Caractérisations expérimentale et numérique du comportement hydro-mécanique d'un matériau hétérogène : mélange de poudre/pellets de bentonite

The present investigation deals with the hydro-mechanical behavior of a mixture composed of pellets and powder of MX80 bentonite with a proportion of 80/20 in dry mass. This is one of the studied materials by the French Institute for Radiation protection and Nuclear Safety (IRSN) within the SEALEX project, which aims at investigating the long-term performance of swelling clay-based sealing systems in the context of geological high-level radioactive waste disposal. This study has been conducted by following an experimental program covering different scales. Firstly, the microstructure changes while wetting of a single pellet was investigated by combining MIP results with μ-CT observations. Results revealed that swelling of a pellet is due to the development of cracks, with significant development between 38 and 9 MPa of suction, combined to swelling of bentonite grains, which is governed by the hydration mechanisms of smectite at nano-scale. The application of suctions below 9 MPa leads to a significant decrease of the platelet thickness and to an increase in the disorder of the platelet assembly. Water retention tests, swelling pressure tests and suction controlled oedometer tests on the pellet/powder mixture were performed. Similar water retention properties were observed for the mixture under constant-volume condition and pellet under free swelling condition under suctions higher than 4 MPa, suggesting that physico-chemical suction prevails on capillary suction. At lower suctions, constant-volume condition defined a lower water retention capacity because of the disappearance of macro-pores. Lower yield stress values than the common pure bentonite mixtures were found for the pellet/powder mixture for non-zero suctions, showing that the volume change behavior is governed by the rearrangement and crushing of pellets, and the loss of the granular structure in the case of zero suction. Two mock-up tests were performed, aiming at studying two extreme cases at a global dry density of 1.49 Mg/m3: a homogeneous pellet/powder mixture fabricated by following a special protocol, and a strong heterogeneous sample. Results revealed that the radial swelling pressure depends strongly on the local pellet/powder distribution combined with the evolution of the hydration front. An anisotropy swelling was found in both cases, being the axial swelling pressure lower than the radial one. Moreover, different values of axial pressure were found between the two tests, even though they have the same global dry density of samples. In parallel, μ-CT observations were carried out on the mixture while wetting, revealing a homogeneous sealed sample after 100 days of hydration. No density gradients were identified at the investigated resolution (50 μm/voxel) after this long time of hydration. A new damage model, which takes into account the development of fissures within a pellet while wetting, was proposed an included to the well-IVknown double porosity Barcelona Expansive Model (BExM) to carry out numerical simulations of one mock-up test. The initial heterogeneous porosity distribution was also considered to reproduce the anisotropy swelling. The experimental results obtained in this study will greatly help well understand the response of seals made up of pellets/powder bentonite mixture in the SEALEX in situ experiment. Moreover, the constitutive model developed taking into account the pellet cracking damage and the initial sample heterogeneity allows significantly improving the prediction of hydomechanical behavior of seals/plugs made up of this mixture, constituting thus an useful tool for the safety assessment of the nuclear waste disposal systemCette thèse porte sur le comportement hydromécanique d’un mélange de poudre