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Swelling of clay–sulfate rocks is a serious and devastating geo-hazard, often causing damage to geotechnical structures. Therefore, understanding underlying swelling processes is crucial for the safe design, construction, and maintenance of infrastructure. Planning appropriate countermeasures to the swelling problem requires a thorough understanding of the processes involved. We developed a coupled hydro-mechanical (HM) model to reproduce the observed heave in the historic city of Staufen in south-west Germany, which was caused by water inflow into the clay–sulfate bearing Triassic Grabfeld Formation (formerly Gipskeuper = “Gypsum Keuper”) after geothermal drilling. Richards’ equation coupled to a deformation process with linear kinematics was used to describe the hydro-mechanical behavior of clay–sulfate rocks. The mathematical model is implemented into the scientific open-source framework OpenGeoSys. We compared the model calculations with the measured long-term heave records at the study site. We then designed a sensitivity analysis to achieve a deeper insight into the swelling phenomena. The synthetic database obtained from the sensitivity analysis was used to develop a machine learning (ML) model, namely least-squares boosting ensemble (LSBoost) model coupled with a Bayesian optimization algorithm to rank the importance of parameters controlling the swelling. The HM model reproduced the heave observed at Staufen with sufficient accuracy, from a practical point of view. The ML model showed that the maximum swelling pressure is the most important parameter controlling the swelling. The other influential parameters rank as Young’s modulus, Poisson’s ratio, overburden thickness, and the initial volumetric water content of the swelling layer.

Landslides are more widespread than any other geological events on Earth. Due to their steep topography and contrasted weather conditions, volcanic regions are especially prone to water-triggered landslides. Indeed, examples of volcanic slope failures abound. Regions endowed with volcanic soils are often densely populated and landslides are causing devastating impacts including loss of human lives, damage to critical infrastructure and disruption to livelihoods. There is growing concern that the intensifying effects of climate change on the hydrological cycle – changing the amount and frequency of rainfall and meltwater input – will exacerbate shallow-seated landslide susceptibility. Soil is the weakest material involved in landslide-related disasters, and soil properties are pivotal in determining the susceptibility of a slope to mass movement. Volcanic soils have unique, but hitherto poorly constrained, hydraulic and mechanical properties. Depending on the volcanic parent materials and weathering conditions, these soils can display clay fractions of different mineralogical composition which likely influence their hydraulic and mechanical properties.Our study aims to advance understanding of the relationships between the hydraulic and mechanical properties of volcanic soils. We sampled undisturbed volcanic soils in Tenerife (Spain) and Ecuador characterized by different mineralogies in order to measure their microstructural and hydraulic properties (particle size distribution ; pore size distribution ; hydraulic conductivity ; water retention curve). We also determined the mechanical properties (shear strength) of these soils and quantified the effect of soil water content on these properties conducting triaxial tests at different moisture level. The mineralogical analysis performed reveal clay fractions either enriched in allophanes or halloysites for the different sampled sites. The allophanic soils display large porosities and water retention values, whereas halloysites-rich soils are less efficient to retain water but seem to conduct it faster. Halloysites-rich soils also show higher, but more water content dependent shear strengths. Indeed, maximum shear stresses reached during triaxial tests are largely increased with drying while allophanic soils’ shear strength are less impacted by a decreased water content. This could be explained by the aggregation of allophane particles in clumps during drying, causing a reduction of shear strength offsetting the classic increasing shear strength due to the increased capillary effects.

Shrinkage of expansive soils may cause serious problems to geotechnical and geo-environmental structures. In this study, the shrinkage behaviour of compacted clay treated with 4% lime was investigated. The samples of compacted clay without lime treatment were also tested for comparison. Besides, changes in the microstructure were analysed by mercury intrusion porosimetry tests on both treated and untreated samples. The results showed that the shrinkage curve of untreated sample is close and almost parallel to the saturation line, prior to approaching the compacted shrinkage limit (shrinkage limit for compacted soils). By contrast, the shrinkage curve of lime-treated soil is located leftwards and exhibits a higher compacted shrinkage limit, evidencing a clear microstructure effect. Further analysis showed that large aggregates with large pores are formed by lime treatment and these large pores play an important role in the shrinkage behaviour of soils. In addition, the curing time effect seems to be insignificant on the shrinkage behaviour of lime-treated soil.

