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An underground research laboratory (URL) was built at Bure, France, for conducting in situ experiments, which aims to demonstrate the feasibility of constructing and operating a High Level Waste disposal facility in the Callovo–Oxfordian (COx) claystone formation. During the URL construction, excavation of the galleries unavoidably induced a damaged zone, namely excavation damaged zone (EDZ). The ventilation within the gallery makes the EDZ under partially saturated condition. For the assessment of the long-term safety, it is necessary to investigate the time-dependent behaviour of damaged and unsaturated COx claystone. In this study, intact COx claystone was first artificially damaged in the laboratory, then its strain rate-dependent volume change behaviour was investigated through one dimensional CRS (constant rate of strain) compression test with controlled suction. The results from suction equilibration indicate that the damaged claystone exhibits a higher water content and a larger void ratio. Further examination shows that the damaged claystone has a slightly lower degree of saturation at the same suction of 9 MPa. This is due to the pre-existing micro-cracks inside the damaged claystone. The results from the CRS tests show that the volume change behaviour (compression index and yield stress) is suction-dependent. Moreover, at the same suction, the yield stress increases with the increase in strain rate, which can be described with a linear correlation in a log–log plane. This phenomenon is consistent with the observations on other soils. A suction-dependent viscous parameter C A is thus obtained, which corresponds to the slope of this linear correlation. Furthermore, the compression and viscous parameters were determined and analysed for the damaged claystone, allowing a loading-collapse curve to be defined, which accounts for the effects of strain rate and suction. [ABSTRACT FROM AUTHOR]

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 fractures network generated during the excavation of underground research facilities can induce stress redistribution and alteration of flow and transport properties, becoming preferential paths for releasing radionuclides into the host rock. Nevertheless, in the long term, the fracture can be sealed through the resaturation of water coming from the rock as a function of its self-sealing potential. Despite the large number of experimental studies that have proven the self-sealing capacity of clay rocks, very few attempts have been made to describe and predict the phenomenon numerically. This may be due to the difficulty of measuring the initial hydro-mechanical conditions. Besides, samples artificially fractured in the laboratory can be disturbed by the preparation process itself, which can alter the hydro-mechanical state. This paper addresses that issue by bridging the gap between experiments and numerical modeling. Representative experimental tests performed on Callovo–Oxfordian Claystone (COx) are used to offer a hydro-mechanical fracture law taking into account the self-sealing capacity of the material. Implementing such a model in a finite element code allows its validation through comparison with laboratory tests. Furthermore, the role of the initial fracture size and the evolution of water permeability during the wetting/drying process is investigated. Due to its transmissivity, injected water can penetrate the rock, initially reaching the damaged zone around the fracture before spreading through the entire sample. This progression is accounted in the constitutive equation and represented numerically. Nevertheless, a larger initial crack leads to reduced recovery rates. These results match the experiments, offering a valuable perspective in the modeling of self-sealing in in situ conditions. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation of the swelling behaviour of a compacted bentonite–sand mixture subjected to hydration under constant volume conditions. Contrary to previous studies, the tested sample was isotropically compacted before being hydrated under constant volume conditions until full saturation was reached. The total axial pressure, total radial pressures at four different heights of the sample, and injected water volume were recorded over time. The experimental data reveal a complex and non-uniform evolution of the axial and radial stresses over time, as well as anisotropy of the total stresses, which persist at the saturated equilibrated state. To gain further insights, a numerical analysis was performed using an advanced hydromechanical framework for partially saturated porous media, accounting for the evolving microstructure of the material. The complex evolution of the total axial and radial pressures with time is attributed to the advancing hydration and swelling front in the sample, along with the development of irreversible strains. The good agreement between the numerical results and the experimental data enables validation of the developed framework. Implications for engineered barriers in deep geological disposal of radioactive waste are discussed. Highlights: The swelling behaviour of an isotropically compacted bentonite-based material under constant-volume conditions is investigated. Hydration of the sample generates stress heterogeneity and anisotropy, which persist at the saturated equilibrated state. Numerical modelling shows that the complex evolution of the total axial and radial pressures can be attributed to the advancing hydration and swelling front in the sample, along with the development of irreversible strains. The relationship between the local dry density and the radial stress seems to follow the global dry density-swelling pressure trend determined on small-scale samples. [ABSTRACT FROM AUTHOR]

