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International audience; The Callovo-Oxfordian (COx) claystone is considered as a potential host rock in the French concept of high level radioactive waste disposal at great depth. To better understand and to complement existing published data on the thermo-hydro-mechanical behaviour of the COx claystone, an experimental program was carried out by using a hollow cylinder triaxial device specially developed for low permeability materials. Special care was devoted to the saturation of the specimens that was made under stress conditions close to in-situ, and to conditions ensuring full drainage thanks to a reduced drainage length and low shear rate. Tests were carried out under in-situ, half to in-situ and twice the Terzaghi mean effective in-situ stress at 25°C and 80°C to investigate the effects in the close field of the temperature elevation due to the exothermic nature of the waste. Some radial permeability tests were conducted at various temperatures. The data obtained showed that there is little effect of temperature on the elastic parameters determined, whereas a tendency to a decrease in shear strength was noted, compatible with the few published data. Temperature also appeared to have little effect on the intrinsic permeability, with higher flows mainly due to the decrease in water viscosity.

In this study, the drying-induced microstructure evolution of compacted silt with different salinities is investigated using mercury intrusion porosimetry and environmental scanning electron microscope. The pore size distribution (PSD) of specimens compacted near optimum (w = 17%) exhibited bimodal characteristics. Upon drying (w = 8%), the PSD changed to a trimodal pattern with the appearance of a new nano-pore population. This suggested the development of nano-fissures, which could occur in the clay fraction and at the interface between clay particles and silt/sand grains due to the clay shrinkage. With further drying (w = 3%), the nano-pores disappeared and the PSD recovered to bimodal characteristics, suggesting that the nano-fissures created were enlarged until they became micro-fissures. The salinity seemed to decrease the frequency of the drying-induced micro-pores due to the enhanced mechanical strength of salted soil by the soil aggregation resulting from the compression of the diffuse double layer and a possible cementation effect produced by precipitated salt. However, the salinity effect was relatively low, owing to: (a) the low clay fraction (15·7%) and the low-activity clay minerals (illite, chlorite and kaolinite); (b) the low specific surface area (24 m2/g), which limited the salinity effect on the diffuse double layer.

International audience; In thermal-related engineering such as thermal energy structures and nuclear waste disposal, it is essential to well understand volume change and excess pore water pressure buildup of soils under thermal cycles. However, most existing thermo-mechanical models can merely simulate one heating–cooling cycle and fail in capturing accumulation phenomenon due to multiple thermal cycles. In this study, a two-surface elasto-plastic model considering thermal cyclic behavior is proposed. This model is based on the bounding surface plasticity and progressive plasticity by introducing two yield surfaces and two loading yield limits. A dependency law is proposed by linking two loading yield limits with a thermal accumulation parameter nc, allowing the thermal cyclic behavior to be taken into account. Parameter nc controls the evolution rate of the inner loading yield limit approaching the loading yield limit following a thermal loading path. By extending the thermo-hydro-mechanical equations into the elastic–plastic state, the excess pore water pressure buildup of soil due to thermal cycles is also accounted. Then, thermal cycle tests on four fine-grained soils (natural Boom clay, Geneva clay, Bonny silt, and reconstituted Pontida clay) under different OCRs and stresses are simulated and compared. The results show that the proposed model can well describe both strain accumulation phenomenon and excess pore water pressure buildup of fine-grained soils under the effect of thermal cycles.

International audience; An accurate monitoring of axial and radial thermal strains of specimens of the Callovo–Oxfordian claystone, heated under constant confining stress conditions close to that prevailing in the Bure Underground Research Laboratory at 490 m depth, evidenced a particular behavioural feature. The effect of the transverse isotropy of the claystone on thermal strains was not only confirmed, but it was also shown that the thermo-elasto-plastic overall volumetric response was mainly due to the axial response, in which some influence of the adsorbed water is suspected. Conversely, the reversible radial response observed along a temperature cycle could be modelled through a simple homogenisation method that confirmed that it was governed by the thermo-elastic expansion/contraction response of the constitutive minerals. Hence, the simultaneous occurrence of elastic thermal radial strains and elasto-plastic thermal axial strains was evidenced.

International audience; To better understand the salinity effect on the compaction behaviour of soil, standard Proctor compaction test was conducted on soil samples with different salinities. Matric suction and small strain shear modulus, Gmax, were determined and pore size distribution was also investigated on samples statically compacted at different water contents. Results showed that with the decrease of soil salinity from initial value of 2.10‰ (g of salt/ kg of dry soil) to zero, the maximum dry density increased and the optimum water content decreased, whereas there was no significant change with the increase of soil salinity from 2.10‰ to 6.76‰. Interestingly, it was observed that, Gmax also decreased when the soil salinity decreased from initial value of 2.10‰ to zero and kept almost constant when the soil salinity increased from 2.10‰ to 6.76‰, for dry samples with similar matric suction and also for samples compacted at optimum and on wet side whose matric suctions were slightly different due to the difference in remolded water content. Furthermore, the effect of salinity on compaction behaviour and Gmax decreased for samples compacted from dry side to wet side. The pore size distribution exhibited bi-modal characteristics with two populations of micro-pores and macro-pores not only for samples compacted on dry side and at optimum state, but also for those compacted on wet side. Further examination showed that the modal size of micro-pores shifted to lower values and that of macro-pores shifted to higher values for saline soil compared to the soil without salt.

International audience; In the laboratory, the instantaneous profile method has been routinely applied to determine the hydraulic conductivity of unsaturated compacted clayey soils. But the application of this method is often limited by the large-dimensions of water content probes such as TDR (Time Domaine Reflectometry) probes. Indeed, the existing commercial TDR probes are too large to match the small dimensions of laboratory samples. Furthermore, this method is difficult to be applied to natural stiff clays or other stiff materials (lime/cement treated soils for instance) owing to the surface irregularity created during the sample preparation. The voids between soil surface and testing cell would give rise to side wall leakage during water infiltration under constant-volume condition. This paper attempts to solve these problems by using resistant cohesive filament tape to wrap the soil sample for sidewall leakage prevention and keep the specimen in constant-volume condition. A set of home-made miniature TDR probes were developed for volumetric water content monitoring, together with a set of relative humidity sensors for suction monitoring. As expected, the hydraulic conductivity of stiff Teguline clay which was taken from the area of Albian Paris Basin was found decreasing with the increase of suction. Moreover, the value of hydraulic conductivity at zero suction determined by extrapolation is very close to that determined directly by constant head method. This confirms the promising use of miniature TDR probes in water content monitoring, as well as the effectiveness of resistant filament tape in sidewall leakage prevention under constant-volume condition. The successful incorporation of miniature TDR probes and resistant filament tape extends the common instantaneous profile method to small laboratory samples and stiff materials.

