Search

In this study, the compression behaviour of unsaturated damaged COx claystone is investigated by performing high pressure oedometer tests with controlled suction. The damaged oedometer samples were prepared by shearing the claystone in triaxial conditions. Four high pressure oedometer tests were conducted using different controlled suctions. The effects of suction and damage on the swelling and compression were investigated in oedometer. Results show that the intact claystone exhibits a larger swelling strain than that of the damaged one at the same imposed suction. The damaged claystone at a lower suction displays a smaller yield stress and higher compressibility, and exhibits a larger compression index Cc than the intact one, evidencing the effect of damage. In addition, the step compression index Cc* increases with the increase of vertical stress, while deceases with the decrease of suction. This indicates the dependencies of compression behaviour on vertical stress and suction.

Some (saturated) claystones, considered as host rocks for deep radioactive waste disposal, are sensitive to drying and wetting. Wetting may result in swelling and damage during the resaturation of specimens for mechanical testing. Given that, due to evaporation, specimens may have lost some pore water during coring, transport, storage and trimming prior to lab testing, it is required to re-saturate them by reinjecting the evaporated water content, compressing the pore space under constant water content, or by using a combination of both. It is important to characterize the hydromechanical processes involving suction changes under total stress loading in order to adapt preparation methods and minimize sample disturbances. We present a novel experimental setup to test claystone specimens under isotropic compression at constant water content, while measuring suction, and we discuss the data of three specimens testedat different initial suctions and degrees of saturation. Interestingly, the path followed in the (decreased) suction vs (increased) total confining stress diagram was observed to be close to the Gens-Alonso “Neutral Line” (a line inclined of 45° in a suction/mean total stress diagram, illustrating similar effects of changes of both variables). In other words, even in close to saturated conditions, changes in confining stress resulted in comparable changes in suction. The data obtained indicate that applying a total stress of 12.2 MPa during resaturation keeps the material on the compressive side of the Neutral Line, limiting swelling-induced sample disturbance.

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.

The effect of technological voids on the swelling pressure of compacted MX80 bentonite-Callovo-Oxfordian (COx) claystone mixture was investigated by simulating the technological voids with a pre-defined space between the compacted disks of the mixture. Both axial and radial swelling pressures were monitored. After the tests, the microstructure of samples at different positions was investigated using mercury intrusion porosimetry (MIP), together with the determination of dry density and water content. Results showed that two main processes, filling and homogenization, occurred during soil hydration. In the filling process, the initial technological voids were gradually filled and the axial swelling pressure tended to increase. In the homogenization process, the samples had a sealing zone and a swelling zone. The sealing zone was characterized by a lower dry density than the expected final dry density while the swelling zone was characterized by a larger one. From the MIP results, the sealing zone showed larger inaccessible-pore, medium-pore, and large-pore void ratios and a lower small-pore void ratio than the swelling zone. Over time, the medium and large pores in the sealing zone were compressed, while the small pores in the swelling zone decreased. The stabilized axial swelling pressure followed a unique relationship with the expected final dry density. Moreover, the swelling pressure anisotropy was found to decrease as the technological voids increased. Key words: bentonite-claystone mixture, technological voids, swelling pressure, swelling anisotropy, microstructure. L'effet des vides technologiques sur la pression de gonflement du melange compacte de bentonite MX80 et de pierre argileuse Callovo-Oxfordien (COx) a ete etudie en simulant les vides technologiques avec un espace predefini entre les disques compactes du melange. Les pressions de gonflement axiales et radiales ont ete surveillees. Apres les tests, la microstructure des echantillons a differentes positions a ete etudiee par porosimetrie au mercure (<< MIP >>), ainsi que la determination de la densite seche et de la teneur en eau. Les resultats ont montre que deux processus principaux, soit le remplissage et l'homogeneisation, se produisent pendant l'hydratation du sol. Au cours du processus de remplissage, les vides technologiques initiaux ont ete progressivement combles et la pression de gonflement axial a eu tendance a augmenter. Dans le processus d'homogeneisation, les echantillons avaient une zone de scellement et une zone de gonflement. La zone de scellement etait caracterisee par une densite seche inferieure a la densite seche finale prevue, tandis que la zone de gonflement etait caracterisee par une densite plus importante. D'apres les resultats de la MIP, la zone de scellement presentait des indices des vides de pores inaccessibles, moyens et grands plus eleves et un indice des vides de pores petits plus faible que la zone de gonflement. Au fil du temps, les pores moyens et larges de la zone de scellement se sont comprimes, tandis que les petits pores de la zone de gonflement ont diminue. La pression de gonflement axial stabilisee a suivi une relation unique avec la densite seche finale attendue. De plus, il a ete constate que l'anisotropie de la pression de gonflement diminuait a mesure que les vides technologiques augmentaient. Mots-cles : melange bentonite-argile, vides technologiques, pression de gonflement, anisotropie de gonflement, microstructure., 1. Introduction In the context of deep geological disposal for radioactive waste, pre-compacted blocks of bentonite-based materials are often considered as possible sealing and backfilling materials thanks to their high [...]