To clarify the mechanism of stiffness change by pozzolanic reaction for lime-stabilised soils during curing, it is essential to avoid the effect of water evaporation in this process. In this study, changes of small strain shear modulus Gmaxand total suction of a lime-treated silt during curing were investigated, with water content kept constant to avoid the effect of water loss. The soil samples were prepared with 2% lime, and then compacted dry of optimum (17%) and wet of optimum (22%). The Gmaxvalues were determined at various curing times using bender elements and the suction was measured using a hygrometer WP4. Results show that a two-phase development of Gmaxoccurs for the lime-treated samples. The Gmaxincreases slowly in the first phase, corresponding to the flocculation process, but then Gmaxstarts to increase significantly in the second phase owing to the substantial cementitious compounds produced through pozzolanic reaction. Interestingly, the suction changes follow the same two-phase pattern, suggesting that the production of cementitious compounds increases the soil stiffness; it also modifies the soil microstructure, leading to soil water redistribution and thus changes in suction.

International audience; Like bricks of compacted bentonite/sand mixtures, mixtures made up of pellets and powder of bentonite are considered as a possible material to make the sealing plugs used to fill up galleries and ensure long-term watertightness in deep radioactive waste disposal. Pellets/bentonite mixtures have a low permeability, high swelling capacity, good radionuclide retention capability and operational advantages in terms of placement in the galleries. Following a previous in-depth characterisation of bentonite pellets/powder mixture conducted by the same group, an investigation of the water-retention properties and microstructure changes of a bentonite pellet subjected to wetting/drying cycles under free swelling conditions was carried out by means of mercury intrusion porosimetry (MIP) and X-ray microtomography. A complete description of the changes in water content, void ratio and degree of saturation of the pellet was provided. Data showed that the free swelling of the pellet is due to the combined effect of both crack propagation at the macro scale, and the swelling of bentonite grains, governed by hydration mechanisms along the smectite faces at the nano scale. Significant development of a crack network is observed between 38 and 9 MPa. For suctions below 9 MPa, there is a significant decrease of the platelet thickness and an increase in the disorder of the platelet assembly, resulting in the average MIP entrance pore radius increasing to 0·4 μm within the expanded bentonite grains.

International audience; Experimental results for the cracking of heterogeneous clay samples during desiccation are reported, and an associated numerical model is developed for comparison. The clay samples contain embedded rigid inclusions to induce heterogeneous strain fields during drying. The crack paths and local strain fields are monitored during the desiccation process using digital image correlation. A numerical phase field model for crack initiation and propagation is introduced and compared with the experimental results. A qualitative agreement is found for the obtained crack paths, whereas discrepancies remain for the local strain fields. A discussion regarding the comparison between the experimental results and model is provided.

Besides standard approaches, the soil water retention curve (SWRC) can be estimated either from pore size distribution obtained by mercury intrusion porosimetry or from the soil freezing characteristic curve. These methods are based on simplified laws such as Young-Laplace for capillary suction or ClausiusClapeyron for cryo-suction. These laws might not be valid for clayey sands where the presence of clay particles would induce other water retention mechanisms. This study aims to assess the clay content's effect on the validity of these two methods in determining the SWRC of clayey sands. Clay sands were prepared at different clay contents by mixing pure sand with kaolin clay prior to compaction at the Proctor maximum dry density. Five clay contents (dry mass of clay divided by dry mass of soil) were considered (0, 5, 10, 15, and 20%). The reference SWRC was obtained by standard suction measurements. The results show that SWRCs estimated from mercury intrusion porosimetry and soil freezing characteristic curve are generally in agreement with the reference SWRC. However, the results obtained by the soil freezing characteristic curve are limited in terms of suction range (500 to 5 MPa) which is not appropriate for clayey sands, while those from mercury intrusion porosimetry show significant discrepancy because of the structure heterogeneity obtained by Proctor compaction.

Blocks of compacted bentonite/sand mixture, with a 40/60 proportion in dry mass, are being studied as an elementary component in the construction of seals in the French concept for high-levelradioactive waste disposal. In the disposal, the seal will be subjected to water infiltration from the host rock. Water retention behaviour in the mixture is governed by bentonite. Nevertheless, the impact of the sand fraction, especially at high concentrations, is still unclear. The water retention curve is found experimentally for the wetting path by using vapour equilibrium technique. Moreover, NMR relaxometry tests executed atfull saturation state are presented. As a result of the comparison with experimental data reported in the past, for different mixtures -ranging from pure bentonite to 20/80 bentonite/sand mixtures-, it is concluded that all porosity in the mixture can be correctly attributed to bentonite. This conclusion simplifies mixtures to equivalent pure bentonite specimens with a dry density equal to the bentonite dry density (𝜌𝑑,𝑏) of the mixture. Moreover, an original method to compare NMR relaxation times for different bentonite/sand -or pure bentonite- mixtures is proposed. It provides the longitudinal relaxation time (T1) in saturated mixtures as a function of 𝜌𝑑,𝑏 and the montmorillonite concentration in bentonite.