International audience; Drained triaxial testing is challenging in low-permeability claystones (10−20 m2). This paper presents a method of testing low-permeability clay rocks in a standard triaxial cell. In this system, the resaturation of the specimen and the drainage conditions were enhanced by reducing the drainage length to 19 mm, the specimen radius. To do so, two geotextiles were placed around the top and bottom ends of the specimen, with no connection between them. Resaturation was hence performed by forcing water infiltration into the specimen from the upper and lower geotextiles, with a maximum infiltration length of around 19 mm, resulting in reasonable saturation durations. High-precision local measurements of radial strains were also achieved by ensuring direct contact between the LVDT rod and the specimen through the membrane. A poroelastic numerical calculation was carried out, and it was shown that, with these drainage conditions, a strain rate of 6.6 × 10−8 s−1 was satisfactory to ensure good drainage when shearing claystone specimens. After a check test made on a low-permeability sandstone with well-known mechanical characteristics, two tests were carried out to investigate specimens of the Callovo-Oxfordian claystone, a possible host rock for deep geological disposal in France. The results compare well with other published data from drained triaxial tests.

International audience; Lime treatment can enhance the workability and hydro-mechanical properties of soil through different physical-chemical reactions. Nevertheless, the beneficiary effect of lime treatment can be altered when the soil was exposed to wetting-drying cycles, depending on the wetting fluid and soil state. In the present study, the changes in small strain shear modulus (Gmax) and microstructure of a compacted lime-treated silt under wetting-drying cycles were studied. The untreated state of soil was also considered for comparison. Meanwhile, the effects of wetting fluid (deionized water and synthetic seawater) and maximum soil aggregate size (Dmax = 0.4 mm for S0.4 and 5 mm for S5) were investigated. Results showed that Gmax increased significantly for the lime-treated soil over curing, and increased slightly for the untreated soil. The untreated specimens were softened and damaged by wetting-drying cycles, while the lime-treated specimens exhibited good resistance with the pore size distributions almost kept reversible along the wetting-drying paths. The subsequent more intensive drying resulted in a significant fabric alteration with occurrence of shrinkage-related fissures of the clay part. However, these fissures were almost healed with rewetting. Thereby, the Gmax of lime-treated soil showed a constant decreasing trend with wetting and an increasing trend with drying. Moreover, Gmax decreased slightly with wetting-drying cycles for the lime-treated specimens wetted by deionized water, suggesting that the wetting-drying indeed softened the soil. However, Gmax increased for the lime-treated specimens wetted by synthetic seawater, due to the more production of cementitious compounds promoted by salts. The lime-treated specimens S0.4 wetted by synthetic seawater had higher Gmax than those wetted by deionized water, while the wetting fluid had insignificant effect on the Gmax of specimens S5 due to the limited promotion of pozzolanic reaction and negligible soil aggregation induced by salts.

For the French conventional railway lines, a layer, namely interlayer, was created in the substructure mainly by the interpenetration of ballast grains and subgrade soils. The in situ investigation indicated that the content of coarse grains decreases over depth. In this study, the resilient modulus and damping ratio of the unsaturated lower part of the interlayer soil were investigated at six different volumetric inclusion contents fv (volumetric ratio of dry coarse grains to the whole sample) by carrying out cyclic triaxial tests following a multi-step loading procedure. The results show that the two dynamic parameters (resilient modulus and damping ratio) change significantly at the beginning of loading and the variation rate decreases as the number of cycles increases. Besides, a mean characteristic volumetric inclusion content fv–cha ranging from 27.8% to 28.9% was identified, separating two zones with different inclusion effects. To verify this observation, X-ray microcomputed tomography (μCT) scans were conducted on as-compacted samples. The results obtained strongly support the existence of fv–cha: when fv ≤ fv–cha, the matrix of fines constitutes the skeleton of the sample with inclusions floating in it, leading to slight changes of the two dynamic parameters with fv. By contrast, when fv > fv–cha, the inclusions dominate the skeleton of the sample, giving rise to significant changes of the two dynamic parameters with fv.

International audience; A masonry wall is a composite structure characterized by a large variety in geometrical and material parameters. The determination of the effective macroscopic properties, through the homogenization scheme, depends on a great number of variables. Thus, in order to replace heavy numerical simulation, in this paper, the use of artificial neural networks (ANN) is proposed to predict elastic membrane and bending constants of the equivalent Love-Kirchhoff plate of hollow concrete blocks masonry wall. To model the ANN, a numerical periodic homogenization in several parameters is used. To construct the model, five main material and geometrical input parameters are utilized. Multilayer perceptron neural networks are designed and trained (with the best selected ANN model) by the sets of input-output patterns using the backpropagation algorithm. As a result, in both training and testing phases, the developed ANN indicates high accuracy and precision in predicting the equivalent plate of a hollow masonry wall with insignificant error rates compared to FEM results.

International audience; The degenerate scale issue for D boundary integral equations and boundary element methods has been already investigated for Laplace equation, antiplane and plane elasticity, bending plate for Dirichlet boundary condition. Recently, the problem of Robin and mixed boundary conditions and of piecewise heterogeneous domains have been considered for the case of Laplace equation. We investigate similar questions for plane elasticity for more general boundary conditions. For interior problems, it is shown that the degenerate scales do not depend on the boundary condition. For exterior problems, the two degenerate scales (homogeneous medium) or two of them (heterogeneous medium) are tightly linked with the behavior at infinity of the solutions. The dependence of this behavior on the boundary conditions is investigated. We give sufficient conditions for the uniqueness of the solution. Numerical applications are provided and validate the set of theoretical results.

International audience; Based on long term investigations of the microstructure of various geomaterials as 14 different as sensitive low plastic clays from Eastern Canada, unsaturated compacted silt from 15 the Paris area, highly plastic compressible deep marine clay from the Gulf of Guinea, 16 compacted MX80 bentonite and the Callovo-Oxfordian claystone, considered as possible 17 barriers in deep geological radioactive waste disposal, two fundamentals and distinct nano 18 and micro mechanisms governing their macroscopic volume changes have been identified. In 19 low plastic structured clays and dry compacted soils, in which an aggregate microstructure has 20 been evidenced, the decrease in volume under mechanical compression result from the 21 collapse of inter-aggregates pores in an ordered manner, from the larger to the smaller, with 22 no effect on the intra-aggregate porosity. The soil skeleton can hence be modelled as an 23 elastic fragile porous matrix, affected by the ordered collapse of its pores. Things are different 24 in plastic soils, due to the significant reactivity of the montmorillonite minerals to changes in 25 water content that results in significant changes of the initial porous matrix. The ordered 26 adsorption of layers of water molecules in clay platelets with respect to the suction or stress 27 applied, evidenced long time ago in Soil science through X-ray diffraction techniques, appear 28 to be able to help better understanding various phenomena like the compression of plastic 29 compressible soils and the hydration of compacted bentonites. Interestingly, it also applies to 30 understand the effects of smectite minerals in the volume changes behaviour of the Callovo-31 Oxfordian claystone. These two mechanisms at nano and microscopic scales hence provide a 32 better understanding of the macroscopic volume changes of a large variety of natural and 33 compacted clayey soils and rocks. 34 Keywords: clayey soils and rocks; microstructure; montmorillonite; pore size distribution; 35 nano scale; water adsorption. 36 37 2

International audience; 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.

