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This work aimed to understand the mechanical behavior of siliceous and calcareous sand materials under uniaxial confined compression loading at high stress levels. For this purpose, a series of quasi-oedometric compression tests were conducted on sand materials, to examine the effects of grain size, nature, and moisture contents on the soil crushability and the compression behavior, using an upgraded thick pressure vessel device that can reach mean stress up to 500 MPa. All samples were prepared using an aspect ratio of 1:1 (diameter: height), placed inside a high strength steel vessel, and compressed at a uniform axial displacement rate of 5 µm/s. The vessel is instrumented with multiple strain gauges allowing for the characterization of the hydrostatic and deviatoric behavior of each test. The results of quasi-oedometric tests, conducted on these types of sand, up to a passive confinement of 500 MPa, show that particle breakage is enhanced by the presence of water. It was noticed that, for siliceous sand, smaller particles break more than larger particles, and that the calcareous grains manifest a rapid response to axial stress compared to siliceous sand. Testing various soil properties shows a great potential to better characterize the sensitivity to breakage of soils. Lastly, a post-mortem analysis of samples before and after testing, using the X-ray micro-tomography technique, was applied to study the mechanical damage of sand specimens.

A pipe buried into sand was subjected to a surface loading in order to study its mechanical behaviour. The test was conducted at small scale, in a tank filled with sand, while the pipe was instrumented with strain gauges and inner displacements sensors. The paper presents the soil vertical settlement, and pipe response to the load, in terms of deformations, stresses, and radial displacements. The radial displacements are computed using a specially developed methodology and allow to present the deformed shape of the pipe, with limited direct measurements. Results show that under this loading the vertical diameter of the pipe decreases, while the horizontal one increases, but to a lower degree. The deformed shape is more complex than the ellipse often considered. The pipe deformed shape exhibits a rapid change in curvature near the crown, often referred as “heart shaped deformation” in the literature. The overall values of stresses reached remain far from the elastic limit of the material used, and smaller than the impact of the inner gas pressure for this type of relatively thick pipe of moderate diameter.

The development and construction of offshore wind farms requires the correct estimation of the friction that can be mobilised at the rock/grout interface. In conventional studies, the shear behaviour of a joint is usually investigated with laboratory tests under constant normal load/stress (CNL), however, in engineering practice, direct shear testing under constant normal stiffness (CNS) has been proved to be more realistic in the assessment of the development of the side shear resistance in rock grouted pile design. In this work, an extensive experimental campaign on the shear response of a weak carbonate rock (limestone) interface with grout is presented, in the frame of offshore wind turbines. First, basic mechanical testing is performed on the two interface materials in order to evaluate their mechanical properties. The output of these tests reveals not only the contrasting properties of the two interacting materials, but also the decreased response of the limestone in the presence of water. A series of monotonic shear tests (both under CNL and CNS conditions) on wet rough limestone/grout interfaces reveal the high impact of adhesion between the two materials to the mechanical response. Based on the monotonic results, a number of CNS shear tests under cyclic loading takes place, where different failure modes are observed dilatant and contractant response. The variability of the failure mode is strongly related not only to the adhesion created with the cast grout, but also to the limestone’s micro-structural heterogeneity that manifests already after consolidation. The post-shear morphological state of the interface is analysed, while the variability of the failure surface and the presence of water gouge creation do not allow a clear correlation of the morphologfy to the mechanical response. Overall, the response of this type of weak rock interface where the properties of the grout are significantly higher, is governed by the behaviour of the rock.

