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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]

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 [...]

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.

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.