International audience; 22 23 The ConVergence-ConFinement (CV-CF) method is widely used in conventional tunneling at 24 a preliminary stage of the design. In this method, the rock-support interaction is simplified by 25 means of a two-dimensional plane-strain assumption. However, when the ground exhibits large 26 deformation and/or when the support is very stiff and installed close to the tunnel face, the 27 results obtained with the CV-CF method may significantly differ from those obtained using 3D 28 numerical computations. The strong interaction taking place between the rigid lining and the 29 rock mass is not considered in the most common use of the CV-CF method. Some 30 improvements of the CV-CF method as the so-called implicit methods have been developed in 31 order to better account for this interaction. 32 In this paper, the applicability of the CV-CF methods is discussed for full face excavation 33 tunneling with a stiff support system. An in-depth comparison between plane-strain closed form 34 solutions and numerical results which properly accounts for the 3D effects at the vicinity of the 35 tunnel face is carried out. The range of application of the different approaches of the CV-CF 36 method is discussed. Finally, some simple empirical formula which can be used in preliminary 37 design for a large range of ground conditions are proposed. 38 39

The effects of temperature on the water transport properties of intact and damaged Opalinus (OPA) clay are investigated by using a recently developed hollow cylinder triaxial cell [Monfared (Int J Rock Mech Min Sci 48:637–649, 2011b)] that allows full saturation and drainage conditions in low-permeability clays and shales. The volumetric response of saturated OPA clay sample during a drained heating test shows an irreversible contraction after a temperature threshold. The permeability tests which are performed before and after the heating test show that the induced irreversible sample contraction by thermal loading reduces the permeability of OPA clay sample. In order to study the effect of temperature on the permeability of a damaged sample of OPA clay, the permeability tests are performed on a saturated sample previously sheared by a standard drained triaxial loading. The test results show no significant effect of shear-type damage on the permeability of the sample at 25 °C and 80 °C. The experimental results presented in this paper show the crucial role of the thermally induced strains on sample permeability. Thermo-elastic dilation leads to a slight increase of the permeability, whereas thermoplastic contraction leads to a reduction.

International audience; In the present work, the shear behaviour of soils and the soil–concrete interface is investigated through direct shear tests at various temperatures. A conventional direct shear apparatus, equipped with a temperature control system, was used to test sand, clay, and the clay–concrete interface at various temperatures (5, 20, and 40 °C). These values correspond to the range of temperatures observed near thermoactive geostructures. Tests were performed at normal stress values ranging from 5 to 80 kPa. Results show that the effect of temperature on the shear strength parameters of soils and the soil–concrete interface is negligible. A softening behaviour was observed during shearing of the clay–concrete interface, which was not the case with clay specimens. The peak strength of the clay–concrete interface is smaller than the ultimate shear strength of clay.

International audience; The pile (and micropile) group effect in sand remains a key issue in civil engineering. This paper describes an experimental study carried out using a calibration chamber and an instrumented model pile installed by jacking. A specific methodology is proposed to study group effect through a generic pile within the group. A pile group can be modeled by a group of five piles, one central pile and four adjacent piles. The central pile represents a generic pile within a group with a large number of piles, and the calibration chamber enables the performance of a parametric study to evaluate the global and local (tip resistance and shaft friction) group effects and quantify them using the efficiency coefficient factors relative to tip resistance and shaft friction. The effect of parameters, such as the pile-to-pile spacing ratio and the installation order on pile group response has been investigated. The instrumented generic pile is designed to study independently tip resistance and shaft friction.The results show that the installation order has a significant influence on the pile group response. Some recommendations from laboratory tests are proposed to optimize pile installation in the case of a group and benefit from group effect.

International audience; This paper aims to model the thermo-mechanical behavior of saturated clays under undrained conditions. Classic thermo-hydro-mechanical formulations obtained using different approaches were compared and discussed, showing their compatibility and differences. A thermo-elasto-plastic model called TEAM, using a two-surface approach, was developed for saturated clays under undrained conditions in the framework of thermo-poro-mechanics. The aim of the model is to predict a smooth transition between the elastic and elastoplastic states. Two additional physical parameters, namely volumetric thermal expansion coefficient and pore water compressibility, were back analyzed from the results of the undrained heating tests. By simulating experiments found in the literature, it was shown that this model is appropriate in capturing the evolution of pore water pressure of saturated clays under non-isothermal undrained conditions.

International audience; Methane hydrate is being considered as a potential future energy source but may at the same time constitute a considerable geo-hazard. In the present study, methane hydrate bearing sand sediment was created by pressurizing methane gas into previously moistened, then chilled, packed sand specimen (excess gas method). The excess gas was then replaced by water at high pressure. Afterward, a heating/cooling cycle was applied under undrained conditions, in order to completely dissociate gas hydrates and then recreate them inside the specimen. Finally, the pore pressure was reduced to the atmospheric pressure to dissociate gas hydrates. The whole process was performed in a magnetic resonance imaging (MRI) system, allowing the determination of water and/or gas and hydrate quantity (and spatial distribution) at various times. The MRI signal was finally analyzed to interpret various processes in sand sediment: initial hydrate formation, heating-induced hydrate dissociation, cooling-induced hydrate re-formation, and depressurizing-induced hydrate dissociation.

By the end of 2018, 42 years after the landing of the two Viking seismometers on Mars, InSight will deploy onto Mars’ surface the SEIS (Seismic Experiment for Internal Structure) instrument; a six-axes seismometer equipped with both a long-period three-axes Very Broad Band (VBB) instrument and a three-axes short-period (SP) instrument. These six sensors will cover a broad range of the seismic bandwidth, from 0.01 Hz to 50 Hz, with possible extension to longer periods. Data will be transmitted in the form of three continuous VBB components at 2 sample per second (sps), an estimation of the short period energy content from the SP at 1 sps and a continuous compound VBB/SP vertical axis at 10 sps. The continuous streams will be augmented by requested event data with sample rates from 20 to 100 sps. SEIS will improve upon the existing resolution of Viking’s Mars seismic monitoring by a factor of ∼2500 at 1 Hz and ∼200000 at 0.1 Hz. An additional major improvement is that, contrary to Viking, the seismometers will be deployed via a robotic arm directly onto Mars’ surface and will be protected against temperature and wind by highly efficient thermal and wind shielding. Based on existing knowledge of Mars, it is reasonable to infer a moment magnitude detection threshold of Mw∼3 at 40∘ epicentral distance and a potential to detect several tens of quakes and about five impacts per year. In this paper, we first describe the science goals of the experiment and the rationale used to define its requirements. We then provide a detailed description of the hardware, from the sensors to the deployment system and associated performance, including transfer functions of the seismic sensors and temperature sensors. We conclude by describing the experiment ground segment, including data processing services, outreach and education networks and provide a description of the format to be used for future data distribution., Space Science Reviews, 215 (1), ISSN:1572-9672, ISSN:0038-6308