International audience; The effect of water distribution on heat conduction in unsaturated structural clayey soils with double porosity system is investigated in the present study. A dual-porosity model consisting of intra-aggregate and inter-aggregate pores is developed to describe the water distribution within intra-aggregate and inter-aggregate pores. Heat transfer using this model is then numerically simulated to determine the soil effective thermal conductivity. The obtained values are compared with available experimental data. The results show that the model can predict the accurate soil thermal conductivity values at various water contents and densities. On one hand, the model shows a higher soil thermal conductivity when water content and/or dry density are higher. On the other hand, the model shows an effect of water distribution on the soil thermal conductivity; this later is higher when water is preferentially distributed within aggregates than between aggregates. In addition, the model can give a direct visualization of heat transfer mechanisms in unsaturated clayey soils with double porosity system. In fact, heat conduction is dominant in the wet region of the dual-porosity space. This study provides both a useful predictive model of thermal conductivity and a better understanding on the physical mechanism of heat conduction in unsaturated structural clayey soils or other multiphase media with double porosity system.

DEM-simulated model cohesive assemblies of spherical grains of diameterd, with contact tensile strengthF0, once prepared in loose states, are quasistatically subjected to growing isotropic pressureP, and then to triaxial compression, maintaining lateral stresses σ2= σ3=Pwhile increasing axial stress σ1=P+qand strain є1. Reduced pressureP*=d2P/F0varies from 0.1 (cohesion dominated case, for which systems typically equilibrate with solid fraction Ф ≃ 0.35), to large values for which the cohesionless behavior is retrieved. In triaxial compression, while the moderate strain response (є1~ 0.1) is influenced by initial coordination numbers and mesoscale heterogeneities, the approach to the critical state, as bothq(deviator) and Ф steadily increase, gets slower for smallerP*. Critical ratioq/P strongly increases for decreasingP*, as roughly predicted in an “effective stress” scheme. Anomalously small elastic moduli are observed in the gel-like structures. While extensive geometric rearrangements take place, no shear banding is observed. Loose cohesive granular assemblies are thus capable of large quasistatic stable plastic strains and ductile rupture.

International audience Accurate prediction of thermal regime (i.e., the variation of soil temperature, θ, and heat flux, ) to investigate the migration of thermal energy in soil mass, poses challenge to the geoenvironmental engineers while dealing with various thermo-active structures. In this context, several numerical approaches have been attempted to solve the heat transfer equation (HTE) for conduction to predict the thermal regime. However, most of them need accurate boundary condition defined and involve complicated numerical approach which is inconvenient for the practising engineers. Hence, an attempt has been made in the present study to develop a simplified, but numerically efficient, approach based on the finite difference method (FDM) to analyse one dimensional heat transfer in dry sands when the end boundary is not defined. Furthermore, a time dependent initial boundary condition has been applied to this model to simulate similar experimental condition and results have been compared vis-à-vis those obtained from the experiment and COMSOL Multiphysics ® to validate the proposed approach.

International audience; Bentonite pellet-powder mixtures are candidate materials for sealing the galleries in deep geological repositories for radioactive waste. In the present work, swelling pressure tests are performed on pellet-powder mixtures with different powder contents. Results highlight the influence of the initial granular structure on the mechanical behaviour of pellet-powder mixtures. In mixtures with a low density powder in the inter-pellet porosity, the macroscopic response of pellet-powder mixtures is identical to that of a pellet assembly with no powder. A model is proposed to describe the hydromechanical behaviour of pellet-powder mixtures. The formulation considers two distinct states of the material, Granular and Continuous. In the Granular state, the pellets control the mechanical behaviour of the mixture. In the Continuous state, both pellets and powder contribute to the mechanical behaviour of the mixture. In the Granular domain, the material behaviour is described by constitutive laws proposed after a numerical study using Discrete Element Method. In the Continuous domain, the material behaviour is described by a modified Barcelona Basic Model. Transition between the two domains depends on the density of the powder phase and suction. The model is implemented in a Finite Element Method code, and the swelling pressure tests performed in the laboratory are simulated with a single set of parameters. These results improve the knowledge on the behaviour of bentonite pellet-powder mixtures during hydration in repository conditions.