The French National Radioactive Waste Management Agency is leading the design of a deep geological radioactive waste disposal to be located in the Callovo-Oxfordian formation. At the disposal main level, large-diameter galleries will ensure access to the storage cells and connection between zones. After a long operational period, the disposal will be closed by sealing structures located at some key positions to ensure post-closure safety over the long term. The seals are intended to prevent the flow of water and the migration of radionuclides from the disposal to the biosphere throughout the entire storage life. Thus, the long-term safety of the disposal relies to a large extent on the performance of these structures. The paper presents the work carried out to assess and simulate the phenomena underlying the response and performance of a large-diameter sealing concept under real disposal conditions. The complexity of the problem is addressed by considering in the simulations large-scale 3D geometries, advanced constitutive models, complex coupled phenomena, key geometric details at decimetre scale, and all the phases from the excavation to the post-closure period (~ a few thousand years). Highlights: The three-dimensional hydro-mechanical modelling of a large structure concept, foreseen to seal access galleries in a clayey rock deep disposal, has been carried out. Model considers large geometry (order of tens of meters), anisotropy, constitutive laws of different complexity, interfaces, and relevant geometrical details (order of cm). Results evidence a progressive hydration and swelling of the sealing core during more or less 2000 years after disposal closure. The concrete plugs placed at both ends of the sealing structure move longitudinally, but core density and swelling pressure do not change significantly in seal central part. Swelling core recompresses partially the excavation-induced damaged zone in the area of lining deposition. [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]

The swelling behaviour of expansive clays is strongly related to the interaction between clay particles. In this paper, a series of constant-volume swelling pressure and mercury intrusion porosimetry (MIP) tests were carried out on MX80 bentonite/Callovo-Oxfordian (COx) claystone mixtures with different bentonite fractions and dry densities. Results show that the swelling pressure increased linearly with the increase in dry density. Additionally, at a given dry density, the larger the bentonite fraction, the larger the swelling pressure. From the MIP results, the inter-particle pore volume decreased with the increases of bentonite fraction and dry density. To link the macroscopic swelling behaviour with the microstructure features, a new method was proposed, allowing the determination of the average inter-particle distance from the pore size distribution. Moreover, a linear relationship was identified between the swelling pressure and the average inter-particle distance in a semi-logarithmic plane, regardless of the bentonite fraction and montmorillonite content. [ABSTRACT FROM AUTHOR]

This paper investigates the swelling mechanism of Callovo-Oxfordian (COx) claystone grain in its mixtures with bentonite which are potential candidate for engineered barriers in the French high-level radioactive waste disposal. The swelling pressures of MX80 bentonite/COx claystone mixtures with various bentonite fractions and dry densities were first experimentally determined by carrying out infiltration tests under constant-volume conditions. According to the experimental results, the claystone void ratio after hydration in the mixtures was estimated by taking into account the bentonite/claystone grain interaction. Then, the volume change of claystone grain under different pressures applied by swollen bentonite was determined. The volumetric strain of claystone grain was found to decrease linearly with the increase in pressure. This allowed the indirect determination of the swelling pressure of claystone grain. The determined value was found to be comparable to that estimated from the relationship between the void ratio and the swelling pressure in the direction perpendicular to the stratification of intact COx claystone, confirming the identified swelling mechanism of bentonite/claystone mixtures. [ABSTRACT FROM AUTHOR]

A pre-compacted MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been proposed as a candidate sealing material in a French deep geological repository for high-level radioactive waste. An in-depth understanding of the hydraulic property of this material is essential for accessing the over-pack corrosion and nuclide migration along the gallery. In this study, water flow in the compacted bentonite/claystone mixture was investigated by carrying out infiltration and water retention tests under constant-volume conditions alongside detailed microstructure observation. The evolution of total water hydraulic conductivity with suction was experimentally determined based on the instantaneous profile method. It was observed that the measured total hydraulic conductivity increased slowly at high suctions, then quickly at low suctions and finally approached the saturated value. A new method was developed based on the evolving pore structure, accounting for both vapour and liquid-water hydraulic conductivities. The total hydraulic conductivity predicted by the proposed method agreed well with the measured value, indicating the reliability of the predictive method. The vapour hydraulic conductivity was larger than the liquid-water one at a suction larger than 15·2 MPa, while liquid-water hydraulic conductivity became dominant at a suction lower than 15·2 MPa. In addition, based on the predicted vapour and liquid-water hydraulic conductivities, a numerical model was set up to simulate the water infiltration process. Comparison between the simulation and the measurement showed close agreement, validating the two-phase water flow mechanism in the bentonite/claystone mixture. [ABSTRACT FROM AUTHOR]