International audience; This study presents a full-scale model investigation on variations of soil stress in a geosynthetic-reinforced pile-supported track bed at various water levels and loading cycles, with four testing procedures: water level rising, cyclic loading at high water level, water level lowering, and cyclic loading at low water level. The soil arching effect was revealed, characterized by higher stress above the pile cap. With the water level rising and loading cycles increasing at high water level, this effect becomes more pronounced, until a peak value of dynamic stress concentration ratio is reached. The stable state of soil arching is obtained earlier near the crown of soil arching, but this arching effect develops more significantly at the foot of soil arching. With the water level lowering and loading at low water level, the soil arching effect remains steady, with slightly changed dynamic stresses in the track bed. The geogrid shows a significant impact on the load transfer mechanism for the quasi-static stress: the quasi-static pile-cap stress presents higher values below the geogrid, whereas the opposite trend is observed for the water-bag (subsoil) area. Nevertheless, this mechanism is not obvious with respect to the dynamic stress, with the values showing no distinct difference above and below the geogrid.; Cette étude présente une étude de modèle grandeur nature sur les variations des contraintes du sol dans des plates-formes sur pieux renforcées par géosynthétique à différents niveaux d’eau et cycles de chargement, avec quatre procédures d’essai : élévation du niveau d’eau, chargement cyclique à niveau d’eau élevé, abaissement du niveau d’eau et chargement cyclique à niveau d’eau bas. L’effet de voûte du sol a été révélé, caractérisé par une contrainte plus élevée au-dessus du chapeau de pieu. Lorsque le niveau d’eau augmente et que les cycles de chargement augmentent à niveau d’eau élevée, cet effet devient plus prononcé, jusqu’à ce qu’une valeur maximale du rapport de concentration des contraintes dynamiques soit atteinte. L’état stable des voûtes du sol est obtenu plus tôt près du sommet de voûte du sol, mais cet effet de voûte se développe de manière plus significative au pied de voûte du sol. Avec l’abaissement et le chargement du niveau d’eau à un niveau d’eau bas, l’effet de voûte du sol reste stable, avec des contraintes dynamiques légèrement modifiées dans la plate-forme de la voie. La géogrille montre un impact significatif sur le mécanisme de transfert de charge pour la contrainte quasi-statique : la contrainte quasi statique de chapeau de pieu présente des valeurs plus élevées sous la géogrille, tandis que la tendance inverse est observée pour la zone du sous-sol. Néanmoins, ce mécanisme n’est pas évident pour la contrainte dynamique, les valeurs ne montrant aucune différence nette au-dessus et en dessous de la géogrille

International audience; An energy approach is proposed as a complement to the stress approach commonly considered for investigating soil desiccation cracking. The elastic strain energies before and after crack initiation are estimated by both numerical and analytical solutions. The energy released by cracking is then compared with the fracture energy to discuss crack initiation conditions. This leads to combined energy and stress conditions for crack initiation following Leguillon's theory. An approximate analytical solution is derived from a variational formulation of the porous elastic body equations. A cohesive zone model and finite element code are used to simulate crack propagation in an unsaturated porous body. This analysis shows that the energy criterion is reached before the stress criterion, and this can explain unstable crack propagation at the beginning. The approximate analytical solution allows predicting correctly the crack depth and opening in its initiation stage.

International audience; The water retention, compressibility and swelling properties of a pellet/powder bentonite mixture were investigated in the laboratory. The water retention capacity was determined under constant-volume condition using a specially designed cell allowing vapor exchange in all directions; suction controlled oedometer tests were carried out for the compressibility investigation; both constant-volume and swell-consolidation methods were applied for the swelling pressure determination. The vapor equilibrium technique was used for suction control. Comparable results were found for the water retention capacity of the pellet/powder mixture determined under constant-volume condition and one single pellet under free swelling condition for suctions higher than 4 MPa, suggesting that the suction was mainly controlled by the micro-pores inside the pellets/powder grains and the swelling of the clay particles was accommodated by the existing macro-pores. On the contrary, at lower suctions, the constant-volume conditions gave rise to a lower water retention capacity for the mixture, indicating the restricted clay particle swelling after the filling-up of the existing macro-pores. Oedometer compression curves revealed a different behavior for low and high suctions: at high suctions, the volume change behavior was governed by the pellets rearrangement combined with possible pellets crushing upon loading. By contrast, at low suctions, the mixture lost its initial granular structure during suction decrease; thus, the volume change behavior became similar to that of compacted bentonite. Compared to compacted bentonite, the pellet/powder mixture showed a lower yield stress. A lower value of swelling pressure was found with the constant-volume method, confirming the limitation of the swell-consolidation method in determining soil swelling pressure.

International audience; An offshore wind farm will be developed in France at the Fecamp site covering in excess of 67 km². In this location, medium to high density chalk containing flint nodules is found. In this paper, some of the hydro-mechanical properties of this specific marine chalk are presented, aiming to characterize the first meters of the chalk more precisely for the design of gravity base foundations. A series of un-drained static and cyclic triaxial tests carried out on 100 mm and 300 mm diameter specimens of weathered chalk are presented. Specimens have been recovered from 100 mm, 300 and 500 mm diameter vibrocore sections. Results show, upon undrained static tests, that the material presents essentially dilative behaviour and usually generates negative excess pore water pressures during shear. Upon undrained cyclic tests, the material shows a progressive development of fairly significant positive excess pore water pressures and relatively high values of axial strains. Results also indicate that the excess pore water pressure dissipation, after a sequence of cycles, induces a densification of the material which results, upon post-cyclic undrained static shear, in a more dilative response resulting in higher values of undrained shear strength.

International audience; Non-compacted pellet/powder bentonite mixtures are considered as candidate sealing plug materials in geological disposals of radioactive waste. Due to its nature, this mixture is characterized by a heterogeneous porosity network, which is responsible for its complex hydro-mechanical (HM) behaviour. The French Institute of Radioprotection and Nuclear Safety (IRSN) has investigated this mixture within the SEALEX project. Both in situ large-scale and laboratory small-scale experiments were carried out. This paper presents the results of a small-scale mock-up test at 1/10th scale of the in situ experiments, in which the pellet/powder mixture was saturated from both sides (top and bottom of the specimen). Both swelling pressure and relative humidity were monitored at several positions of the specimen. Different responses from the sensors were found, depending on the local porosity as well as the evolution of the hydration front. The HM response of the mixture is strongly conditioned by the initial pellet/powder distribution, which depends on the protocol followed for the specimen preparation. After 800-day hydration, an anisotropy was found between the axial and the radial swelling pressures, due to the presence of larger void at the top of the sample and the friction at the cell wall. The sample was still heterogeneous after 800-day hydration mainly due to the initial heterogeneous porosity distribution, combined with the effect of friction and the non-saturation of the mixture. The evolution of injected water with time revealed that the sample was not full saturated after 800-day hydration.

International audience; In the French conventional railway substructure, the interpenetration of ballast grains and subgrade fine soils that occurred over years of operation has created a new layer, namely, an interlayer. The in situ investigation on this interlayer showed that the content of ballast grains decreases over depth, with the lower part characterized by a matrix of fines with inclusions of coarse grains. In this study, the permanent deformation of a material simulating the lower part of interlayer soil was investigated at six different volumetric inclusion contents fv (volumetric ratio of the inclusion grains to the total sample) by performing cyclic triaxial tests. The results indicate that at a given cycle, the permanent deformation decreases with the increase of fv, and the evolution of permanent deformation is strongly influenced by the loading history. To further investigate the effect of inclusion content, a method that allows eliminating the influence of loading history and estimating the end-stage permanent deformations at different stress levels was applied. Quadratic polynomial and bilinear fitting methods were used to fit the variations of the estimated end-stage permanent deformations with fv, showing a proper similarity between the two methods at high loading amplitudes (higher than 15 kPa). Furthermore, the testing results revealed the existence of a characteristic volumetric inclusion content fv-cha by bilinear fitting method, separating two zones with different inclusion effects. This observation is strongly supported by the X-ray microcomputed tomography (μCT) scans performed on the as-compacted samples. When fv ranges from 0 to 20% (smaller than fv-cha), the fines constitute the skeleton of the sample, and the permanent deformation decreases rapidly with the increase of fv. By contrast, when fv increases up to 35–45% (larger than fv-cha), the inclusions dominate the skeleton of the sample, leading to a slight decrease of permanent deformation with fv.