Soft soil subjected to cyclic loading typically exhibits an increase in excess pore pressure under undrained condition which brings the soil to an overconsolidated state. Then, under a subsequent large number of cycles (e.g. more than one million) which also takes time, the creep at overconsolidated state influences the cyclic effect and thus results in a complicated long-term undrained behavior. This paper aims to clarify this long-term undrained behavior of overconsolidated clay. The reconstituted samples are prepared from natural samples retrieved in the north of France. First, the shear strength characteristics along monotonic triaxial stress paths are identified. Then load control cyclic tests on overconsolidated specimens are conducted in fully saturated and undrained conditions. Small cyclic deviatoric stresses are applied in order to investigate more particularly the behavior under a very large number of cycles, during which an unusual pore pressure evolution is observed. To explain this, undrained triaxial creep tests are performed on reconstituted specimens with different values of OCRs under some specified stress states. The evolutions of axial strain, excess pore pressure, stress ratio, stress path, plastic strain rates and stress dilatancy during undrained creep are discussed. The additional undrained creep tests also show that two processes are simultaneously acting in a competitive manner: increase in the pore pressure due to the cyclic loading and decrease in the pore pressure because of creep.

Waste mining rock and rockfill materials contain coarse clasts that could easily reach more than 1 m in size. Small scaling techniques for mechanical testing on such coarse materials require altering the particle-size distribution (PSD), by reducing maximum grain size to be able to fit a sample in a laboratory device. To capture the stress−strain behaviour, it is assumed that the reduced PSD has to be parallel to the prototype grading. However, individual grain properties could change with particle size, such as shape and crushing strength. Parallel scaling techniques have been widely applied in rockfill materials, however, the effects of particle-size correlations have been rarely considered, and its effects remain not well understood. This paper presents experimental data on particle-size correlations with both particle shape and particle strength, together with triaxial tests on parallel graded samples of a shale rockfill material. The results show that inverse particle-size−strength correlation results in decreasing particle crushing in finer samples, while particle size−shape correlation could contribute to increase particle crushing in finer samples comprising more elongated grains. Depending on characteristic particle size, one of these opposed trends will prevail and control the material behaviour.

We present a study on compaction band in a high-porosity limestone (Tuffeau de Maastricht) based on comparison and analysis of X-ray tomography observations and computational multiscale simulations. We employ a hierarchical multiscale approach coupling the finite element method (FEM) with the discrete element method (DEM) to simulate the formation of compaction bands in Tuffeau de Maastricht. A high-porosity RVE is prepared according to X-ray tomography observations of material microstructure, and its grain-scale parameters are calibrated by data from laboratory isotropic compression and triaxial compression tests. Triaxial compression tests are simulated by FEM as a boundary value problem to observe compaction bands. The generated RVE is embedded into each integration point of the FE mesh, receiving displacement gradient as DEM boundary conditions, and is solved accordingly to produce the required mechanical responses for FEM computation without assuming phenomenological constitutive relations. The simulated global mechanical responses of the triaxial tests are found to show qualitative agreement with the experimental data. The evolution of compaction band patterns in the simulation matches remarkably well with the experimental observations in terms of fields of porosity and incremental strains. Both show two compaction fronts initiating from the two ends of the specimen and propagating toward the middle. By virtue of the multiscale approach, useful microstructural information is further extracted from the simulations to offer cross-scale insights into compaction bands. The study confirms that significant debonding accompanied by collapse of macropores and grain rearrangements is the major microstructural mechanisms causing the formation of compaction bands in high-porosity Tuffeau de Maastricht.

Investigating the mechanical properties of rockfills requires laborious, time-consuming, and expensive large-scale testing. Therefore, practitioners frequently adopt design parameters based...

International audience; Grain breakage during compression and shearing is one important mechanism responsible for irrecoverable changes of the mechanical properties of granular materials. Here we present results of triaxial tests on limestone fragments under some monotonic and cyclic stress paths and we investigate the relationships between the progression of grain breakage, the plastic work and the evolution of the critical state line. Using the plastic work concept, we propose a method for determining grain shape and grain breakage indices, and we show how grain breakage influences the critical stress state, and hence the mechanical behaviour. The validity of the relationships is then verified on different granular assemblies (granite fragments and quartz sands), although further tests remain necessary to validate them under more generic stress paths.

