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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]

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]

In the conventional French railway substructures, a layer namely interlayer was created naturally by the interpenetration of ballast and subgrade. The lower part of this interlayer is characterized by a matrix of fines with inclusion of coarse grains in it. Moreover, the coarse-grain content is decreasing over depth. In this study, the effect of volumetric content of coarse grains f v (volumetric ratio of coarse grains to the whole sample; grain size > 2 mm) on the mechanical behavior of the lower part interlayer soil was investigated by carrying out monotonic triaxial tests. The results showed that increasing the volumetric content of coarse grains f v led to more significant dilatancy. The shear strength also increased with the increase of f v and the increasing trend appeared to be much more significant when f v >20%. Examination of the variations of peak deviator stress with f v revealed the existence of a characteristic value of f v - cha  ≈ 27% that separates two zones with different coarse-grain effects: a fine-fine contact dominating zone when f v  ≤ 27% and a grain-grain contact dominating zone when f v  > 27%. To clarify the observed phenomena, X-ray microcomputed tomography (μCT) scans were performed on the as-compacted samples, allowing the coarse-grain distribution inside the as-compacted sample to be visualized. It was found that when f v ranged from 0% to 10%, the skeleton of the sample was mainly constituted by fines with the coarse grains floating in the fines matrix. By contrast, when f v reached 20%, the coarse grains formed some aggregates but the fine-fine contacts were still dominating. When f v increased up to 35%–45%, the skeleton of the sample turned to be dominated by the coarse grains. This suggests that the characteristic volumetric content of coarse grains identified through monotonic triaxial test results corresponds to the transition of soil fabric from fine-fine contacts to grain-grain contacts. [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]

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Abstract: In the track substructure of ancient railways in France, a fouled ballast layer has often been created with time. The mechanical behaviour of this coarse soil was studied in the laboratory using a large-scale triaxial cell. The soil taken from the fouled ballast layer of an ancient railway was re-compacted to a dry density of 2.01Mg/m3 at three water contents (4, 6, and 12%) corresponding to three values of the initial degree of saturation (32, 48, and 100% respectively). Both monotonic and cyclic triaxial tests were performed under constant water content conditions. The experimental results gave the following evidence of the significant effect of the water content on the soil mechanical behaviour: (i) the lower the compaction water content, the higher the shear strength; (ii) a permanent axial strain of 0.4% was found after a large number of cycles at a water content of 4%, while it was 1.4% at the higher water content of 6%. For the saturated soil specimen, failure was even observed after a limited number of cycles. Based on the results obtained, a constitutive model for permanent deformation was elaborated, that accounts for the stress level, the number of cycles and the soil water content. [Copyright &y& Elsevier]

Abstract: Soil air permeability is one of the most important parameters which govern the aeration in agricultural soils and thus has a significant effect on the plant growth and crop production. Therefore, it appears important, when analysing the effect of soil compaction due to agricultural machinery, to correlate air permeability with soil capacity parameters such as air-filled porosity, degree of saturation, water content, etc. In the present work, the relationship between air permeability, soil capacity parameters and vertical stress was analysed by performing confined uniaxial compression tests accompanied by air permeability measurements. Three French soils having different textures were studied. Tests were performed on remoulded and undisturbed soils, at various initial dry bulk densities and water contents. For the remoulded soils, the air permeability has been found strongly correlated with the applied vertical stress for sandy loam; by contrast, no obvious correlation could be established for clay. As far as the undisturbed soils are concerned, the air permeability could be correlated with the air-filled porosity for sandy loam and silty–clayey loam but also no evident correlation could be established for clay. Examination of an existing model predicting the air permeability from the air-filled porosity using one tortuosity/connectivity parameter showed that this parameter varies in a small range for sandy soils and in a larger range for clayey soils. [Copyright &y& Elsevier]

Abstract: In order to better understand the heave observed on the railway roadbed of the French high-speed train (TGV) at Chabrillan in southern France, the swelling behaviour of the involved expansive clayey marl taken from the site by coring was investigated. The aim the study is to analyse the part of heave induced by the soil swelling. First, the swell potential was determined by flooding the soil specimen in an oedometer under its in-situ overburden stress. On the other hand, in order to assess the swell induced by the excavation undertaken during the construction of the railway, a second method was applied. The soil was first loaded to its in situ overburden stress existing before the excavation. It was then flooded and unloaded to its current overburden stress (after the excavation). The swell induced by this unloading was considered. Finally, the experimental results obtained were analyzed, together with the results from other laboratory tests performed previously and the data collected from the field monitoring. This study allowed estimating the heave induced by soil swelling. Subsequently, the part of the heave due to landslide could be estimated which corresponds to the difference between the monitored heave and the swelling heave. [Copyright &y& Elsevier]

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]

• Grey-scale soil image is an indicator of a soil density field. • The grain size distribution (GSD) is predicted by up-scaling the grey-scale image. • It is done by treating only densities above the fixed threshold while up-scaling. • The representation of grains at different resolutions follows a multifractal law. • This multifractal GSD model shows rather good agreement with dry sieving data. Previous works related to the application of the multifractal theory for analyzing the grain size distribution (GSD), showed the potential of this approach to deal with this complex issue. However, absence of the practical application of this kind of statistical analysis raised some doubts among the soil scientists. Compared to the experimental dry sieving method, which is based on mass representations of different grain sizes, the approach presented in this work relies on the analysis of grain densities (density indicators) scanned by means of X-ray CT (Computed Tomography). By reducing the resolution of the scanned soil image(s), the cumulative representation of solid particles equal to or larger than the actual discretization element can be determined, and described analytically by means of the universal multifractals (UM). For validation of the new UM approach, the X-ray CT results of three different soils were used: the volcanic substrate covering Green Wave (a green roof of Champs-sur-Marne in France), and two horizons of the soil collected from the low land mountain area of Sierra de Guadarrama in Spain. Comparison between the proposed UM model and the experimental data of these three materials confirms that the GSD can be reasonably well predicted from the scanned images of soils covering wide range of grain sizes. The UM model, unlike the fractal-based models, accounts for fractal dimension that depends on grain size, and hence, based on the preliminary results presented in this work, it could be rather useful in case of multi-modal soils whose GSD curves are described with multiple fractal dimensions. [ABSTRACT FROM AUTHOR]