International audience; Although not the prime focus of the InSight mission, the near-surface geology and physical properties investigations provide critical information for both placing the instruments (seismometer and heat flow probe with mole) on the surface and for understanding the nature of the shallow subsurface and its effect on recorded seismic waves. Two color cameras on the lander will obtain multiple stereo images of the surface and its interaction with the spacecraft. Images will be used to identify the geologic materials and features present, quantify their areal coverage, help determine the basic geologic evolution of the area, and provide ground truth for orbital remote sensing data. A radiometer will measure the hourly temperature of the surface in two spots, which will determine the thermal inertia of the surface materials present and their particle size and/or cohesion. Continuous measurements of wind speed and direction offer a unique opportunity to correlate dust devils and high winds with eolian changes imaged at the surface and to determine the threshold friction wind stress for grain motion on Mars. During the first two weeks after landing, these investigations will support the selection of instrument placement locations that are relatively smooth, flat, free of small rocks and load bearing. Soil mechanics parameters and elastic properties of near surface materials will be determined from mole penetration and thermal conductivity measurements from the surface to 3–5 m depth, the measurement of seismic waves during mole hammering, passive monitoring of seismic waves, and experiments with the arm and scoop of the lander (indentations, scraping and trenching). These investigations will determine and test the presence and mechanical properties of the expected 3–17 m thick fragmented regolith (and underlying fractured material) built up by impact and eolian processes on top of Hesperian lava flows and determine its seismic properties for the seismic investigation of Mars’ interior.

International audience; A numerical model for thermo-hydro-mechanical strong couplings in an elasto-plastic Cosserat continuum is developed to explore the influence of frictional heating and thermal pore fluid pressurization on the strain localization phenomenon. This model allows specifically to study the complete stress–strain response of a rock specimen, as well as the size of the strain localization zone for a rock taking into account its microstructure. The numerical implementation in a finite element code is presented, matching adequately analytical solutions or results from other simulations found in the literature. Two different applications of the numerical model are also presented to highlight its capabilities. The first one is a biaxial test on a saturated weak sandstone, for which the influence on the stress–strain response of the characteristic size of the microstructure and of thermal pressurization is investigated. The second one is the rapid shearing of a mature fault zone in the brittle part of the lithosphere. In this example, the evolution of the thickness of the localized zone and the influence of the permeability change on the stress–strain response are studied.

International audience; In the Sahelian region, the West African Monsoon (WAM) is an important phenomenon for land water storage evolution, as demonstrated by The Gravity Recovery and Climate Experiment (GRACE) estimations. The Monsoon leads to an annual increase of the water mass. However, GRACE data also displays the existence of a semi-annual cycle whose its origin is still uncertain. This cycle is characterized by a gain of water mass at the beginning of the dry season. In this study, 10-days GRACE data are used to understand the characteristics of this semi-annual cycle. Investigations of the rainfall events, rivers discharge peaks, and the Lake Chad water level variations suggest that they are not at the origin of this cycle. However, MODIS evapotranspiration data display an increase each 6 months, during the rainy season, and at the same time as the semi-annual cycle estimated by GRACE. This increase occurs in regions where the amount of clays at the surface exceeds 30%. The link between both signals and the proportion of clays at the surface leads us to the conclusion that the seasonal variation of the vertical permeability of clays controls the amount of water present in the unsaturated zone.

In this paper we study the phenomenon of localization of deformation in fault gouges during seismic slip. This process is of key importance to understand frictional heating and energy budget during an earthquake. A infinite layer of fault gouge is modeled as a Cosserat continuum taking into account Thermo-Hydro-Mechanical (THM) couplings. The theoretical aspects of the problem are presented in the companion paper (Rattez et al., 2017a), together with a linear stability analysis to determine the conditions of localization and estimate the shear band thickness. In this Part II of the study, we investigate the post-bifurcation evolution of the system by integrating numerically the full system of non-linear equations using the method of Finite Elements. The problem is formulated in the framework of Cosserat theory. It enables to introduce information about the microstructure of the material in the constitutive equations and to regularize the mathematical problem in the post-localization regime. We emphasize the influence of the size of the microstructure and of the softening law on the material response and the strain localization process. The weakening effect of pore fluid thermal pressurization induced by shear heating is examined and quantified. It enhances the weakening process and contributes to the narrowing of shear band thickness. Moreover, due to THM couplings an apparent rate-dependency is observed, even for rate-independent material behavior. Finally, comparisons show that when the perturbed field of shear deformation dominates, the estimation of the shear band thickness obtained from linear stability analysis differs from the one obtained from the finite element computations, demonstrating the importance of post-localization numerical simulations.

International audience; A Thermo-Hydro-Mechanical (THM) model for Cosserat continua is developed to explore the influence of frictional heating and thermal pore fluid pressurization on the strain localization phenomenon. A general framework is presented to conduct a bifurcation analysis for elasto-plastic Cosserat continua with THM couplings and predict the onset of instability. Furthermore, the presence of an internal length in Cosserat continua enables to estimate the thickness of the localization zone. This is done by performing a linear stability analysis of the system and looking for the selected wavelength corresponding to the instability mode with fastest finite growth coefficient. These concepts are applied to the study of fault zones under fast shearing. For doing so, we consider a model of a sheared saturated infinite granular layer. The influence of THM couplings on the bifurcation state and the shear band width is investigated. Taking representative parameters for a centroidal fault gouge, the evolution of the thickness of the localized zone under continuous shear is studied. Furthermore, the effect of grain crushing inside the shear band is explored by varying the internal length of the constitutive law.

In this paper, some of the hydro-mechanical properties of a specific marine chalk are discussed. A series of undrained static and cyclic triaxial tests carried out on specimens recovered from 100, 300 and 500 mm diameter VibroCore sections. Results show that, upon undrained cyclic tests, the material presents a progressive development of significant positive excess pore water pressures and relatively high values of axial strains., Dans cette communication, on présente le comportement hydromécanique d'une craie marine. Une série d'essais triaxiaux statiques et cycliques de type non drainé ont été réalisés, sur des éprouvettes récupérées à partir de VibroCores de 100, 300 et 500 mm de diamètre. Lors des essais de chargements cycliques non drainés, le matériau développe progressivement des surpressions positives significatives accompagnées d'une accumulation assez importante de la déformation axiale.