International audience; To better understand results of SEALEX in situ tests carried out at Tournemire Underground Research Laboratory, the hydromechanical behavior of a pellet/powder MX80 bentonite mixtures prepared at a dry density of 1.49 Mg/m3 were investigated by means of microfocus X-ray computed tomography (μ-CT) observations and laboratory small scale infiltration tests. Radial and axial swelling pressures as well as relative humidity were monitored while wetting. Two configurations were considered: for the first, a pellet/powder mixture was prepared following a specific protocol to minimize initial structural heterogeneity; the second one was specially designed to study a strong heterogeneous mixture distribution. μ-CT observations performed on the two samples during hydration revealed an apparently homogeneous sample for the first mixture after 100 days of hydration. For the second specimen, several voids were still observed after 40 days of hydration. A comparison was made between the in situ and mock-up tests. It was observed that the evolutions of radial and axial swelling pressures depend on the initial heterogeneous distribution of the mixture. This heterogeneity is due to the different dry density values at the vicinity of the different sensors. The final values of axial swelling pressures were different for both configurations for the same global dry density.; Pour mieux comprendre les résultats des tests in situ SEALEX effectués au laboratoire de recherche souterrain de Tournemire, le comportement hydromécanique de mélanges de bentonite MX80 pastille/poudre préparés à une densité sèche de 1,49 Mg/m3 a été étudié au moyen d’observations de microtomographie aux rayons X (μ-CT) et des tests d’infiltration à petite échelle en laboratoire. Les pressions de gonflement radiales et axiales ainsi que l’humidité relative ont été contrôlées pendant le mouillage. Deux configurations ont été envisagées : pour la première, un mélange pastille–poudre a été préparé selon un protocole spécifique afin de minimiser l’hétérogénéité structurelle initiale ; le second a été spécialement conçu pour étudier une forte distribution de mélanges hétérogènes. Les observations de μ-CT effectuées sur les deux échantillons au cours de l’hydratation ont révélé un échantillon apparemment homogène pour le premier mélange après 100 jours d’hydratation. Pour le second échantillon, plusieurs vides ont encore été observés après 40 jours d’hydratation. Une comparaison a été faite entre les tests in situ et maquette. Il a été observé que les évolutions des pressions de gonflement radiales et axiales dépendent de la distribution hétérogène initiale du mélange. Cette hétérogénéité est due aux différentes valeurs de densité sèche à proximité des différents capteurs. Les valeurs finales des pressions de gonflement axiales étaient différentes pour les deux configurations pour la même densité sèche globale.

International audience; Bentonite pellet mixtures are candidate material for the sealing of galleries in radioactive waste disposals. The hydromechanical behaviour of assemblies of bentonite pellets is investigated upon partial hydration through (1) suction-controlled swelling pressure tests in the laboratory and (2) discrete element method simulations. The combination of these experimental and numerical approaches highlights that, before the mixture homogenization, the swelling pressure develops in two phases. The first phase is characterized by an increase in contact forces between pellets. The second phase is characterized by plasticity at contacts between pellets and is controlled by the progressive decrease in pellet strength and stiffness upon hydration. In addition, numerical results highlight that the swelling pressure measured in the laboratory can be influenced by the sample preparation, the cell size, and the diameter of the pressure sensor.

The aim of this paper is to investigate the long-term hydro-mechanical behaviour of a highly heterogeneous MX80 bentonite pellet/powder mixture (80/20 in dry mass), which is one of the candidate sealing materials in deep geological repositories. In spite of the operational advantages related to the use of the mixture, structural heterogeneities resulting from the installation process constitute a matter of concern and require special approaches to describe adequately the material behaviour during hydration. In this study, a double structure formulation taking into account the initial structural heterogeneity of the material as well as damage to pellets upon wetting is proposed. The formulation is applied to the modelling of a 1/10 mock-up of Sealex large-scale tests. Hydraulic and mechanical parameters are determined from an extensive laboratory programme carried out on a single pellet of bentonite and on the pellet/powder mixture. To ensure an adequate analysis of the test, the initial heterogeneous structural distribution of the material is determined by image analysis of microfocus X-ray computed tomography observations. The model allows the anisotropic swelling behaviour of the mixture to be satisfactorily reproduced when accounting for the spatial variation of the material's initial porosity. In particular, the long-term (decade) hydro-mechanical behaviour of the mixture can be well described. Detailed analysis of the modelling results demonstrates the existence of dry density gradients at the long term and their influence on swelling pressure anisotropy.