For the deep geological disposal of radioactive waste in France, an Underground Research Laboratory has been constructed in the formation of Callovo-Oxfordian (COx) claystone at a depth of about 490 m. During the excavation, damage of host rock occurred around the openings. To investigate the hydro-mechanical behaviour of damaged COx claystone in the laboratory, it is necessary to propose a method for the preparation of damaged sample because field sampling appears difficult. In this study, such a method was proposed by performing triaxial test loading paths (constant mean stress and constant confining pressure). The anisotropic behaviour of COx claystone was revealed by isotropic loading. The strain–stress curve from triaxial tests exhibited elasto-plastic pattern. Both the shear modulus G and the Young's modulus E decreased slightly with the increase of deviator stress, suggesting the damage behaviour. Interestingly, all results from this study and from the literatures showed that at 90% of the peak deviator stress qmax, the ratio ET,0.9/EI (where ET,0.9 represents the tangent modulus at 90% of qmax and EI represents the initial tangent modulus) is close to 0.22, suggesting that the tangent modulus ET is a good indicator of damage level for the COx claystone sample. Observation using X-ray computed tomography and Mercury intrusion porosimetry showed that the damaged sample is characterised by the creation of significant micro-cracks after the triaxial loading. This finding suggests that ET can be used to control the damage level in triaxial cell for preparing damaged COx claystone samples. Highlights: ET was used to analyse the evolution of damage during shearing. A unique relationship was found between the normalised ET/EI and the normalised q/qmax. A new method was developed to prepare damage claystone. Microstructure observations from CT and MIP tests indicated qualitatively the significant micro-cracks developed inside COx claystone. [ABSTRACT FROM AUTHOR]

Compacted bentonite/sand mixtures are often considered as sealing/backfilling materials in deep geological disposal for radioactive waste. This study investigates the swelling pressure of compacted bentonite/sand mixtures with different bentonite fractions and dry densities upon hydration under constant-volume conditions. It was found that the swelling pressure of bentonite/sand mixtures is strongly dependent on their bentonite fraction and final montmorillonite dry density. For the samples with a bentonite fraction larger than a critical value, there is a linear relationship between the swelling pressure and final montmorillonite dry density. By contrast, for the samples with a bentonite fraction lower than the critical value, the sand particles will be skeletonized after strong compaction and lead to a heterogeneous distribution of montmorillonite, resulting in a larger swelling pressure compared to the samples with higher bentonite fractions. The maximum sand skeleton void ratio, corresponding to that at which the sand skeleton is just formed, is used to estimate the critical montmorillonite dry density and the critical bentonite fraction. Comparison between the test data and the estimated results shows good agreement, indicating the relevance of the identified swelling mechanism for compacted bentonite/sand mixtures. [ABSTRACT FROM AUTHOR]