The purpose of this study is to investigate the effective behaviour of a micro-cracked material whose matrix bulk and shear moduli are ruled by a linear viscoelastic Burgers model. The analysis includes a detailed study of randomly oriented and distributed cracks displaying an overall isotropic behaviour, as well as aligned cracks resulting in a transversely isotropic medium. Effective material properties are approximated with the assumption that the homogenized equivalent medium exhibits the characteristics of a Burgers model, leading to the identification of short-term and long-term homogenized modules in the Laplace–Carson space through simplified formulations. The crucial advantage of this analytical technique consists in avoiding calculations of the inverse Laplace–Carson transform. The micromechanical estimates are validated through comparisons with FE numerical simulations on 3D microstructures generated with zero-thickness void cracks of disc shape. Intersections between randomly oriented cracks are accounted for, thereby highlighting a potential percolation phenomenon. The effects of micro-cracks on the material's behaviour are then studied with the aim of providing high-performance creep models for macrostructure calculations at a moderate computation cost through the application of analytical homogenization techniques. [ABSTRACT FROM AUTHOR]

Examination of the microstructure and mineral composition of Callovo-Oxfordian (COx) claystone revealed the presence of clay matrix intruded by a large amount of mineral inclusions such as carbonate and quartz. This claystone can be damaged when wetted/dried due to the swelling/shrinkage of clay matrix, or when subjected to mechanical loading due to the loss of bonds formed among mineral grains. The damage of COx claystone was evidenced in the literature by analysing its pore size distribution curves after wetting/drying and mechanical loading, obtained from the mercury intrusion porosimetry (MIP) tests. In this paper, a damage coefficient accounting for the physics of damage (i.e. generation of cracks) was adopted, corresponding to the proportion of cracks in the total porosity. This coefficient enables the description of the effect of cracks on the overall mechanical behaviour of COx claystone. Based on the MIP results, the evolution of the damage coefficient with the effective stress and plastic volumetric strain was identified for COx claystone, during both the hydration and the oedometer compression processes. [ABSTRACT FROM AUTHOR]

Components of the so-called “multiple-barrier system” from the waste form to the biosphere include a combination of waste containers, engineered barriers, and natural barriers. The Engineered Barrier System (EBS) is crucial for containment and isolation in a radioactive waste disposal system. The number, types, and assigned safety functions of the various engineered barriers depend on the chosen repository concept, the waste form, the radionuclides waste inventory, the selected host rock, and the hydrogeological and geochemical settings of the repository site, among others. EBS properties will evolve with time in response to the thermal, hydraulic, mechanical, radiological, and chemical gradients and interactions between the various constituents of the barriers and the host rock. Therefore, assessing how these properties evolve over long time frames is highly relevant for evaluating the performance of a repository system and safety function evaluations in a safety case. For this purpose, mechanistic numerical models are increasingly used. Such models provide an excellent way for integrating into a coherent framework a scientific understanding of coupled processes and their consequences on different properties of the materials in the EBS. Their development and validation are supported by R&D actions at the European level. For example, within the HORIZON 2020 project BEACON (Bentonite mechanical evolution), the development, test, and validation of numerical models against experimental results have been carried out in order to predict the evolution of the hydromechanical properties of bentonite during the saturation process. Also, in relation to the coupling with mechanics, WP16 MAGIC (chemo Mechanical AGIng of Cementitious materials) of the EURAD Joint Programming Initiative focuses on multi-scale chemo-mechanical modeling of cementitious-based materials that evolve under chemical perturbation. Integration of chemical evolution in models of varying complexity is a major issue tackled in the WP2 ACED (Assessment of Chemical Evolution of ILW and HLW Disposal cells) of EURAD. WP4 DONUT (Development and improvement of numerical methods and tools for modeling coupled processes) of EURAD aims at developing and improving numerical models and tools to integrate more complexity and coupling between processes. The combined progress of those projects at a pan-European level definitively improves the understanding of and the capabilities for assessing the long-term evolution of engineered barrier systems.