International audience; The paper presents a study of the thermo-mechanical behavior of energy diaphragm wall. A physical model, which consists of a small-scale concrete diaphragm wall equipped with a heating exchange pipe, was used. A heating test was performed where hot water (at 50 °C) was circulated through a heat exchange pipe for 75 h. The results show that the temperatures in the wall and in the soil increased quickly during the first 20 h and reached stabilization at the end of the experiment. The temperature increase induced increase of axial strain in the wall and earth pressure at the soil/wall interface. In addition to the experiment, a numerical model, using finite element analysis, was used to predict the behavior of the wall during this experiment. The good agreement between the numerical and the experimental results allows the main phenomena that took place to be explained; heating induces thermal expansion of the wall that results in the modification in stress in the wall and at the soil/wall interface. In addition, since the pipe was located closer to one side of the wall, the thermal expansion of the wall was not homogenous, and the wall bent during heating.

Loess covers large portions of Earth's surface. Loess deposits are typically composed of silt with clay and fine sand particles and usually reach a few meters in thickness. Thermal conductivity of loess varies over a relatively large range from 0·07 to 2 W/(m.K), depending on the particle composition, soil texture and also moisture content. In this study, intact loess was sampled at shallow depth (1 m) in Northern France. Suction, volumetric moisture content and thermal conductivity of soil were measured simultaneously during the application of wetting/drying cycles to the sample. The results show that the volumetric moisture content significantly influences the soil thermal conductivity. As is the case for the water-retention curve, a hysteresis loop can be observed when plotting the thermal conductivity against suction. However, no hysteresis appears when the thermal conductivity is plotted as a function of the degree of saturation.

International audience; This paper presents a numerical study on the desiccation cracking process of clayey soil. The initiation and propagation of cracks were investigated using finite element code, including the damage-elastic cohesive fracture law to describe the behaviour of cracks. The coupling between the hydraulic behaviour (moisture transfer in the soil matrix and in the cracks) and the mechanical behaviour (volume change of the soil matrix and development of cracks) were also considered. The results of a laboratory experiment performed on clay soil, taken from a literature review, were used to evaluate the numerical modelling. The results show that the code can reproduce the main trends observed in the experiment (e.g., shrinkage related to drying, crack development). In addition, the numerical simulation enables the identification of other phenomena, such as the evolution of suction and stress related to drying and the development of a single crack. These phenomena are difficult to observe experimentally.

International audience; A small-scale pile has been developed in the laboratory to investigate the thermo-mechanical behavior of energy piles subjected to a significant number of thermal cycles. The pile (20 mm external diameter), installed in dry sand, was initially loaded at its head to 0, 20, 40 and 60% of its ultimate bearing capacity (500 N). At the end of each loading step, 30 heating/cooling cycles were applied. The long-term behavior of the pile was observed in terms of head settlement, axial force profile, soil and pile temperature, and stress in soil. The results evidence the irreversible settlement of the pile head induced by thermal cycles under constant load head. In addition, the incremental irreversible settlement that accumulates after each thermal cycle decreases when the number of cycles increases. The evolution of irreversible pile head settlement versus number of cycles can be reasonably predicted by an asymptotic equation.

This work aims to investigate the aggregate size effect on changes in mineralogical composition and microstructure of lime-treated compacted soils. Three soil powders with different maximum aggregate sizes (Dmax = 5, 1 and 0.4 mm) were prepared prior to the treatment with 2% of lime. X-ray diffraction (XRD), environmental scanning electron microscope (Env. SEM) coupled with chemical analysis using energy dispersive X-ray spectrometry (EDX) and mercury intrusion porosimetry (MIP) were used to analyse untreated and treated samples at various curing times. Crystallized C-S-H on tobermorite form was identified in the lime-treated soil prepared with large aggregates after one year curing, and an evident increase in nanopores

International audience; The behavior of model granular materials (glass beads) wetted by a small quantity of liquid forming capillary bridges is studied by one-dimensional compression test combined with X-ray computed tomography (XRCT) observation. Special attention is paid to obtain very loose initial states (initial void ratio of about 2.30) stabilized by capillary cohesion. XRCT-based analyses involve spherical particle detection adapted to relatively low-resolution images, which enable heterogeneities to be visualized and microstructural information to be collected. This study on an ideal material provides an insight into the macroscopic compression behavior of wet granular materials based on the microstructural change, such as pore distance distribution, coordination number of contacts, coordination number of neighbors and number of contacts per grain.