et pellets de bentonite MX80 avec une proportion 80/20 en masse sèche. Il s’agit d’un matériau étudié par l’Institut de Radioprotection et de Sûreté Nucléaire (IRSN) dans le cadre du projet SEALEX qui a pour objectif principal la vérification de l’efficacité des dispositifs de scellement ou des barrières ouvragées dans le system du stockage géologique des déchets radioactifs. Le comportement hydromécanique du matériau à différentes échelles a été étudié par différents essais en laboratoire. Premièrement, les changements à l’échelle microstructurale d’un seul pellet de bentonite durant l’hydratation a été abordée à l’aide de deux techniques : la porosimétrie au mercure et la tomographie aux rayons-X. Les résultats ont montré que le gonflement d’un pellet peut être expliqué par deux mécanismes : la création des fissures surtout à des succions entre 38 et 9 MPa, et le gonflement des grains de bentonite, correspondant à l’hydratation des smectites à l’échelle nano. A des succions inférieures à 9 MPa, une diminution de l’épaisseur des feuillets d’argile et une augmentation du désordre des ceux-ci sont observées. Des essais de rétention d’eau, de pression de gonflement et de compression à l’odomètre à succion contrôlée ont été effectués sur le mélange de poudre et pellets. Les propriétés de rétention d’eau sous conditions de volume constant et pour un seul pellet sous conditions de gonflement libre apparaissent similaires pour des valeurs de succion supérieures à 4 MPa. Cela implique que la succion physico-chimique est prédominante devant la succion capillaire. Pour des valeurs de succions plus basses, une capacité de rétention plus faible a été observée sous conditions de volume constant, à relier à la disparition des macro-pores par le gonflement des grains de bentonite. Des valeurs de pression de préconsolidation plus petites que celles des mélanges de bentonite pure ont été obtenues pour des succions non-nulles, montrant l’effet granulaire des pellets dans le mélange. Deux colonnes d’infiltration ont été réalisés afin d’étudier deux cas extrêmes avec une densité sèche globale identique (1.49 Mg/m3). Avec la première colonne, un mélange de poudre et pellets relativement homogène, fabriqué en suivant un protocole spécial a été étudié. En revanche, un mélange fortement hétérogène a été fabriqué dans la deuxième colonne d’infiltration. Les résultats montrent que la pression de gonflement radiale dépend fortement de la distribution des pellets et de la poudre ainsi que de l’évolution du front d’hydratation. Une anisotropie de gonflement a été observée dans les deux cas, avec la pression de gonflement axiale inférieure à celle radiale. De plus, la valeur finale de pression de gonflement axiale est différente pour les deux colonnes, bien que les deux échantillons aient fabriquées avec la même densité sèche globale. En parallèle, plusieurs observations à la tomographie aux rayons-X ont été réalisées sur le mélange de pellets et poudre pendant l’hydratation. Un mélange complètement homogène a été observé après 100 jours d’hydratation à l’échelle étudiée (50 μm/voxel). Un nouveau modèle d’endommagement qui prend en compte des fissures observées au sein du pellet pendant hydratation a été développé en adaptant le Barcelona Expansive Model (BExM). L’essai d’infiltration sur l’échantillon relativement homogène a été simulé avec succès en utilisant le modèle développé. L’hétérogénéité initiale de la porosité a été aussi considérée dans la simulation afin de reproduire l’anisotropie de gonflement. Les résultats expérimentaux obtenus dans le cadre de cette étude permettent de mieux comprendre la réponse des ouvrages de scellement avec le mélange de pellets et poudre de bentonite dans le projet SEALEX. De plus, le modèle développé, qui prend en compte des fissures observées au sein du pellet et l’hétérogénéité initiale du matériau, permettra d’améliore