Several experimental methods are used to investigate the swelling capacity and transport properties of re-sealed macro-cracked Callovo–Oxfordian (COx) claystone, particularly in the absence of confining pressure. Six COx claystone samples from four different geological locations of the Bure basin (France) are tested, five of which are macro-cracked and one remains intact. Sample swelling occurs, during re-hydration with liquid water, leading to the measurement of an apparent swelling pressure. The latter is continuously recorded with a dedicated device. The asymptotic apparent swelling pressure of macro-cracked UT (transitional unit) COx is approximately 1 MPa, while it varies from 3 to 5 MPa for macro-cracked UA (clayey unit) COx. Quantitative X-ray diffraction (QXRD) analysis demonstrates that the amount of smectite, which is a swelling clay, is weakly correlated with apparent swelling pressure. Surprisingly, the interstratified illite/smectite with lower smectite content is highly correlated to apparent swelling pressure. Nitrogen isotherms data imply that the Gurvich total pore volume (VGurvich) and specific surface area (SSA) are highly linearly correlated with the low smectite content interstratified phase. This means that the distribution of smectite strongly affects the swelling capacity of COx. Moreover, nitrogen sorption is an easier and more effective technique than QXRD for assessing COx swelling capacity, since both VGurvich and SSA have been proven as effective indicators. For both UT and UA COx, self-sealing can cause significant reductions in water permeability (Kw). In particular, UA COx shows higher sealing efficiency and faster kinetics compared to UT COx. After sealing, the equivalent crack aperture (calculated from Poiseuille's law) decreases from tens of microns to less than 1 micron. According to the gas breakthrough tests, the gas breakthrough pressure (GBP) of re-sealed macro-cracked COx is of the same order of magnitude as the equivalent capillary pressure of residual crack. This indicates that the gas migration in the re-sealed cracked COx claystone mainly occurs through the residual crack and is 'a priori' controlled by capillary processes. Highlights: A dedicated device was designed to study the transport and swelling properties of a sealed COx claystone under low confinement. The apparent swelling pressure and water permeability were continuously recorded during the sealing test due to water injection in fractures. Water permeability of samples decreased by orders of magnitude to almost meet the intact material permeability. Gas breakthrough experiments performed on sealed samples evidenced gas flow through the residual cracks. [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. [ABSTRACT FROM AUTHOR]

Pre-compacted MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been considered as a promising sealing/backfilling material in deep geological repository for high-level radioactive waste in France. When the pre-compacted blocks are emplaced in the gallery, technological voids can remain. After the infiltration of groundwater from the host rock, they will swell freely, filling the technological voids, and then undergo further hydration under constant-volume conditions. From the perspective of the storage safety, it is essential to understand the effects of technological voids and hydration time on the hydro-mechanical behaviour of such pre-compacted blocks. In this work, a series of infiltration tests at various hydration times was carried out on compacted MX80 bentonite/COx claystone mixture with different technological voids. The evolutions of the swelling pressures in axial and radial directions as well as the hydraulic conductivity were monitored while wetting. After the predetermined hydration times, the dry density, water content and suction at different positions were determined, together with the microstructure investigation using mercury intrusion porosimetry. It was observed that the soils close to initial voids swelled and filled the initial voids upon contact with water, with significant increases in large-pore and medium-pore void ratios. From the variation of dry density profile with time, compression and swelling zones could be identified: in the compression zone, the soils corresponding to the initial soils were subjected to compression, with decrease in large-pore and medium-pore void ratios over time, whereas in the swelling zone, the soils with a higher dry density than the expected final one underwent further swelling, with large-pore void ratio increasing until saturation and then a slight decrease due to water redistribution in the soil. Owing to the soil density heterogeneity, the axial swelling pressure and hydraulic conductivity of samples with voids were slightly larger than those of samples without voids; contrarily, the radial swelling pressure was lower than that of the samples without voids. [ABSTRACT FROM AUTHOR]

X-ray micro-tomography (XMT) is an efficient technique for non-destructive imaging of morphological structures. It is suitable for monitoring local displacement fields of heterogeneous materials. Digital volume correlation (DVC) methods are widely used for the quantification of local strain fields from highly contrasted XMT images. The aim of this work is to investigate strain localization and cracking process in a hard clayey rock. For this purpose, three-dimensional images have been taken by in situ X-ray micro-tomography on a tested sample under different loading levels and time steps. These images are analyzed with a DVC based method to calculate both local strain fields and averaged global strains. In particular, the progressive localization of strain field with applied stress and creep time is investigated in relation with material heterogeneities. It is found that the strain field localization as well as the cracking process is clearly influenced by the presence of stiff inclusions, pores, weak clayey zones and layered microscopic structure of claystone. [ABSTRACT FROM AUTHOR]

Wetting–drying cycles are likely to induce damage of claystone. To evaluate and to prevent micro-cracking impacts on Callovo-Oxfordian (COx) argillite, which is, in France, the potential host rock of structures containing nuclear wastes, 18 months of tests were carried out on different series of samples. These tests consisted of gas permeability and Young's modulus measurements. They were carried out on dry material, as a reference state, previously submitted to successive cycles of wetting–drying. The results show without any ambiguity that repeated hydric cycles lead to damage i.e. additional micro-cracking that induces an increase in gas permeability (possibly by 2–3 orders of magnitude compared with the intact one) and decrease in Young's modulus by 5–15%. This means that strong precautions have to be followed to keep stable the initial material water content prior to the tests and to study a material representative of its in-situ state. [ABSTRACT FROM AUTHOR]