International audience; As pore-scale morphologies and spatial distribution (pore habits) of natural gas hydrates in marine sediments considerably affect their physical/mechanical properties, they have extensively been investigated by X-ray computed tomography (XRCT) and especially synchrotron X-Ray computed tomography (SXRCT). While both image spatial and scan temporal resolutions are being improved over time, it is still challenging to distinguish water from methane hydrate in an image due to their low absorption contrast. In this study, methane hydrate formation and growth in wet sand were observed at submicron/micron scale using SXRCT. Saline water (Potassium iodide-KI) was used in order to improve the image contrast. Evolutions of methane hydrate morphologies and distribution at both the pore and sample scales were observed. Results are discussed based on various mechanisms related to material behaviors and experimental conditions, e.g. suction variation during methane hydrate formation, local temperature gradient in the sample, and particularly the interaction of X-rays with the sample. Methane hydrate formation is interpreted as a dynamic process, favoring the Ostwald ripening. Furthermore, morphologies and pore habits of methane hydrates under excess-gas and excess-water conditions are discussed. Some recommendations are finally given for further studies on methane hydrates-bearing sediments via XRCT or SXRCT.

International audience; In engineering practice, energy pile foundations are often designed for the lifetime of the building. Thermal exchange between a pile and the surrounding soil depends on the annual energy needs of the building, as heating mode in winter and cooling mode in summer. Thus, energy pile foundations will undergo a heating–cooling cycle per year. In the present work, an experimental method based on a small-scale pile model installed in saturated clay was used to study the thermo-mechanical behaviour of energy piles under thermal cycles. Thirty cycles were applied (to represent a 30-year period if the daily cycles are neglected) while a constant pile head load was maintained. Four tests were performed corresponding to pile head loads equal to 0, 20, 40 and 60% of pile resistance. The results obtained show an increase in irreversible pile head settlement with the thermal cycles. In order to interpret the experimental results better, the finite-element method is used to simulate the experiments numerically. This allows the important role of pile thermal contraction/expansion in the pile–soil interaction under thermo-mechanical loading to be highlighted.

International audience; Pellet-based expansive clay materials are considered as a sealing material for closing the galleries in radioactive waste disposal concepts. In repository conditions, the granular mixture progressively homogenises upon hydration by the host rock pore water. The present study focuses on the material behaviour before homogenisation. A grain-scale experimental characterisation is first performed in the laboratory. A model describing the hydromechanical behaviour of a pellet is proposed based on the experimental results. Then, suction-controlled swelling pressure tests are performed in the laboratory. Using Discrete Element Method (DEM) and the model proposed for a single pellet, the tests are successfully simulated. It is highlighted that (i) the swelling pressure evolves in two phases in the investigated suction range, controlled by the granular structure of the mixture; (ii) wall effects at the laboratory scale affects the material response; (iii) measurement variability associated to the sensor diameter is non-negligible; (iv) DEM is a valuable tool able to provide insight into the material behaviour.

Bentonite-based pellet materials are considered as a sealing material for the isolation of galleries in the French radioactive waste disposal concept, owing to operational convenience. The influence of the granular nature of the material is studied through Discrete Element Method (DEM) simulations. Each pellet is modelled individually and represented by a sphere of same mass and density as real pellets. Swelling pressure tests of pellet mixtures, carried out at laboratory scale, are simulated using a model describing the hydromechanical behaviour of a single pellet upon suction decrease. The mixture behaviour is satisfactorily reproduced upon hydration from 89 MPa (initial state) to 7 MPa of suction. The model is then used to study the behaviour of large granular assemblies of bentonite pellets on the same hydration path. Results highlight that the mixture assembling process, the pellet strength and stiffness, and the mixture density have an influence on the swelling pressure development upon hydration. Numerical results obtained through DEM simulations will be of interest for future Finite Element Method simulations of the full hydration path using double structure models where pellets correspond to the microstructural level.