International audience; This paper describes research done on a new testing device including its laboratory test procedure; namely, a laboratory punching test for cementitious backfill materials that evaluates their excavatability with a pick. Three-dimensional and two-dimensional numerical studies using a finite element method were carried out according to calibrated parameters of the material obtained by triaxial tests. This made it possible to study the behavior of the backfill material under action induced by a tool (a pick) as well as to optimize the geometry of the punching test specimen. This paper provides detailed information and results for the experimental study of this laboratory test. Attention was particularly focused on energy relating to displacement as well as types of ruptures. The behavior of these types of materials was found to be greatly influenced by gravel in the material. Repeatability of the test was also studied. Finally, a method for this punching test of backfill materials is also provided.

This paper investigates the influences that steel type, in situ soil properties, water table depth, pile diameter, roughness and driving procedures have on the ageing behaviour of piles driven in sand. Tension tests have been performed on 51 open-ended steel micro-piles, with 48 to 60 mm outside diameter, driven at well-established research sites at Larvik in Norway, Dunkirk in France and Blessington in Ireland to better understand the processes that control axial capacity set-up trends in the field. Mild steel, stainless and galvanised steel micro-piles were driven and left to age undisturbed for periods of between 2 h and 696 days before being subjected to first-time axial tension load tests. In addition to reporting and interpreting these experiments, further investigations of the sites’ geotechnical profiles are reported, including new piezocone and seismic cone penetration soundings as well as laboratory tests. Integration with earlier ageing studies at the same sites with larger (340 to 508 mm outside diameter) open-ended steel piles driven to 7 to 20 m embedments and experiments that varied the piles’ initial surface roughness shows that corrosion, pile scale, roughness, the bonding of soil particles and the driving process can all be highly significant. New insights are gained into the mechanisms that control the axial capacity of piles driven in sand.

This study presents an application of the Gaussian mixture (GM) method for soil classification using multiple cone penetration tests (CPT). Compared to the hard clustering methods, the GM model classifies the CPT data by representing the probability density function of observed variables as a mixture of multivariate normal distributions. A GM model based expectation maximization (EM) algorithm with Bayesian information criterion (BIC) for selecting the optimal number of clusters is developed using six real CPT data performed at Dunkerque site in the north of France. The classification results are compared with the classical CPT based interpretation using the non normalized soil behavior type (SBT) index together with the Robertson chart. The results show that the GM model is able to identify accurately the soil layers. In addition, the combination of all CPTs, rather than considering them separately, may improve the soil layers identification because all the site information is considered.

Up to now, the effect of strain rate on the adhesive bond shearing resistance of stiff clay, which plays a fundamental role in constitutive modeling of such kind of soils, remains unclear. This paper aims to experimentally investigate the strain rate dependency of the adhesive bond shearing resistance of stiff clay. Both undisturbed and reconstituted overconsolidated samples of Merville stiff clay from the Flanders region in the north of France are prepared with different overconsolidation ratios (OCR, up to 56). Oedometer tests and undrained triaxial tests under different confining stresses on samples with different OCRs are conducted. The effect of inter-particle bond is gradually investigated by comparing test results of undisturbed and reconstituted samples with the same OCR. All results suggest a uniqueness of the strain rate dependency in one-dimensional and triaxial conditions with different OCRs for the reconstituted clay, whereas the strain rate dependency of shear strength of overconsolidated clay is found to be higher in undisturbed samples than in reconstituted samples due to inter-particle bonding. This new finding is formulated as the adhesive bond shearing resistance against the strain rate, which should be useful for improving the time-dependent modeling of highly overconsolidated stiff clays.