International audience; This paper deals with the resilient behavior of the interlayer soil which is created mainly by the interpenetration of ballast and subgrade soils. The interlayer soil studied was taken from a site in the southeast of France. Large-scale cyclic triaxial tests were carried out at three water contents (w = 4, 6 and 12 %) and three fines contents corresponding to 5, 10 % subgrade added to the natural interlayer soil and 10 % fine particles (

International audience; The aggregate size of soils is an essential parameter in lime treatment: the thermo-hydro-mechanical behaviour of lime-treated soils can be greatly affected by the aggregate size of soils before treatment. In particular, the compression behaviour and the pore-size distributions of treated soils are expected to be different with different aggregate sizes, leading to different compressibility and permeability of soils. This study aims to investigate the aggregate size effects on the compressibility and air permeability of a lime-treated soil by performing air permeability measurements in oedometer compression tests on a silt with two different maximum aggregate sizes (Dmax = 5 and 0.4 mm) and treated with 2% quicklime. The tests were performed on both untreated and treated samples at various curing times (t = 7, 28, 60 and 90 days). Results showed that lime treatment significantly improves the compression behaviour of soil due to both the flocculation process and the pozzolanic reaction. The treated samples prepared with smaller aggregates have higher oedometer modulus, suggesting a better lime distribution and the production of more cementitious compounds. Air permeability of soil is strongly related to the aggregate size: the soil with smaller aggregates can have a value of one order of magnitude lower than that with larger aggregates. Further examination showed that in the case of smaller aggregates, the morphology of macro-pores and the efficiency of air flow are more sensitive to lime treatment.

International audience; The Callovo-Oxfordian (COx) claystone is considered as a potential host rock for high-level radioactive waste disposal at great depth in France. Given the exothermic nature of radioactive wastes, a temperature elevation planned to be smaller than 100°C will affect the host rock around the disposal cells. To gain better understanding of the thermal volumetric response of the COx claystone, a new thermal isotropic compression cell was developed with particular attention devoted to monitoring axial and radial strains. To do so, a high precision LVDTs system ensuring direct contact between the LVDT stem and the claystone sample through the membrane was developed. A short drainage length (10 mm) was also ensured so as to allow full saturation of the sample under stress conditions close to in-situ, and fully drained conditions during compression. High precision strain monitoring allowed to observe a volumetric creep under stress conditions close to in-situ. A drained heating test under constant stress carried out afterwards up to 80°C exhibited a thermo-elastic expansion up to a temperature of 48°C, followed by thermoplastic contraction at higher temperature. Creep volume changes, that appeared to be enhanced by temperature, were modelled by using a simple Kelvin-Voigt model, so as to estimate the instantaneous response of the COx claystone and to determine its thermal expansion coefficient. The temperature at which the transition between thermal expansion and contraction appeared is close to the maximum burial temperature of the Callovo-Oxfordian claystone, estimated at 50°C. This is in agreement with what has been already observed on the Opalinus Clay by Monfared et al. (2013) that was interpreted as a thermal hardening phenomenon, showing that the material kept the memory of the highest temperature supported during its geological history.

InSight’s Seismic Experiment for Interior Structure (SEIS) provides a unique and unprecedented opportunity to conduct the first geotechnical survey of the Martian soil by taking advantage of the repeated seismic signals that will be generated by the mole of the Heat Flow and Physical Properties Package (HP3). Knowledge of the elastic properties of the Martian regolith have implications to material strength and can constrain models of water content, and provide context to geological processes and history that have acted on the landing site in western Elysium Planitia. Moreover, it will help to reduce travel-time errors introduced into the analysis of seismic data due to poor knowledge of the shallow subsurface. The challenge faced by the InSight team is to overcome the limited temporal resolution of the sharp hammer signals, which have significantly higher frequency content than the SEIS 100 Hz sampling rate. Fortunately, since the mole propagates at a rate of $\sim1~\mbox{mm}$ per stroke down to 5 m depth, we anticipate thousands of seismic signals, which will vary very gradually as the mole travels. Using a combination of field measurements and modeling we simulate a seismic data set that mimics the InSight HP3-SEIS scenario, and the resolution of the InSight seismometer data. We demonstrate that the direct signal, and more importantly an anticipated reflected signal from the interface between the bottom of the regolith layer and an underlying lava flow, are likely to be observed both by Insight’s Very Broad Band (VBB) seismometer and Short Period (SP) seismometer. We have outlined several strategies to increase the signal temporal resolution using the multitude of hammer stroke and internal timing information to stack and interpolate multiple signals, and demonstrated that in spite of the low resolution, the key parameters—seismic velocities and regolith depth—can be retrieved with a high degree of confidence.

International audience; Saturated, isotropic, poroelastic materials are classically described by their elastic stiffness, one Biot coefficient and one Biot modulus (Coussy 2010; Dormieux, Molinari, and Kondo 2002). The situation becomes more complex for unsaturated, isotropic, poroelastic materials that require multiple Biot coefficients and moduli (Coussy and Brisard 2009). These poroelastic coefficients are dependent and linked by several linear relationships. Micromechanical estimates of these coefficients have been proposed by several authors (Ulm, Constantinides, and Heukamp 2004; Pichler and Hellmich 2010). However, these estimates may fail to fulfill the linear relationships that relate the exact poroelastic coefficients. This might be regarded as an undesirable inconsistency of the model. In this work, we propose new, consistent (in the sense that the above mentioned linear relationships are preserved) estimates of the poroelastic coefficients. Our point of departure is the principle of Hashin and Shtrikman, suitably extended to eigenstressed materials (Bornert et al. 2001). Adopting stress-polarization fields that are similar to the eigenstress-free case (Willis 1977; Ponte Castañeda andWillis 1995) allows us to derive variational estimates of the poroelastic coefficients, which can be shown to fulfill all known linear relationships required from the exact values. After outlining the derivation within the general framework of eigenstressed, heterogeneous materials, the results will be specialized to poroelasticity. This will lead to a variational justification of the ad-hoc pore isodeformation assumption (Coussy and Brisard 2009).

International audience; Clays adsorb water following discrete steps in which water is roughly structured in layers. The resulting discrete hydration states can be viewed as phases separated by unstable domains and the change of hydration state can be viewed as a phase transformation. Here, we identify stable basal spacings and the domains of (meta-) stability through the analysis of confining pressure isotherms obtained from molecular simulations performed in grand canonical ensemble. This ensemble mimics drained conditions. Through thermodynamic analysis, we can explain the coexistence of different hydration states at clay particle scale and provide kinetics arguments to the hysteresis observed with respect to traction and compression. The gained insights into the mechanical behavior of the elementary constituents of clays may lead to a better understanding of the behavior of clay-rich rocks and soils.

International audience; In this work, we propose an extension of the phase field model for the modeling of hydraulic fracturing or cracking in heterogeneous saturated porous media. The involved extensions comprise: (a) taking into account interfacial damage between the inclusions and the matrix; (b) modeling fluid flow within both matrix cracks and interfacial cracks; (c) the possibility to handle geometries of the heterogeneous media in the form of regular grids of voxels e.g. as obtained from experimental imaging techniques. The developed numerical framework is based on the phase field method with a regularized description of both bulk and interface discontinuities, extended to a fully coupled hydro-mechanical framework. Both 2D and 3D examples are presented for hydro-mechanical microcracking initiation and propagation in voxel-based models of complex heterogeneous media with interfacial damage between the inclusions and the matrix.