International audience; A simple and accurate model for predicting the elastic properties of gas hydrate-bearing sediment (GHBS) is proposed and validated against experimental data. It is developed on the basis of the homogenization theory for multiphase composite. Unlike the classical homogenization techniques those fix a homogenization scheme (e.g. self-consistent, Mori-Tanaka, DEM, etc.) for a given microstructure, the proposed model considers a flexible homogenization scheme that adapts with the change of the microstructure of the material. The idea is to modify the elastic properties of the reference matrix with respect to the microstructure change of the mixture. Such modification is ensured by adapting the theoretical method to available experimental data. The derived method is proved to be very powerful as it can fit laboratory measured data on gas hydrate-bearing sand formed by different methods including excess gas and excess water methods and log data taken from various sites. Such feature cannot be satisfied by the existent models those consider the hydrate phase as a cement phase or a pore-filling phase. A very good agreement between the proposed method and experimental data offer a simple and accurate ways to predict the saturation of gas hydrate using sonic log data or 3D seismic data.

International audience; In this study, methane hydrate-bearing sand (MHBS) was created in the laboratory following two methods in order to obtain two types of gas hydrate morphology in sandy sediment. The hydrate morphology in the sediment was assessed by measuring the compressional wave velocity combined with models to predict the wave velocities of the sediment containing gas hydrates. The mechanical properties of the MHBS were investigated by triaxial compression tests. The results obtained by the compressional wave velocity show that after saturating the MHBS sediment (created by the excess gas method) with water, the methane hydrates are partly or completely converted from grain contacts to pore spaces depending on the hydrate saturation (ranging from 0 to 50%). A subsequent temperature cycle completes this conversion process for high hydrate saturation. The results obtained with the triaxial compression tests show higher shear strength, a higher secant Young’s modulus, and a higher dilation angle at higher hydrate saturation. In addition, the effects of hydrate saturation on the mechanical properties of the MHBS obtained by the two procedures (with and without the thermal cycle) are similar at low hydrate saturation. The effect of gas hydrate morphologies can only be detected in the case where the conversion (and/or redistribution) of gas hydrates from grain contacts to pore spaces is not complete (at high hydrate saturation).

International audience; Mixtures made up of bentonite powder and pellets are a possible candidate for making sealing plugs used in deep radioactive waste disposal due to their low permeability, high swelling capacity, favourable properties with respect to radionuclide retention and operational advantages in terms of placement in situ, which is much easier than that of pre-compacted bricks of bentonite/sand mixture. It is therefore essential to better understand their hydro-mechanical behaviour to optimize the design of the repository. In this context, the French Institute for Radiation Protection and Nuclear Safety (IRSN) has launched the SEALEX project (SEALing performance EXperiments) in which this work has been conducted. Once the initially heterogeneous unsaturated powder/pellet (80/20) MX80 bentonite mixture is put in place, these sealing materials will be subject to coupled hydro-mechanical loadings: hydration due to the infiltration of pore water from the natural barrier and mechanical confinement resulting from the engineered barriers. The present work focuses on the different scales of the material: at the macroscopic scale, it is characterised by a heterogeneous distribution of pellets and powder of bentonite; at the microscopic scale, it is studied by several techniques (MIP, μ-CT observations and SEM). From MIP results, a typical bimodal distribution was found for both pellet and powder. From μ-CT and SEM observations, a heterogeneity was revealed in the internal structure of the pellet: heterogeneous density distribution of the clay minerals and presence of several high density elements.

International audience; The Callovo-Oxfordian (COx) claystone is considered in France as a possible host rock for the disposal of high level long lived radioactive waste at great depth. During the operational phase, the walls of the galleries and of the disposal cells will be successively subjected to desaturation induced by ventilation followed by resaturation once the galleries are closed. To better understand this phenomenon, a sound understanding of the water retention properties of the COx claystone is necessary. Following a previous study by the same group, this paper presents an investigation of microstructure changes of COx claystone under suction changes. Microstructure was investigated by means of mercury intrusion porosimetry tests on freeze-dried specimens previously submitted to various suctions. Along the drying path, the initial microstructure, characterised by a well classified unimodal pore population around a mean diameter value of 32 nm, slightly changed with the same shape of the PSD curve slightly moved towards smaller diameters (27-28 nm) at suctions of 150 and 331 MPa, respectively. The infra-porosity too small to be intruded by mercury (diameter smaller than 5.5 nm) reduced from 4.3% to 3.3%. Oven-drying reduced the mean diameter to 20 nm and the infra-porosity to 1%. Wetting up to 9 MPa suction leads to saturation with no significant change of the PSD curve, whereas wetting at zero suction gave rise to the appearance of a large pore population resulting from the development of cracks with width of several micrometers, together with an enlargement of the initial pore population above the mean diameter. The concepts describing the step hydration of smectites (by the successive placement within the clay platelets along the smectite faces of 1, 2, 3 and 4 layers of water molecules with respect to the suction applied) appeared relevant to better understand the changes in microstructure of the COx claystone under suction changes. This also allowed to better define the status of water in claystones and shales containing smectite, with a distinction made between the water adsorbed within the clay platelets and the free inter-platelet water, involved in hydro-mechanical couplings through changes in pore pressure and water transfers.