Pre-compacted MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been proposed to backfill and seal the underground galleries for radioactive waste disposal in France. While emplacing these pre-compacted blocks, technological voids are created between the blocks and the host rock and among the blocks themselves. It is expected that homogenization process will take place over time for the structure constructed with pre-compacted blocks upon hydration. This study investigated the boundary friction effect on such a process. Results showed that after the filling of technological voids, the soil far from the technological voids would swell further, while those near the voids would be compressed under the welling pressure generated by the soil behind, resulting in an increase in homogeneity in terms of dry density distribution. However, this homogenization process would stop after a certain time. Further examination showed that the homogenization process ended when the maximum boundary friction force became equal to or higher than the vector sum of swelling forces in the radial direction. Based on the force equilibrium and the mass conservation, the final dry density distribution was estimated. Comparison between the estimation and the measurement showed a good agreement, indicating the relevance of the identified mechanism related to boundary friction. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Mixtures composed of MX80 bentonite and crushed Callovo-Oxfordian (COx) claystone have been considered as one of the promising sealing/backfilling materials in French deep geological disposal for radioactive waste. One of the key issues about these mixtures is hydraulic conductivity. In this study, the swelling pressure and hydraulic conductivity of bentonite–claystone mixtures with different bentonite fractions and dry densities were determined by carrying out infiltration tests under constant volume conditions. The results indicated that as the bentonite fraction and dry density increased, the overall swelling pressure increased and the hydraulic conductivity decreased. Based on the assumption that the stress between bentonite and claystone grains was equal to the overall swelling pressure, the void ratios and volumetric fractions of bentonite and claystone in the mixture were determined. According to the relationships between hydraulic conductivity and void ratio for pure bentonite and claystone, the overall hydraulic conductivities of the bentonite–claystone mixtures were then calculated using five analytical models (parallel, series, Maxwell–Eucken 1, Maxwell–Eucken 2, and effective medium theory models) corresponding to different potential composite structures, allowing appreciation of the composite structures of bentonite–claystone mixtures with various bentonite fractions. For specimens with ≥70% bentonite, the bentonite formed a matrix with the claystone dispersed within it, for specimens with <40% bentonite, the matrix was formed by swollen claystone with the bentonite dispersed into the matrix, and for specimens with 40%–70% bentonite, the bentonite and claystone were randomly distributed, without being continuous or dispersed. Based on the defined composite structure of the bentonite–claystone mixture, a new method is proposed for estimating the overall hydraulic conductivity. A comparison between estimation and measurement showed the relevance of the proposed method. [ABSTRACT FROM AUTHOR]

Creep tests are commonly performed to characterize time-dependent deformation of geological materials. Classical measuring methods are not suitable for long term tests and not able to provide full three-dimensional strain fields. In this study, Digital Volume Correlation (DVC) is applied to X-ray micro-tomography (XRMT) images from creep tests on a hard clayey rock. In situ uniaxial compression creep tests are performed under different levels of stress and with different loading orientations with respect to the structural anisotropy of rock. Based on the XRMT images taken during the creep tests, DVC is applied to compute the full three dimensional strain fields and global averages strains of tested samples. The effects of bedding planes and hard inclusions on the non-uniform distribution of strains are analyzed. [ABSTRACT FROM AUTHOR]

There are several methods for the determination of soil swelling pressure. In this study, such common methods were assessed using intact Callovo-Oxfordian (COx) claystone samples extracted in the direction perpendicular to the bedding plane, in the French Underground Research Laboratory in Bure. Higher swelling pressure was found with the swell-consolidation method, the swelling pressures determined by other methods being comparable. The swelling line that defined the swelling limit was lower than the consolidation curve, confirming the limitation of the swell-consolidation method in determining soil swelling pressure. Moreover, the swelling pressure corresponding to the in situ void ratio was expected to be 12 MPa–14 MPa, while the value corresponding to the sample void ratio after sampling was estimated between 0.3 and 1.9 MPa, much lower than the expected one. This large difference might be attributed to the claystone rebound due to unloading and claystone damage due to sampling. It could be also due to the natural material variability in terms of mineralogy and dry density. Further studies are, thus, needed for clarifying this point. [ABSTRACT FROM AUTHOR]

