Néanmoins, le comportement doit représenter une interaction sol / structure couplée axial-latéral puisqu’un pieu rigide présente un mode de déformation en rotation plutôt qu’une déflexion. Le mode de déformation exige en outre une formulation différente des mécanismes, incluant la distribution des moments le long du pieu ainsi que la prise en compte de la résistance au cisaillement de la base du pieu.Ce travail présente un modèle numérique visant à prendre en compte le couplage axial / latéral pour un monopieu installé dans du sable. Les essais de pénétration au cône effectués dans la chambre de calibration traités avec la méthode ICP, et la base de données des propriétés du sable de Fontainebleau NE34 disponible dans la littérature fournissent les paramètres constitutifs pour la définition du modèle.Tout d'abord, un modèle Eléments Finis PLAXIS 3D présente un pieu modèle dans la configuration de la chambre d'étalonnage avec des conditions aux limites représentatives et le comportement constitutif du sable. La géométrie du modèle de pieu ainsi que l’intensité des charges respectent les relations de similitudes entre modèle et prototype. Une masse constante placée en tête du pieu représente la charge verticale (la charge permanente de la structure de l'éolienne). Une charge ponctuelle latérale simplifiée représente les charges environnementales complexes, agissant à un certain excentrement par rapport au niveau de la fondation. Il en résulte une combinaison de charges verticale (V), latérale (H) et un moment (M), étudiée dans les cas de chargement monotone et cyclique. Différents cas de charge combinés dans les limites des capacités portantes horizontale et verticale représentent le comportement général du pieu modèle.Les variations de contraintes normale et de cisaillement à proximité de l’interface pieu/sol ont été étudiées, afin de caractériser cette interaction. L’étude des contraintes en plusieurs points stratégiques du modèle EF a fourni une base pour le calepinage des capteurs de pression utilisés dans la partie expérimentale. Une méthodologie permettant de formuler l’évolution des contraintes latérale et de cisaillement à proximité de la surface du pieu, en prenant en compte l’interaction couplée, est présentée.Deuxièmement, un modèle de macro-éléments locaux (LME), un assemblage de ressorts non linéaires formulé à l'aide d'une boîte à outils Matlab ATL4S, présente l'interaction sol-pieu avec la considération du couplage inhérent à différentes profondeurs. Les résultats du modèle PLAXIS définissent la base d'un modèle LME à l'échelle du modèle correspondant. Les résultats obtenus dans les deux études numériques démontrent l’importance de l’interaction couplée vertical-latéral pour une fondation rigide de type monopieu. Un travail de similitude concernant fournit une relation entre l’échelle du prototype et le modèle en laboratoire. Il aide en outre en comparaison et à la validation avec les résultats expérimentaux et numériques disponibles dans la littérature.

Monopile is the most common foundation system for offshore wind turbine structures, statistically about 80, as per Wind Europe, 2018 report. A pile can be defined as flexible or rigid depending on the embedded length to diameter ratio (Le/D) and the relative stiffness of pile and soil. The existing codes for pile design are mainly developed for flexible piles, whereas monopile for new offshore wind turbines typically falls in the rigid category. The dominant complex cyclic wind and waves loads on offshore wind turbine structure and consequently on the monopile foundation, act in the lateral direction. The API design procedure, representing the lateral and vertical soil response through uncoupled non-linear springs, is developed for flexible piles and is recognised as conservative for rigid piles. The behaviour shall represent a coupled vertical and lateral soil-structure interaction because a rigid pile presents rotation deformation mode instead of deflection. The deformation mode further demands a different formulation mechanism, including the distributed moments along the pile shaft and shear & moment behaviours at the pile base as an essential part of the soil response investigations.This work presents the numerical models aimed to address the limited understanding of coupling consideration of monopile installed in sand. The cone penetration tests performed in the calibration chamber, data treatment with ICP method and available Fontainebleau sand NE34 properties database in the literature provide the constituent parameters for model definition.First, a PLAXIS 3D finite element model presents the model pile in the calibration chamber configuration with representative boundary conditions and the constitutive behaviour of the sand. The model pile geometry and load magnitudes are the outcomes of a similitude relationship with a representative prototype. A constant mass placed on the pile head represents the vertical load (the dead weight of the wind turbine structure). A simplified lateral point load represents the complex environmental loads, acting at a distance above foundation level, represents the lateral and moment load at mudline. Thus, vertical (V), lateral (H) and moment (M) collectively represent the combined load, investigated in both monotonic and cyclic loading cases. Different combined loading case in the limits of horizontal and vertical load capacities represent the overall behaviour of the model pile. The observation of normal and shear stress changes close to the pile-soil interface at different depths quantify the pile-soil interaction. The response investigation at some strategic stress points in the FE model soil volume provides a basis for soil-stress transducers (SSTs) layout plan in the experimental soil volume. A methodology to formulate the lateral and shear stresses evolution close to the pile surface as representative of the coupled interaction is presented.Second, a local-macro element (LME) model, an assembly of non-linear springs formulated using a Matlab toolbox ATL4S, presents the soil-pile interaction with inherent coupling considerations at different embedment depths. The PLAXIS model outcomes define the basis for a corresponding model scale LME model. The obtained results from both numerical investigations demonstrate the significance of vertical-lateral coupled interaction as a set of hypothesised equations representing the coupled interaction for rigid monopile foundation. A similitude work provides a relation between a prototype scale and the lab-scale monopile model. It further aids in comparison and validation with the available experimental and numerical