Modelling the long-term hydro-mechanical behaviour of a bentonite pellet/powder mixture with consideration of initial structural heterogeneities

International audienceThe 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 behaviourduringhydration.Inthisstudy,adoublestructureformulationtakingintoaccounttheinitial 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 mechanicalparametersaredeterminedfromanextensivelaboratoryprogrammecarriedoutonasingle 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 behaviourofthemixturetobesatisfactorilyreproducedwhenaccountingforthespatialvariationofthe 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

Investigation of the hydro-mechanical behaviour of a pellet/powder MX80 bentonite mixture using an infiltration column

International audienceNon-compacted pellet/powder bentonite mixtures are considered as candidate sealing plug materials in geological disposals of radioactive waste. Due to its nature, this mixture is characterized by a heterogeneous porosity network, which is responsible for its complex hydro-mechanical (HM) behaviour. The French Institute of Radioprotection and Nuclear Safety (IRSN) has investigated this mixture within the SEALEX project. Both in situ large-scale and laboratory small-scale experiments were carried out. This paper presents the results of a small-scale mock-up test at 1/10th scale of the in situ experiments, in which the pellet/powder mixture was saturated from both sides (top and bottom of the specimen). Both swelling pressure and relative humidity were monitored at several positions of the specimen. Different responses from the sensors were found, depending on the local porosity as well as the evolution of the hydration front. The HM response of the mixture is strongly conditioned by the initial pellet/powder distribution, which depends on the protocol followed for the specimen preparation. After 800-day hydration, an anisotropy was found between the axial and the radial swelling pressures, due to the presence of larger void at the top of the sample and the friction at the cell wall. The sample was still heterogeneous after 800-day hydration mainly due to the initial heterogeneous porosity distribution, combined with the effect of friction and the non-saturation of the mixture. The evolution of injected water with time revealed that the sample was not full saturated after 800-day hydration

Impact of initial structural heterogeneity on long-term swelling behavior of MX80 bentonite pellet/powder mixtures

International audienceTo 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

In-depth characterisation of a mixture composed of powder/pellets MX80 bentonite

International audienceMixtures 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

Water-retention properties and microstructure changes of a bentonite pellet upon wetting/drying; application to radioactive waste disposal

International audienceLike 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

Characterization of water retention, compressibility and swelling properties of a pellet/powder bentonite mixture

International audienceThe water retention, compressibility and swelling properties of a pellet/powder bentonite mixture were investigated in the laboratory. The water retention capacity was determined under constant-volume condition using a specially designed cell allowing vapor exchange in all directions; suction controlled oedometer tests were carried out for the compressibility investigation; both constant-volume and swell-consolidation methods were applied for the swelling pressure determination. The vapor equilibrium technique was used for suction control. Comparable results were found for the water retention capacity of the pellet/powder mixture determined under constant-volume condition and one single pellet under free swelling condition for suctions higher than 4 MPa, suggesting that the suction was mainly controlled by the micro-pores inside the pellets/powder grains and the swelling of the clay particles was accommodated by the existing macro-pores. On the contrary, at lower suctions, the constant-volume conditions gave rise to a lower water retention capacity for the mixture, indicating the restricted clay particle swelling after the filling-up of the existing macro-pores. Oedometer compression curves revealed a different behavior for low and high suctions: at high suctions, the volume change behavior was governed by the pellets rearrangement combined with possible pellets crushing upon loading. By contrast, at low suctions, the mixture lost its initial granular structure during suction decrease; thus, the volume change behavior became similar to that of compacted bentonite. Compared to compacted bentonite, the pellet/powder mixture showed a lower yield stress. A lower value of swelling pressure was found with the constant-volume method, confirming the limitation of the swell-consolidation method in determining soil swelling pressure

Analysis of the structural changes of a pellet/powder bentonite mixture upon wetting by X-ray computed microtomography

International audiencePellet/powder bentonite mixture is one of the candidate materials for sealing plugs in deep geological high-level radioactive waste disposal. This note presents an investigation on the structure changes of this mixture occurring during the saturation process by means of X-ray computed micro-tomography. The test was performed in an infiltration column (60 mm in inner diameter and 120 mm in height). Water was supplied to the two ends of the column and the changes of the sample morphology were observed during a period of 100 days of hydration. Digital Volume Correlation (DVC) technique was used to determine the vertical displacement field of the bentonite powder. A pressure transducer was used to measure the axial swelling pressure during the hydration. The results show that the initial distribution of powder in the inter-pellet pores was not homogeneous; the powder filled almost completely the pores in the zones close to the two ends while air-filled inter-pellet voids were observed in the middle of the column specimen. When water started to infiltrate inside the specimen from both ends, the pellets and the powder grains started to swell (because of the swelling properties of smectite, the principal mineral of bentonite) and filled the voids. That induced at the same time increase of swelling pressure and downward movement of powder grains. The results allowed a better understanding on the hydro-mechanical couplings, at the pellet scale, in the pellet/powder bentonite mixture upon wettin