International audience; This work is devoted to an experimental investigation of the poroelastic behavior of the Callovo–Oxfordian claystone, a potential host rock for the deep underground repository of high-level radioactive waste in France. Drained, undrained, pore pressure loading and unjacketed tests were carried out in a specially designed isotropic compression cell to determine the poroelastic parameters of fully saturated specimens. Great care was devoted to the saturation procedure, and small loading rates were used to ensure full drainage conditions in drained and pore pressure tests (0.5 kPa/min) and in the unjacketed test (2 kPa/min). High-precision strain measurements were performed by ensuring direct contact between the LVDT stems and the specimen. An analysis in the framework of transverse isotropic poroelasticity provided the Biot effective stress coefficients b1 (perpendicular to bedding) between 0.85 and 0.87 and b2 (parallel to bedding) between 0.90 and 0.98 under different stress conditions (pore pressure 4 MPa, total isotropic stresses of 14 and 12 MPa, respectively). A set of equivalent isotropic poroelastic parameters was also determined and a very good compatibility between the results of different tests was found, giving confidence in the parameters determined. The unjacketed test provided a directly reliable measurement of the unjacketed modulus (Ks = 21.7 GPa) that was afterward confirmed by an indirect evaluation that showed the non-dependency of Ks with respect to the stress level. These parameters were obtained for specimens cored and trimmed in the laboratory. A parametric study was then conducted so as to provide an estimation of the parameters in situ, i.e., not submitted to the damage supported by laboratory specimens. A minimal value b = 0.77 seems to be a reasonable lower bound for the equivalent isotropic Biot parameter.

Compacted bentonite-based materials are often used as buffer materials in radioactive waste disposal. When the compacted bentonite blocks are emplaced, technological voids related to different interfaces involving the buffer material are created, and their hydro-mechanical behaviour is of primary importance for the safety of disposal. In this study, the hydraulic resistance of the interface between compacted MX80 bentonite and Boom Clay is investigated in the laboratory using an injection cell. The results obtained show that when water is injected, the technological gap is quickly reduced due to the bentonite swelling. When water pressure reaches the hydraulic resistance of the interface, hydraulic fracturing takes place with a drastic pressure decrease. After fracturing, water injection continues and bentonite continues to swell. A higher subsequent pressure is needed to produce a new hydraulic fracturing. After a certain time, the hydraulic resistance becomes high enough so that no further fracturing occurs, suggesting that the technological gap is sealed.

International audience; The conventional railway substructure in France was built by emplacing ballast directly on subgrade. Over years of operation, the interpenetration of ballast and subgrade created a soil layer between them. Under different conditions, this naturally formed layer, namely interlayer, can contain different quantities of fine particles, becoming more or less sensitive to changes in water content. As the water content changes are governed by the hydraulic behavior of the interlayer soil, assessing the influence of fine particle content on the hydraulic behavior of interlayer soil is of importance. To this end, the hydraulic behavior of an interlayer soil taken from Senissiat (near Lyon, France) was investigated using two infiltration columns, a large-scale column equipped with tensiometers and a time domain reflectometer (TDR) for suction and volumetric water content measurements, respectively, and a smaller column equipped with high-capacity tensiometers only. Different fines contents were considered and wetting-drying cycles were applied to the soil specimens. The hydraulic conductivity was determined by applying the instantaneous profile method. The results obtained showed that (i) hysteresis exists for both the soil water retention curve and the hydraulic conductivity changes with suction; (ii) the effect of wetting-drying cycles is insignificant; (iii) adding 10% fine particles to the natural interlayer soil gives rise to changes in the soil water retention curve but does not induce significant changes in hydraulic conductivity; (iv) the unsaturated hydraulic conductivity of interlayer soil with 10% fine particles added is close to that of soil sieved at 2 mm, suggesting that the unsaturated hydraulic conductivity of interlayer soil is mainly governed by fine particles through the suction effect. By contrast, in a saturated state, the value for the interlayer soil with 10% fine particles added was found to be higher, suggesting that in this case the hydraulic conductivity is mainly governed by the water transfer through macropores.

International audience; This study aims to provide knowledge on the thermo-mechanical behaviour of heat exchanger piles, through a laboratory scale model. The model pile (20 mm in external diameter) was embedded in dry sand. The behaviour of the axially loaded pile under thermal cycles was investigated. After applying the axial load on the pile head, the pile's temperature was varied between 5 °C and 30 °C. Seven tests, corresponding to various axial loads ranging from 0 to 70 % of the pile estimated bearing capacity, were performed. The results on pile head displacement show that heating under low axial load induced heave and cooling induced settlement; the pile temperature-displacement curve was found to be reversible and compatible with the thermal expansion curve of the pile. However, at higher axial loads, irreversible settlement of the pile head was observed after a few thermal cycles. The axial load profile measured by the strain gauges evidenced that the pile head load was mainly transferred to the pile toe. Nevertheless, thermal cycles modified significantly the mobilised skin friction along the pile. The total pressure measured at various locations in the soil mass was also slightly influenced by the thermal cycles.

Fault systems can play a significant role regarding several risk issues related to CO2-injection-induced mechanical perturbations: fluid flow enhancement or compartmentalization within the reservoir, loss of integrity of the reservoir-caprock systems, potential triggering of seismicity and generation of new leakage pathways or permeability barriers. Several methods exist to model hydro-mechanical behaviour of fault zones. A first "conventional" approach aims at evaluating the fault response by directly post-processing the results of the large-scale coupled hydro-mechanical simulations. This consists of: estimating the changes of the effective stress field in the reservoir-caprock system during CO2 injection; computing changes of shear and normal stresses acting on the fault plane; comparing them to a fault reactivation criterion. However, this approach does not account for the effects of the presence of the fault on the stress and pressure field in the surrounding rock matrix. More sophisticated (and physically more realistic) modeling strategies have been proposed, which explicitly integrate the fault zone as a distinct element in the large-scale simulation, i.e. by representing it as a linear discontinuity with various hydraulic and mechanical properties. From a CO2 storage perspective, such a model still remains limited regarding two issues: 1. Faults are complex and heterogeneous geological systems, which do not correspond to discrete surfaces as already postulated by many authors. A fault zone is composed of an inner core made of fine material, often impermeable, and where slip is concentrated. It is surrounded by an outer damage zone that often acts as a hydraulic pathway, because of the presence of a fracture network, whose characteristics (fractures' orientation, connectivity, lengths, density, number of fractures' families, etc.) depend on the distance to the core; 2. Chemical interactions (dissolution and precipitation processes, chemically-induced weakening, etc.) between CO2-enriched brine and the minerals constituting the fault zone. can affect the mechanical and transport properties of the faulted/fractured system. In particular, chemo-mechanical processes can either stabilize the system if the compaction rate is increased or destabilize it if new micro-fractures are created. The FISIC project intends to overcome those limitations by accurately modelling the hydro-chemo-mechanical (HCM) complexity of a fault zone. The main goal is to provide appropriate theoretical and numerical models for accurate evaluation of fault stability in the context of CO2 geological storage, i.e. improving the stability analysis of a fault both undertaking pressure increase and alteration due to the presence of an acidic fluid. Four research questions are addressed and the progress regarding each of them is presented: 1. How to represent a fault zone in a tectonic setting, which has a priori been selected far from major potentially seismically-active faults, i.e. a moderate-to-low-deformed setting; 2. What are the fractures' organization within the damage zones of faults, i.e. their spatial distribution? 3. What are the dominant chemo-mechanical processes resulting from aqueous CO2 in fractured/faulted systems? 4. How to integrate (numerically) the hydro-chemo-mechanical (HCM) behaviour of fault zones in large-scale (reservoir or basin-scale) simulations? Regarding the first and second question, a geostatistical approach is adopted based on observations of fault structures at the Cirque de Navacelles. This site, located in the late Jurassic platform carbonates of Languedoc (southern France), can be considered a good analogue for CO2 storage reservoir. The third question is addressed at laboratory scale focusing on the CO2(aqueous)-induced effect on: i. the slow growth of cracks (subcritical fracturing) and their healing; ii. the degradation of mechanical properties such as fracture toughness or shear strength. Finally, the fourth question is numerically addressed by relying on: i. advanced meshing tools developed for converting complex geometries to finite element models; ii. analysis of fracture propagation condition within multiphase fractured porous media; iii. the rheological behavior derived from the experimental study.