Argillaceous rocks have long been selected as privileged host rock candidates for the nuclear waste disposal. For the sake of evaluating the sealing capacity of these natural barriers, one urgent issue is to study the poromechanical properties of them in the long-term gas migration process. As the key parameter of poroelasticity, Biot's tensor of argillaceous rocks should be firstly studied. In this paper, a series of poromechanical tests was firstly carried out to measure Biot's tensor components. 'Non-logical' values that Biot's tensor components were larger than unity (1.0) were obtained in the tests. It was then suspected that the process of injecting pore pressure is not a purely poromechanical problem. In order to deeper study about these values, complementary poromechanical tests were handed on two samples with helium and argon or helium and carbon dioxide (CO2), respectively, to confirm the adsorption-swelling phenomenon. The results showed that Biot's tensor components are obviously affected by the gases used, which is owing to their different adsorption capacities. To evaluate these swelling affect consequences, a series of permeability and triaxial compression tests was performed with different type of gases. The results obtained reveal that gas adsorption-swelling deeply effect the transport and (poro)mechanical properties of argillite. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Based on a theoretical analysis of stress evolution in surrounding rocks of an underground cavity during excavation, a series of lateral decompression (LD) tests are designed and performed in this study to investigate the mechanical behavior of the Callovo-Oxfordian (COx) claystone. The mean stress is kept constant in each LD test. A number of unloading-reloading cycles are included in each LD test. Furthermore, to evaluate the effect of temperature on mechanical strength and elastic properties of the COx claystone, the LD tests are performed under five different values of temperature from 20 to 90 °C. The shear strength as well as failure pattern in the LD tests are analyzed. The variation of elastic moduli is determined and analyzed from the unloading-reloading cycles. The effect of temperature on both the shear strength and elastic moduli is identified and characterized. The COx claystone showed a shear failure in the LD tests, and the failure was characterized by the creation of a localized macro-crack and of a damaged zone with many micro-cracks around the macro-crack. The temperature increase induced a degradation of the shear strength of the COx claystone. An obvious mechanical damage of elastic properties was observed in the COx claystone during the shear stress loading in the LD tests. This damage was more important in the lateral direction than the axial one, leading to an induced anisotropy of the material. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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 in 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 and 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 in 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 micrometres, 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 hydromechanical couplings through changes in pore pressure and water transfers. [ABSTRACT FROM AUTHOR]

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The corrosion of metals contained in intermediate-level long-lived waste (ILW) under reduced chemical condition will lead to the production of hydrogen gas during the post-closure phase of a deep geological repository for radioactive waste. According to previous investigations by Talandier et al. (Proceedings of TOUGH symposium 2006, Berkeley, ), the period of concern covers several 1,000 years after closure of a repository in a clay host rock (Callovo-Oxfordian). The limited hydrogen transport efficiency of the host rock will lead to significant saturation of the concrete waste canister pore space and voids with a gas phase and pressure build-up within the emplacement drifts. On the other hand, the water availability is limited as a result of (i) the low permeability of the clay host rock and (ii) the desaturation of the rock mass close to the drift wall due to the ventilation of the drifts during the operational phase of the repository. In former numerical simulations it was assumed that under the reducing chemical conditions prevailing in the repository, the corrosion rate would be a function of the available metal surface and the temperature only. In this paper, simulation results based on new phenomenological functions are presented, which were implemented in TOUGH2. These allow taking into account (i) a water saturation dependency of the hydrogen generation rate, (ii) the water consumption due to the corrosion process, and (iii) the total metal mass available for corrosion. The paper presents results of 1D radial and 2D vertical simulations of a typical cross-section of a waste emplacement drift and the surrounding rock mass. The interactions between water availability from the low permeable clay, the dependency of the hydrogen generation rates from the water saturation of the waste package and the hydrogen migration in the host rock are demonstrated. [ABSTRACT FROM AUTHOR]

A correction to this paper has been published: https://doi.org/10.1007/s00603-021-02419-w [ABSTRACT FROM AUTHOR]