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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]

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]

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]

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]

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]

As a preferred host rock for the disposal of high-level long-lived radioactive waste in France, Callovo-Oxfordian (COx) argillite inevitably undergoes gas and water intrusion during long-term sealing. In this study, a series of gas and water permeability tests have been performed on dry, partially saturated, and fully saturated COx argillite to investigate the effects of confining pressure and pore pressure on transport properties of this material. It is first found that gas permeability is very sensitive to the sample damage state for dry samples. Due to the presence of microcracks, the relationship between porosity and gas permeability is not evident, while the Klinkenberg effect is clearly observed in intact samples. Moreover, gas permeability of partially saturated samples is very sensitive to water saturation, especially for highly saturated samples. This indicates that the material has a good gas sealing capacity after the host rock is resaturated during the long-term sealing process. For both low- and highly saturated samples, confining pressure and gas pressure have very different effects on gas permeability of partially saturated argillite. Water permeability of argillite varies between 10−20–10−21 m2 (K w), suggesting a good water-sealing capacity. The evolution of K w is more sensitive to the change of pore pressure than to the change of confining pressure, indicating that the effective stress coefficient for water permeability is greater than 1. The intrinsic permeability of this low-permeable clayey rock is discussed at the end of this work. • The water retention capacity of COx argillite was determined to assess its potential correlation with gas permeability. • Gas transport properties of dry and partially saturated COx argillite were studied. • The effects of confining pressure and pore pressure on gas and water permeability were examined. • The water permeability for saturated samples was measured to examine the effects of confining pressure and water pressure. • The intrinsic permeability coefficients of argillite measured with gas and water were compared. [ABSTRACT FROM AUTHOR]

Excavation of the galleries for the geological disposal of radioactive waste can induce an excavation damaged zone (EDZ), which can have consequences on the long-term safety of the repository. In this study, intact Callovo-Oxfordian (COx) claystone taken from the Meuse/Haute-Marne Underground Research Laboratory in France was artificially fractured in different directions, and the hydro-mechanical behaviour was investigated by performing high pressure oedometer test and hydraulic conductivity test. Comparison between the fractured and intact COx samples allows identification of the effects of fracture in terms of swelling, compression, hydraulic conductivity and creep. It appears that the presence of fracture decreases the swelling strain and this phenomenon depends on the infiltration path and the orientation of fracture. In particular, the infiltration path affects the global swelling rate of fractured claystone. The self-sealing of fracture is mainly attributed to the swelling of COx claystone grains in the fracture zone. Lower compression coefficient a , higher compression index C c * and higher secondary consolidation coefficient C α are observed for the fractured claystone, indicating a higher compressibility and creep than the intact one. The compression and creep behaviours depend on both the orientation of fracture and the stress level. Comparison of hydraulic conductivities between fractured and intact claystone indicates that the hydraulic conductivity can be partially recovered due to the collapse of macro-pores in the filled fracture zone. This is also confirmed by analysing the evolution of consolidation coefficients c v with stress. Beyond a stress level about 16 MPa, the fracture effect can be neglected, but the compressibility increases further due to grain breakage. Interestingly, this stress is close to the in-situ stress before excavation. • The presence of fracture decreases the swelling strain and this phenomenon depends on the infiltration path and the orientation of fracture. • The self-sealing of fracture is mainly attributed to the swelling of COx claystone grains in the fracture zone. • The compression and creep behaviours depend on both the orientation of fracture and the stress level. • k of fractured claystone can be partially recovered under high stress due to the collapse of macro-pores in the filled fracture zone. [ABSTRACT FROM AUTHOR]

The Callovo-Oxfordian (COx) claystone has been considered as a possible host rock for deep geological repository of high-level radioactive waste in France. When constructing the repository, it is unavoidable to generate a fractured zone in the vicinity of excavated repository walls. In the assessment of the repository performance, it is important to investigate the role of the fractures on the swelling capacity of COx claystone and to further characterize the fractured zone evolution. In this study, the effect of fracture voids on the swelling behaviour of COx claystone was investigated by simulating the fracture voids with a space created by a limited amount of grains sandwiched between claystone disks, and by performing a series of swelling tests in oedometer under a low vertical stress (0.05 MPa) and swelling pressure tests under constant-volume condition. Results showed that, with water infiltrating into the sample with fracture voids, the grains collapsed before the swelling of claystone disks. The whole sample was composed of swelling zones corresponding to the claystone disks with higher dry density than the expected final value, and of compression zones corresponding to the initial fractures with lower dry density. Moreover, the axial strain and swelling pressure decreased as the fracture void increased. The maximum fracture voids that might be fully filled by the swelling of COx claystone (sealing) were found to be 5.14%–7.65% without considering the long-term creep contribution in the field condition. • Fractures were simulated with soil grains of defined sizes. • Water infiltration led to collapse of soil grains first prior to claystone swelling. • The swelling zones corresponded to claystone disks. • The compression zones corresponded to initial fractures. • A maximum fracture voids that might be sealed was 5.14%–7.65%. [ABSTRACT FROM AUTHOR]