This paper addresses the microstructure change induced by swelling for natural stiff Teguline clays. The predominant clay mineral measured by X-ray diffractometry was identified to be illite and no swelling clay minerals were found. Significant swelling occurs upon wetting, illustrating the process of suction release from a nearly fully saturated condition. Mercury intrusion porosimetry (MIP) and scanning electron microscopy were used to identify the change in microstructure with swelling during wetting. It is found that the soil swelling mainly leads to an increase of macropores (0.15-350 [micro]m), and a slight decrease of micropores (0.006-0.15 [micro]m) and inaccessible pores (350 [micro]m). Furthermore, swelling was found to occur in the direction perpendicular to the bedding plane, indicating an anisotropic swelling behaviour. The creation and development of cracks during wetting were also investigated. It appears that most cracks caused by swelling have the size of macropores (0.15-350 [micro]m) identified by the MIP test. These findings call enough attention to the significant change in microstructure by wetting-induced swelling that can strongly influence the thermohydromechanical properties of stiff clays. Key words: stiff clays, swelling, suction, microstructure, crack, wetting. Resume : Cet article porte sur le changement de la microstructure induit par le gonflement pour les argiles teguline rigides naturelles. Le mineral argileux predominant mesure par diffractometre a rayons X a ete identifie comme etant illite et aucun mineral d'argile gonfle n'a ete trouve. Un gonflement important se produit en le mouillant, illustrant le processus de liberation de succion partir d'une condition presque completement saturee. La porosimetrie intrusion de mercure (<< MIP >>) et la microscopie electronique ;a balayage ont servi ;a identifier le changement de la microstructure avec gonflement lors d'humidification. Il a ete trouve que le gonflement du sol conduit principalement une augmentation des macropores (0,15350 [micro]m), et une legere diminution des micropores (0,006-0,15 [micro]m) et des pores inaccessibles (350 [micro]m). En outre, un gonflement s'est avere se produire dans la direction perpendiculaire au plan de litage, ce qui indique un comportement anisotrope de gonflement. La creation et le developpement des fissures au cours de l'humidification ont aussi ete examines. Il semble que la plupart des fissures provoquees par le gonflement ont la taille de macropores (0,15-350 [micro]m) identifie par l'essai de la MIP. Ces resultats font ressortir suffisamment le changement significatif du gonflement de la microstructure induite par l'humidification qui peut fortement influer sur les proprietes thermohydromecaniques des argiles rigides. [Traduit par la Redaction] Mots-cles : argiles rigides, gonflement, succion, microstructure, fissure, humidification., Introduction When undisturbed samples of natural clays are extracted from a certain depth below the ground surface, suction builds up inside the samples. The induced suction can be very high [...]

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.

Compacted MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been considered as a possible sealing/backfilling material in the French deep geological radioactive waste disposal. The swelling pressure of such mixture is an important factor in the design and long-term safety assessment of deep geological repositories. In this study, constant-volume swelling pressure tests were performed on the mixtures with different claystone fractions and dry densities. The test results show that the swelling pressure of the mixtures decreased with the increasing claystone fraction and decreasing dry density. According to the experimental results, the contribution of claystone to the global swelling pressure was further investigated. It was found that the deformation of claystone and its contribution to swelling pressure was highly dependent on the claystone fraction. As the claystone fraction was larger than 30%, the claystone in the mixture swelled, contributing to the global swelling pressure; On the contrary, as the claystone fraction was less than 30%, the swelling of claystone was inhibited by the bentonite and it worked an inert material without any contribution to the swelling pressure.