International audience; In soil mechanics, the material behaviour is significantly affected by grain crushing. This energy dissipating mechanism, along with the frictional rearrangement of grains, is responsible for the energy loss associated with plasticity. However, the micromechanical factors at the grain scale have not been studied by constitutive models which tend to focus on macroscopic parameters. In this study, the influence of the coordination number on the fragmentation of a single cylindrical grain specimen has been examined with a new device by which a series of multipoint crushing tests has been conducted. Experiments conducted with this device have shown the importance of the contacts number, position, type and force in the fragmentation of the individual grain. The results, analysed by imaging techniques, demonstrate that the existing models treating grain rupture are indeed incapable of reproducing the observed fragmentation. Two types of cracks were distinguished, each corresponding to a different crack mode, depending on the contact arrangement. The results of this study could be integrated into fragmentation models for predicting the occurrence of cracks, and the shape of the resulting fragments.

This paper aims at showing that the reversible behavior of track ballast can be reasonably modeled by a homogeneous elastic layer in the computation of the reversible dynamic response of railway structures subjected to moving train loads. To accomplish this, 2D simulations by means of the discrete element method (DEM) and triaxial tests were carried out. Importantly, these two approaches were performed placing a soft support under the granular specimen. A small number of loading cycles has been considered in both approaches since the long-term settlement of the ballast layer is not the focus of this study, unlike most of the existing research on this topic. In the DEM part, the transfer of the mechanical loads applied to the sample through force chains in the granular medium has been illustrated and discussed based on the numerical responses obtained. In addition, the analysis of the overall response of the grain samples using a homogenized constitutive law shows that the reversible behavior of a ballast layer can be modeled by a continuum approach. However, investigating the effect of the subbase stiffness on the mechanical response of the ballast layer indicates that the homogenized behavior is not intrinsic but depends on the sublayer. These results were confirmed by triaxial tests performed in the laboratory on samples of crushed stones resting on an elastomer mat with different values of the stiffness. Finally, the results from the DEM simulations and the triaxial tests attest that the reversible response of railways can be computed using an elastic layer for the ballast bed provided that an appropriate choice for its apparent modulus is made. The approach that consists of deducing a reversible homogenized behavior of track ballast based on DEM simulations and/or triaxial tests taking into account the support layer looks promising. In future research, more realistic DEM simulations could be envisaged to develop more accurate constitutive laws that could integrate nonlinear and/or anisotropic aspects.

Volumetric strain of a soil element surrounding the pile accumulated under the drained cyclic loading condition is one of major concerns in geotechnical engineering design. In this paper, twenty stress-controlled drained simple shear tests, with 5000 cycles on Fontainebleau sand have been carried out, which aims to investigate four factors influencing cyclic accumulation behaviour: initial void ratio, normal stress, cyclic shear stress ratio and average shear stress ratio. Based on results, the cyclic volumetric strain accumulation for each influencing factor is summarized, which is found highly sensitive to the cyclic shear stress ratio that a bigger magnitude (more than 0.2) leads to a larger degree of densification. In addition, a simple analytical model is then proposed accounting for these factors with seven parameters that can be directly calibrated from laboratory tests. Finally, the applicability of the model is examined by simulating the additional tests through an adoption of the parameters calibrated by the training tests.

This paper presents the results of an experimental study of the particle scale mechanisms that underpin creep, on-going deformations under constant external load, in dry non-cemented sand under 1D oedometric compression loading at 2500 kPa. Traditional observations on the boundary of the sample are complemented with simultaneous measurements of the 3D kinematics of both the entire grain assembly and details of grain-scale mechanisms using synchrotron based X-ray tomography at two different spatial resolutions. Both the continuum response and the local grain scale response are captured using two spatial resolutions, i.e. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${6.5}\,{\upmu }\hbox {m}$$\end{document}6.5μm and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${0.65}\,{\upmu }\hbox {m}$$\end{document}0.65μm respectively. The results, for the first time, illustrate that small displacements measured at the boundary can be the result of rather pronounced fracturing at the individual grain scale.