The long-term safety of shallow disposal facility for short-lived low and intermediate level nuclear waste (LLW/ILW) relies on the performance of engineered barriers (caps and liners). Soil-atmosphere interaction may affect the capping material at shallow depths, due to changes of its coupled hydro-thermal-mechanical behavior under the effect of atmospheric conditions. This study aims to investigate the soil-atmosphere interaction at different time scales in the overburden of an LLW/ILW disposal facility, located in middle France. Actual meteorological data from Valencia El Saler station in Spain, including solar radiation, air temperature, latent heat, rainfall, and actual evaporation, at four different time scales (30 min, daily, weekly and monthly), were applied as the future climate conditions of the studied area based on climate analogues. A numerical approach combining a coupled hydro-thermal model and a soil-atmosphere interaction model was employed. Results show that the employment of meteorological data at a short time scale (30 min) can increase the simulation accuracy by capturing the rainfall events and accelerating the soil-atmosphere interactions. Furthermore, the effect of future climate conditions in the long-term (7 years) on soil hydro-thermal behavior was examined. This study allows a further detailed inspection of the climate's role in the LLW/ILW storage system. [ABSTRACT FROM AUTHOR]

MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been proposed as a sealing/backfilling material in a deep geological repository of radioactive waste in France. A good understanding of the swelling behaviour of this mixture is essential when evaluating the long-term performance of the repository. In this work, the swelling pressure of MX80 bentonite/COx claystone mixture was investigated by the constant-volume method for a full range of bentonite fraction. Results show that the swelling of claystone in the mixture can be inhibited by bentonite and its contribution to the global swelling pressure depends on the bentonite fraction. For the mixture with >70% bentonite, claystone behaves as an inert material, and its contribution to the global swelling pressure can be ignored. However, for the mixture with less bentonite, the swelling of claystone will significantly contribute to the global swelling pressure. A method was proposed allowing the swelling pressure of bentonite/claystone mixture to be predicted in the full range of bentonite fraction. • Claystone's contribution to swelling pressure depends on the bentonite fraction. • The effect of bentonite on claystone's swelling can be described by an inhibition factor. • A method is proposed to predict the swelling pressure of bentonite/claystone mixture. [ABSTRACT FROM AUTHOR]

Abstract: Gases, especially hydrogen, will be generated by corrosion and radiolysis of high radioactive waste containers in deep geological repositories. Pressure build up resulting from those long-term processes may create a risk of geological barrier fracturation and of potential creation of preferential pathways for radionuclide migration. Good knowledge of gas transfer mechanisms in the potential host rock formations is therefore necessary. The present work focuses on Callovo-oxfordian argillites from the Bure (Meuse, France) underground laboratory operated by Andra, the French national agency for radioactive waste management. Its goal is to understand the pore structure of that rock formation as a first step towards characterizing and modelling gas intrusion and transfer. Our approach was to use various techniques in combination. Firstly, mercury intrusion experiments were made. Besides supplying the classical pore size distribution curve, they bear some resemblance with the real-life problem since mercury (a non-wetting phase) invades the porous structure like a gas would. Secondly, the water vapor sorption curve was measured by submitting initially dry samples to atmospheres with increasing vapor content. Comparison of the mercury intrusion/water vapour sorption data sets yielded information on the connectivity of the pore structure. Thirdly, the gas permeability of dry argillites was measured in a specifically designed set-up, using helium as an acceptable (non-detonating) substitute for hydrogen. The main observation was the existence of a large Klinkenberg effect. Finally, all the available information was used to build and constrain a multi-scale models for the pore network using the XDQ technique. The resulting models were used to predict the properties of the material at various water saturation levels. [Copyright &y& Elsevier]