In most of the concept designs for nuclear waste disposals, bentonite-based materials are used to seal underground galleries and shafts. In order to assess the safety of such engineered barriers and to provide a good prediction for the material behaviour, a number of experimental campaigns and constitutive models have been developed mainly focusing on bentonite compacted blocks. On the other hand, in recent years, the use of high density bentonite pellets combined with powdered bentonite has also been considered as an interesting alternative. The bentonite pellets mixtures present a prominent initial heterogeneity level as a result of the inter-pellets porosity, which evolves strongly during hydration. This paper aims to present the numerical modelling strategy adopted for a swelling pressure test on pellets mixture carried by CEA (France). Taking advantage of the finite element code LAGAMINE, the Barcelona Basic model is considered for the bentonite mechanical behaviour and the double porosity model proposed by [1] is used for the water retention behaviour and permeability evolution. Given the already-known complexity of the multiphysical and multiscale coupled processes taking place during bentonite-hydration, the obtained numerical results are in good agreement with experimental measurements. Especially, the non-monotonic evolution of the swelling pressure during the hydration phase is well captured by this model, which is always a challenge for this type of problem.

This laboratory experimental program investigated the impact of variations in the expansive mineral content, the grain size distribution of employed bentonite, the initial dry density and the selected hydration path on the water retention characteristics and swelling properties of processed Callovo-Oxfordian claystone and its mixtures with MX80 bentonite. The French reference concept for the disposal of nuclear waste in deep sedimentary rock formations envisages the reemployment of excavated material as backfill material, which is installed in situ by means of conventional compaction techniques. The investigations were of special interest as the major issues involving in situ compacted backfill materials were portrayed. Experiments showed that the impact of variations in the dry density on the water retention characteristics of all materials vanished as suctions exceeded 100 MPa. The negligible impact of variations in the initial dry density on the collapse behavior of claystone/ bentonite mixtures remained questionable.

Bentonite pellet-powder mixtures are candidate sealing materials in radioactive waste disposal concepts. The mixture is installed in galleries in dry state as a granular material. The material is progressively hydrated by the pore water of the host rock and becomes homogeneous. Before homogenisation, the granular structure controls the material behaviour. In the present work, a modelling approach able to address particular features of pellet-powder mixtures is introduced. Two domains are considered: i) granular, and ii) homogeneous. The material behaviour before homogenisation is studied through Discrete Element Method (DEM) simulations. Constitutive laws for the granular state are proposed from DEM results. The behaviour of the homogenised material is described by a modified Barcelona Basic Model (BBM). Transition from granular to homogeneous states depends on suction and relative volume fractions of pellets and powder. Swelling pressure tests performed in the laboratory are satisfactorily simulated using this approach.

The aim of Beacon is to develop the understanding of fundamental processes that lead to material homogenisation, as well as to improve capabilities for numerical modelling. In earlier assessments of bentonite EBS, the mechanical interaction between the installed bentonite components has been neglected and an “ideal” final state has generally been assumed. Key features of the project are (1) re-evaluation of the available knowledge to extract the crucial data to compile the qualitative and quantitative data and to enhance the conceptual understanding. (2) Enhanced, robust and practical numerical tools based on a good scientific understanding, which have the expected predictive capabilities regarding the evolution of engineered barriers and seals. (3) A developed database with experimental data needed by the quantitative models. (4) Verified calculation tools based on experimental results in different scales. The Beacon project is required for the pan-European objectives at building confidence amongst regulators and stakeholders regarding the performance of the engineered barriers in a geological repository.

In the context of underground exploitation, the behaviour of rocks near galleries and tunnels conditions their stability. Underground drilling generates deformations, damage, fracturing, and significant modification of flow characteristics in the surrounding rock. However, the influence of small-scale characteristics and behaviour on the rock deformations and damage at engineering scale remains a complex issue. Consequently, the multiscale behaviour of a clay rock is modelled starting from the large scale of the excavation damaged zone around galleries and then enriching the approach by considering microstructural characteristics from the scale of mineral inclusions. Lastly, a double-scale numerical framework is considered. It allows to relate small- to large-scale rock behaviour in terms of deformations and material rupture. In fact, the development of damage and cracking at microscale allows to predict large-scale fracturing. The developed method focuses on a claystone in the particular context of long-term management of high-level nuclear wastes by deep geological repository. The results highlight the possibilities of double-scale computing in the prediction of the behaviour of underground engineering structures.