After a field test campaign conducted in Dunkirk (north of France) on open-ended steel piles aiming to study the ageing phenomenon, laboratory scale shear tests were designed to study the behaviour of the sand-steel interface. In order to carry out this laboratory investigation, the direct interface shear apparatus was used for characterizing Dunkirk sand (in dry or unsaturated conditions with about 6% water content as in the field) consolidated on initially smooth mild steel plates at different consolidation time intervals (0, 1 and 7 days) and different consolidation stresses (50, 100, 200 and 300 kPa). The test program also included two normal boundary conditions (Constant normal load CNL and constant volume CV) so that they can be compared to the field results and determine the most approaching configuration. More, the unsaturated condition induced a corrosion of the mild steel plates, causing a layer of sand remaining glued to the plate after removing the shear box. Traces of corrosion were also observed on the lower part of the sand samples (in contact with the plate). These observations lead to the interpretation of an increase of the mechanical properties (local cohesion and increase of the friction angle). In order to follow the evolution of the corrosion for each plate, thickness measurements of the sand layer stuck on the plates were carried out.

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.

Energy piles are double purpose foundation elements used both for transferring loads to the soil and temperature regulation in buildings. The response of the pile-soil interface is influenced by daily and seasonal temperature variations. In order to assess the impact of thermal cycles on the mobilization of shear strength in energy piles, a series of saturated soil-concrete interface direct shear tests were performed in the laboratory for different temperature gradients with a new interface direct shear device adapted for thermomechanical loading. As natural soils are very complex due to a high variability of mineralogy and anisotropy, silica and carbonate sands were chosen in this study. Those sands are considered as the main types of sandy soils commonly met in geotechnics. The experimental campaign is divided in two parts: (i) Concrete-soil direct shear tests at 13°C (constant temperature) to be used as a reference (ii) Concrete-soil direct shear tests after 10 temperature cycles with a gradient ΔT=10°C, under submerged conditions. For these two types of soils, realistic temperature cycles applied between 8 and 18°C cause the overall low contraction of the samples. However the interface friction angles are not significantly modified before and after the temperature cycles. Even if the vertical strains of soils are cumulative along temperature cycles, soil’s strains and friction angle changes are relatively negligible for the temperatures and water content tested, which support the low impact of temperature cycles on the deformation of soil concrete foundation under submerged conditions. These experimental results bring new features which will be implemented in numerical models to study the long-term use of energy piles.

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.

Energy piles are double purpose foundation elements used both for transferring loads to the soil and temperature regulation in buildings. The response of the pile-soil interface is influenced by daily and seasonal temperature variations. In order to assess the impact of the thermal cycles on the mobilization of shear strength, in-situ and laboratory experiments were performed. Three energy piles were instrumented with vibrating cord sensors in the foundation of a waste water treatment plant in the North East (NE) part of Paris (at Sept Sorts, Seine et Marne), France. The deformation of these sensors has been recorded for more than one year and the impact of temperature cycles was assessed. In addition the impact of cyclic thermal loading on soil deformation at the soil-pile interface was also studied in the laboratory using an interface direct shear device equipped with a temperature control system. The preliminary results show that the undisturbed soil temperature below 5 m deep remains constant, around 13 °C and that the response of sand-concrete interface at different temperatures and temperature cycles consists in the overall low contraction of the sand.

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.

Comprehensive series’ of field pull-out tests have been carried out on small diameter (48 to 60mm outside diameter (OD)) open-ended steel driven piles at Larvik in Norway, Dunkirk in France and Blessington in Ireland to investigate the processes that affect the ageing of axially loaded piles in sand. This paper reports new investigations of the sites’ geotechnical profiles and tension tests on corroded mild steel (MS) piles conducted up to 315 days after driving. The potential influences of: (i) pile diameter, (ii) ground water, (iii) steel type and (iv) installation method are assessed and comparisons made with the ageing responses established in earlier tension tests on larger diameter industrial scale steel driven pipe piles. Steel corrosion and bonding of soil particles are shown to be particularly important with the small diameter piles; the driving process is shown to be significant.

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.