The soil-atmosphere interaction was investigated by conducting physical model test in the laboratory and by performing numerical analysis using data from an experimental site. For the physical model test, a large scale environmental chamber was developed. This chamber was instrumented by various sensors allowing the soil suction, volumetric water content, and temperature to be monitored at various depths. In the zone occupied by air, temperature, relative humidity and air rate (wind speed) were monitored. The soil surface temperature was also measured by an infrared thermometer. The soil sample was prepared by compaction and wetted from the soil surface before being dried under controlled conditions of relative humidity, temperature and flow rate of air. A camera fixed above the chamber allowed monitoring the development of cracks. The evaporation rate was calculated based on the temperature and relative humidity data at the inlet and the outlet of the chamber, allowing assessment of the performance of the chamber to simulate the evaporation phenomena. The results show that evaporation is a heat consuming process: in the test condition, both air and soil were cooled by the evaporation process. Cracks developed during soil drying. Analysis of the ‘cracking surface ratio’ and ‘weighted width’ showed that the evolutions of these two parameters are similar: thus, in practice, only one is needed for evaporation analysis. For the numerical analysis, a two-dimensional model of soil-atmosphere interaction was developed and implemented in a fully coupled thermo-hydro-mechanical (THM) code. The soil surface boundary conditions were determined from meteorological data using water balance and energy balance equations. The settlement due to soil-atmosphere interaction in an experimental site was simulated. Comparison between calculation and measurement showed that the THM codes can satisfactorily predict the soil settlement, provided that appropriate evapotranspiration models are implemented.

International audience; In this paper a viscoplastic constitutive model for unsaturated geomaterials is presented. The constitutive laws for the description of the time-dependent mechanical behaviour of saturated and unsaturated geomaterials generalise the isotach approach proposed by Šuklje [52] to the elastoplastic strain hardening based constitutive laws for unsaturated soils. The formulation encompasses strain rate effects and allows the description of the coupled effect of strain rate and suction on the viscoplastic behaviour of geomaterials. The model has been used to simulate tests involving variable stress and/or strain rate conditions, including creep phases and soaking tests. Numerical results demonstrate its reliability in reproducing the general features of the rate-dependent behaviour together with the dependency upon suction. Further tests are currently under consideration to extend model verification to others saturated and unsaturated geomaterials. © 2013 Elsevier Ltd.

International audience; SEALEX is a research project aiming at identifying the key factors that affect the long-term performance of bentonite-based sealing systems with an initial technological void. In this context, a series of in situ experiments have been performed in field conditions. Meanwhile, a small-scale test (1/10) was carried out in controlled conditions in the laboratory, aiming at providing useful information for analyzing the in situ tests in terms of saturation time and sealing effectiveness. In this paper, the results of the small-scale test are presented along with the results from the first in situ test (PT-N1). It was observed that during the saturation process, the evolution of the injected water volume followed a hyperbolic relationship with time in both the laboratory and field conditions. In the laboratory conditions, a decrease in axial swelling pressure occurred due to filling of the technological void. By contrast, this decrease has not been observed in the field conditions. Comparison of the injected water and the axial swelling pressure between the two different scales enabled the definition of a same time upscaling ratio of 2.5 (in situ experiment/small-scale test). Accordingly, the saturation duration of the in situ experiment was estimated to be equal to two years. For the small-scale test, a swelling strain evolution rate of 0.588 mm/day was identified in the case of infiltration from two sides of the sample. This is useful when predicting the evolution of swelling strain in the case of failure of the sealing plug. After filling of an additional 20% void, a swelling pressure of 0.18 MPa was obtained, indicating the favorable sealing capacity of the material after filling the technological void.

International audience; The failure of geomaterials in localised shear bands is one of the most common features in geomechanics. Early studies have provided the necessary criteria for the conditions of localisation, inclination angle with respect to the loading axes and thickness of the shear bands, when loaded at room temperature. This work extends these criteria for the problem of simple shear of a chemically active granular cohesionless material at higher temperatures, where thermal or chemical pressurisation may set in and influence the response of the material. It is deduced that failure occurs at higher positive values of the hardening modulus as temperature increases, that the shear band thickness also depends on the chemical reaction characteristics and that micro-inertia due to grain translations and rotations introduce the necessary rate dependency to regularise the system.

International audience; This paper presents a constitutive model for describing some important features of the behavior of natural stiff clay evidenced experimentally such as the limited elastic zone, the presence of strain hardening and softening, and the smooth transition from elastic behavior to a plastic one. The model, namely ACC-2, is an adapted Modified Cam Clay model with two yield surfaces: similarly to bounding surface plasticity theory, an additional yield surface—namely Inner yield surface—was adopted to account for the plastic behavior inside the conventional yield surface. A progressive plastic hardening mechanism was introduced with a combined volumetric-deviatoric hardening law associated with the Inner yield surface, enabling the plastic modulus to vary smoothly during loading paths. The main feature of the proposed model is that its constitutive equations can be simply formulated based on the consistency condition for the Inner yield surface, so that it can be efficiently implemented in a finite element code using a stress integration scheme similar to that of the Modified Cam Clay model. Furthermore, it is proved to be an appropriate model for natural stiff clay: the simulations of a set of tests along different mechanical loading paths on natural Boom Clay show good agreement with the experimental results.

International audience; To better understand the impact of temperature elevation on the response of the excavation damaged zone around repository cells and galleries for radioactive waste disposal, the combined effects of shear and temperature elevation were investigated in the laboratory on the Cal-lovo-Oxfordian claystone. To do so, a hollow cylinder thermal triaxial cell with short drainage path specifically developed for low permeability rocks was used. Once properly saturated under stress conditions close to in situ, the specimen was sheared along a constant effective mean stress path mimicking the stress path followed during gallery excavation. The shear stress was afterwards released and an undrained heating test was performed on the sheared specimen. It was observed that the temperature increase under undrained conditions led to a thermal increase in pore water pressure resulting in a decrease in mean effective stress that brought back the sheared specimen to failure, evidencing a thermally induced failure. Steady state radial permeability tests performed at various stages of the test demonstrated that the overall permeability of the sheared specimen was comparable to that before shearing, confirming the excellent self-sealing properties of the Callovo-Oxfordian claystone. This shows that, in spite of being possibly remobilised by temperature elevation, the EDZ will keep an overall permeability constant equal to that of the massive rock, keeping the same isolation properties.