results in the literature., Le monopieu est le système de fondations le plus courant pour les structures d'éoliennes en mer, avec une occurrence d'environ 80% d’après le rapport Wind Europe de 2018. Un pieu peut être qualifié de souple ou rigide en fonction de son élancement (Le / D) et de la rigidité relative du pieu et du sol. Les codes existants pour la conception de pieux sont principalement développés pour les pieux flexibles, alors que les monopieux pour les nouvelles éoliennes en mer entrent généralement dans la catégorie des pieux rigides. Les charges dominantes dues au vent et à la houle, de nature cyclique, sur le mât de l’'éolienne et, par conséquent, sur la fondation, agissent dans la direction latérale. La réglementation API, qui représente la réponse du sol sous charges latérales et axiales au moyen de ressorts non linéaires non couplés, est développée pour les pieux flexibles et est reconnue comme étant sécuritaire pour les pieux rigides. Néanmoins, le comportement doit représenter une interaction sol / structure couplée axial-latéral puisqu’un pieu rigide présente un mode de déformation en rotation plutôt qu’une déflexion. Le mode de déformation exige en outre une formulation différente des mécanismes, incluant la distribution des moments le long du pieu ainsi que la prise en compte de la résistance au cisaillement de la base du pieu.Ce travail présente un modèle numérique visant à prendre en compte le couplage axial / latéral pour un monopieu installé dans du sable. Les essais de pénétration au cône effectués dans la chambre de calibration traités avec la méthode ICP, et la base de données des propriétés du sable de Fontainebleau NE34 disponible dans la littérature fournissent les paramètres constitutifs pour la définition du modèle.Tout d'abord, un modèle Eléments Finis PLAXIS 3D présente un pieu modèle dans la configuration de la chambre d'étalonnage avec des conditions aux limites représentatives et le comportement constitutif du sable. La géométrie du modèle de pieu ainsi que l’intensité des charges respectent les relations de similitudes entre modèle et prototype. Une masse constante placée en tête du pieu représente la charge verticale (la charge permanente de la structure de l'éolienne). Une charge ponctuelle latérale simplifiée représente les charges environnementales complexes, agissant à un certain excentrement par rapport au niveau de la fondation. Il en résulte une combinaison de charges verticale (V), latérale (H) et un moment (M), étudiée dans les cas de chargement monotone et cyclique. Différents cas de charge combinés dans les limites des capacités portantes horizontale et verticale représentent le comportement général du pieu modèle.Les variations de contraintes normale et de cisaillement à proximité de l’interface pieu/sol ont été étudiées, afin de caractériser cette interaction. L’étude des contraintes en plusieurs points stratégiques du modèle EF a fourni une base pour le calepinage des capteurs de pression utilisés dans la partie expérimentale. Une méthodologie permettant de formuler l’évolution des contraintes latérale et de cisaillement à proximité de la surface du pieu, en prenant en compte l’interaction couplée, est présentée.Deuxièmement, un modèle de macro-éléments locaux (LME), un assemblage de ressorts non linéaires formulé à l'aide d'une boîte à outils Matlab ATL4S, présente l'interaction sol-pieu avec la considération du couplage inhérent à différentes profondeurs. Les résultats du modèle PLAXIS définissent la base d'un modèle LME à l'échelle du modèle correspondant. Les résultats obtenus dans les deux études numériques démontrent l’importance de l’interaction couplée vertical-latéral pour une fondation rigide de type monopieu. Un travail de similitude concernant fournit une relation entre l’échelle du prototype et le modèle en laboratoire. Il aide en outre en comparaison et à la validation avec les résultats expérimentaux et numériques disponibles dans la littérature.

Offshore structures are designed to resist to severe environmental loads. This manuscript presents experimental and numerical studies on offshore wind turbine pile foundations submitted to static monotonic/cyclic loads. Isolated piles (diameter of 1.8 m and embedded length of 40m) and a four-legged truss structure installed in dense Fontainebleau sand have been tested at 100×g in a geotechnical centrifuge. The behavior of the isolated piles has been characterized under monotonic axial load. The piles have been jacketed at 1×g and 100×g and the influence of the setup method on the bearing capacity has been measured. The stability diagram of a pile has been constructed using cyclic tests, CPT experiments have been performed and the influence of the initial stresses has been studied. Finally, experimental results of the jacket foundation submitted to lateral cyclic loadings are presented. The finite element method and a macro-element for piles have been used to numerically reproduce the experimental results. Constitutive laws are based on the hypoplasticity theory and the initial stresses in the soil have been calculated using the IC-05 method. The numerical results are compared with the experimental and the analytical results from the API standards.; Les structures offshores sont conçues pour résister à des chargements environnementaux sévères. Des études expérimentales et numériques de la fondation d’une éolienne offshore soumise à des chargements statiques monotones/cycliques sont présentées dans ce manuscrit. Des pieux isolés (diamètre de 1,8 m et 40 m de longueur) et une structure en treillis fondée sur 4 pieux ont été testés à 100×g en centrifuge dans un massif de sable de Fontainebleau dense. Le comportement du pieu isolé a été identifié à travers des essais de chargement axial monotone. Différentes méthodes de mise en place de pieu ont été adoptées (installation à 1×g et à 100×g) et leur influence sur la capacité portante a été mesurée. Le diagramme de stabilité du pieu a été construit à partir des résultats des essais cycliques, des essais CPT ont été réalisés et l’influence des contraintes initiales a été étudiée. Des résultats expérimentaux de la fondation jacket soumise à des chargements latéraux cycliques sont enfin présentés. La méthode des éléments finis et un macro-élément pour des pieux ont été utilisés pour reproduire numériquement les résultats expérimentaux. Les lois de comportement adoptées sont basées sur la théorie d’hypo-plasticité et les contraintes initiales dans le sol sont issues de la méthode ICP-05. Les résultats numériques sont confrontés aux résultats expérimentaux et aux résultats analytiques obtenus à partir de la norme API.