This paper presents an experimental and constitutive study of compaction banding in Tuffeau de Maastricht, a bioclastic sedimentary limestone exhibiting up to 52% of porosity. An elasto-plastic constitutive model is used to simulate the mechanical behavior of the limestone within the compaction localization regime. It is shown that the simulated macroscopic behavior is in good agreement with the experimental data. In addition, image processing tools have been used to perform full-field measurements elucidating the mechanics of initiation and propagation of localized compaction zones. These findings emphasize the complex nature of localized compaction in porous rocks and represent a preliminary step towards the integrated use of multi-scale testing and mechanical modeling for their characterization across scales.

This paper discusses the quality of the procedure employed in identifying soil parameters by inverse analysis. This procedure includes a FEM-simulation for which two constitutive modelsa linear elastic perfectly plastic MohrCoulomb model and a strain-hardening elasto-plastic modelare successively considered. Two kinds of optimization algorithms have been used: a deterministic simplex method and a stochastic genetic method. The soil data come from the results of two pressuremeter tests, complemented by triaxial and resonant column testing. First, the inverse analysis has been performed separately on each pressuremeter test. The genetic method presents the advantage of providing a collection of satisfactory solutions, among which a geotechnical engineer has to choose the optimal one based on his scientific background and/or additional analyses based on further experimental test results. This advantage is enhanced when all the constitutive parameters sensitive to the considered problem have to be identified without restrictions in the search space. Second, the experimental values of the two pressuremeter tests have been processed simultaneously, so that the inverse analysis becomes a multi-objective optimization problem. The genetic method allows the user to choose the most suitable parameter set according to the Pareto frontier and to guarantee the coherence between the tests. The sets of optimized parameters obtained from inverse analyses are then used to calculate the response of a spread footing, which is part of a predictive benchmark. The numerical results with respect to both the constitutive models and the inverse analysis procedure are discussed.

X-ray tomography is a very valuable tool for studying the full-field 3D deformation of granular materials. The requirement to stop loading and scan a given state (assumed to be stationary) used in most approaches implies unavoidable stress relaxation during scanning. Since scanning times on laboratory tomographs are normally in the order of 1 hour, the strength of the assumption of a stationary state cannot be tested, which introduces some potential weakness in the interpretation of the rich micro-mechanics observed. This paper presents the kinematics of relaxation of a dry natural sand in a typical oedometric cell used for X-ray scanning, using a synchrotron X-ray source to provide scanning times of around 3 minutes, at two different magnifications. This allows the relaxation of the cell & sand system for the first time to be quantified. Advanced image correlation tools are used to quantify the rearrangements of the soil skeleton during loading and the subsequent relaxation. The results indicate that the magnitude of grain displacements during relaxation, associated to ≈4% reduction in externally measured axial stress under oedometric loading, falls below 0:01D50. It can, therefore, be concluded that the relaxation step required prior to an X-ray scan during an in-situ geomechanical experiment on dry sand does not lead to appreciable uncertainties.

Dans le cadre des projets éoliens offshore, la conception optimisée des fondations de type monopieux forés dans des massifs rocheux carbonatés demande une estimation réaliste de la résistance au cisaillement mobilisable. Dans cette optique, une campagne d’essais de cisaillement d’interface roche / coulis est conduite sur la machine BCR3D qui permet, en particulier, d’appliquer une condition de rigidité imposée sur la normale à l’interface, condition jugée représentative de ce qui se passe au droit du pieu. On présente dans une première partie le dispositif expérimental, la préparation des éprouvettes d’essai et le choix des paramètres expérimentaux. Puis, au travers de différents résultats expérimentaux, on montre l’importance de la rigidité normale dans le développement de la résistance au cisaillement. Dans un second temps, on met en évidence que la rugosité, son évolution sous chargement et le contraste de propriétés mécaniques entre une roche (calcaire tendre ou calcarénite) et le coulis conditionnent le frottement mobilisable. L’élément commun à toutes les observations est la dilatation empêchée de l’interface et l’évolution associée de la contrainte normale à l’interface.