International audience; In the present work, the shear behaviour of soils and the soil–concrete interface is investigated through direct shear tests at various temperatures. A conventional direct shear apparatus, equipped with a temperature control system, was used to test sand, clay, and the clay–concrete interface at various temperatures (5, 20, and 40 °C). These values correspond to the range of temperatures observed near thermoactive geostructures. Tests were performed at normal stress values ranging from 5 to 80 kPa. Results show that the effect of temperature on the shear strength parameters of soils and the soil–concrete interface is negligible. A softening behaviour was observed during shearing of the clay–concrete interface, which was not the case with clay specimens. The peak strength of the clay–concrete interface is smaller than the ultimate shear strength of clay.

International audience; Methane hydrates, naturally formed at high pressure and low temperature in marine and permafrost sediments, represent a great potential energy resource but also a considerable geo-hazard and climate change source. Investigating the grain-scale morphology of methane hydrate-bearing sandy sediments is crucial for the interpretation of geophysical data and reservoir-scale simulations in the scope of methane gas production as methane hydrate morphologies and distribution within the porous space significantly impact their macroscopic physical/mechanical properties. X-ray computed tomography (XRCT) and Synchrotron X-Ray computed tomography (SXRCT) are commonly used to analyze the microstructure of geo-materials. However, methane hydrates exist only at high pressure (up to several MPa) and low temperature (a few °C). This article describes the development of three experimental setups, which aim at creating methane hydrates in sandy sediment, adapted to XRCT and SXRCT observations. The advantages and drawbacks of each setup are discussed. The discussions focus on the effects of the choice of the system to control temperature and pressure on the quality of images. The obtained results would be useful for future works involving temperature and/or pressure control systems adapted to XRCT and SXRCT observations of various geo-materials.

International audience; This paper aims to model the thermo-mechanical behavior of saturated clays under undrained conditions. Classic thermo-hydro-mechanical formulations obtained using different approaches were compared and discussed, showing their compatibility and differences. A thermo-elasto-plastic model called TEAM, using a two-surface approach, was developed for saturated clays under undrained conditions in the framework of thermo-poro-mechanics. The aim of the model is to predict a smooth transition between the elastic and elastoplastic states. Two additional physical parameters, namely volumetric thermal expansion coefficient and pore water compressibility, were back analyzed from the results of the undrained heating tests. By simulating experiments found in the literature, it was shown that this model is appropriate in capturing the evolution of pore water pressure of saturated clays under non-isothermal undrained conditions.

International audience; Methane hydrate is being considered as a potential future energy source but may at the same time constitute a considerable geo-hazard. In the present study, methane hydrate bearing sand sediment was created by pressurizing methane gas into previously moistened, then chilled, packed sand specimen (excess gas method). The excess gas was then replaced by water at high pressure. Afterward, a heating/cooling cycle was applied under undrained conditions, in order to completely dissociate gas hydrates and then recreate them inside the specimen. Finally, the pore pressure was reduced to the atmospheric pressure to dissociate gas hydrates. The whole process was performed in a magnetic resonance imaging (MRI) system, allowing the determination of water and/or gas and hydrate quantity (and spatial distribution) at various times. The MRI signal was finally analyzed to interpret various processes in sand sediment: initial hydrate formation, heating-induced hydrate dissociation, cooling-induced hydrate re-formation, and depressurizing-induced hydrate dissociation.

International audience; An energy approach is proposed as a complement to the stress approach commonly considered for investigating soil desiccation cracking. The elastic strain energies before and after crack initiation are estimated by both numerical and analytical solutions. The energy released by cracking is then compared with the fracture energy to discuss crack initiation conditions. This leads to combined energy and stress conditions for crack initiation following Leguillon's theory. An approximate analytical solution is derived from a variational formulation of the porous elastic body equations. A cohesive zone model and finite element code are used to simulate crack propagation in an unsaturated porous body. This analysis shows that the energy criterion is reached before the stress criterion, and this can explain unstable crack propagation at the beginning. The approximate analytical solution allows predicting correctly the crack depth and opening in its initiation stage.