La plupart des éoliennes offshore sont fondées sur monopieux. L’augmentation constante de la capacité des turbines et l’insuffisance des procédures évoquées dans les documents normatifs, notamment en ce qui concerne la prise en compte des chargements cycliques, posent des défis pour leur dimensionnement. Dans le cas des sites français, la présence de sols et roches carbonatés, dont le comportement est encore mal connu aujourd’hui, rend la tâche encore plus complexe. Le projet SOLCYP+, lancé en 2017 pour une durée de 3 ans, doit permettre de disposer de nouvelles méthodologies fiables et robustes pour le dimensionnement des monopieux de grand diamètre y compris dans le contexte géotechnique du plateau continental français. Cet article présente une description détaillée du projet ANR SOLCYP+. On aborde, tout d’abord, le contexte, la problématique et les objectifs du projet puis, dans un second temps, on décrit plus en détail les quatre principales tâches scientifiques.

International audience; The mechanical properties of sand: stiffness, cohesion and, to a less extent, friction angle can be increased through the process of grouting. A constitutive model adapted for cohesive-frictional materials from a homogenization technique which allowed us to integrate constitutive relations at the grain level has been developed to obtain constitutive equations for the equivalent continuous granular medium. A representative volume was obtained by mobilizing particle contacts in all orientations. Thus, the stress-strain relationship could be derived as an average of the behavior of these local contact planes. The local behavior was assumed to obey a stress-dependent elastic law and Mohr-Coulomb's plastic law. The influence of the cement grout was modeled by means of adhesive forces between grains in contact, which were added to the contact forces created by an external load. The intensity of these adhesive forces is a function of nature and amount of grout present inside the material and can be reduced due to a damage mechanism at the grain contact during loading. In this paper, we present several examples of simulation which show that the model can reproduce with sufficient accuracy the mechanical improvement induced by grouting as well as the damage of the grain cementation during loading..

This paper presents a prospective study for identifying selected parameters of the modified Cam-Clay model representing the behaviour of heavy oil reservoirs. The first part shows that the plastic compressibility, which controls the main recovery mechanism of such reservoirs, can be accurately determined, simultaneously with other parameters, by an inverse analysis of pressure–strain curves. The conditions of the identification procedure mainly involve two tests conducted under different drainage conditions or at different strain rates. The numerical study also establishes the sequence of an original in situ experimental program, in which three dilatometer tests at a relatively great depth (several hundreds of metres) were carried out. The comparison of the experimental data with the numerical computations reveals a significant over-consolidation ratio which does not allow the plastic compressibility to be determined but supports the findings regarding the geological erosion of the site. Copyright © 2006 John Wiley & Sons, Ltd.

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.

L'injection d'un coulis de ciment ultrafin dans un terrain alluvionnaire constitue un traitement de terrain efficace qui permet de construire un ouvrage souterrain sans porter prejudice a l'integrite structurelle du bâti existant. Afin de prendre en compte, d'une maniere rationnelle, l'amelioration des proprietes mecaniques des sols apportee par l'injection, on a conduit une campagne d'essais triaxiaux sur des echantillons vierges de tout traitement puis sur des echantillons injectes en Iaboratoire. On montre comment l'injection de coulis influe sur le comportement mecanique des sols. On propose enfin des facteurs d'amelioration des parametres du modele elastique plastique parfait et la facon dont ils peuvent etre integres dans les calculs d'ouvrages.

International audience; We carried out compression tests on crushable sand to study the effect of creep and relaxation after flooding. We show that there is a consistent framework linking the breakage ratio with the compressibility, which could be useful to develop constitutive models.

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.

We present a method to predict how grain size distribution (gsd) evolves during confined comminution. We describe particle breakage as a probabilistic phenomenon considering the single particle strength and the pdf of interparticle contact forces from DEM simulations. Our results show a good agreement with published experimental data.

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