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International audience; Water evaporation induces large volume change for clayey soils, often causing problems to geotechnical and geoen-vironmental constructions. To better understand this process, an evaporation test on a compacted clay was conducted in a large-scale environmental chamber under controlled atmospheric conditions. Atmospheric parameters (wind speed, air temperature , and relative humidity) and the response of soil parameters (volumetric water content, temperature, soil suction as well as desiccation cracks) were monitored. The results show that the soil temperature is strongly related to the air conditions, evaporation process, and desiccation cracks. Unlike for sand, the evolution of volumetric water content is governed by both the high water retention capacity of clay and the effect of cracks. A three-stage evolution can be observed for not only the actual evaporation rate, but also the surface crack ratio. Thus, the surface crack ratio can be considered as one important parameter in the evaporation analysis taking into account the effect of desiccation cracks.; L’évaporation d’eau induit des variations de volume importantes pour des sols argileux, causant souvent des problèmes aux constructions géotechniques et géoenvironnementales. Pour mieux comprendre ce processus, un essai d’évaporation sur une couche d’argile compactée a été réalisé dans une chambre environnementale à grande échelle avec des conditions atmosphériques contrôlées. Les paramètres atmosphériques (le vent, la température et l’humidité relative) et la réponse des paramètres de sol (la teneur en eau volumétrique, la température, la succion du sol ainsi que la fissuration) ont été suivis. Les résultats montrent que la température du sol est fortement liée aux conditions atmosphériques, au processus d’évaporation et à la fissuration. Contrairement au sable, le développement de la teneur en eau volumétrique est régi par la capacité de rétention d’eau élevée de l’argile et l’effet de la fissuration. Nous pouvons observer une évolution en trois étapes pour non seulement le taux d’évaporation réel, mais aussi pour le rapport fissure/surface. Ainsi, le rapport fissure/surface peut être considéré comme un paramètre important dans l’analyse de l’évaporation en tenant compte de l’effet des fissures de dessiccation

The main drying paths of the water retention curves of lime-treated soils were measured. Aggregate size effect on the water retention property was emphasised. Four soil powders with different maximum aggregate sizes (Dmax = 5, 2, 1 and 0.4 mm) were prepared and mixed with 2% quicklime (by weight of dry soil). Samples were prepared by static compaction at dry side of optimum water content (w = 17%) with a dry density of 1.65 Mg/m3. Suction measurement was performed by a dew-point hygrometer at different curing periods (t = 7, 28 and 90 days). The results obtained show that: i) aggregate size effect is insignificant on the water retention curve of untreated soil in the studied suction range (ca 500 kPa ~ 55 MPa); ii) lime treatment gradually improves the soil water retention capacity with the increasing of curing time; iii) aggregate size effect on water retention property of lime-treated soil becomes significant in the suction range from ca 860 kPa to 9 MPa on the long curing term: treated soils prepared with smaller aggregate size (S0.4 and S1) have a higher water retention capacity rather than soils prepared with larger aggregate size (S5).

Stress release effects in a specimen of claystone extracted at great depth (465 m) in the layer of the Callovo-Oxfordian claystone in Eastern France is examined by mimicking the stress change during extraction in the oedometer. To do so, a specimen first submitted to hydration with swelling impeded is afterwards unloaded at constant water content. Comparable mercury intrusion pore size distribution curves are obtained before and after stress release, showing little change in the configuration of inter-platelets pores within the clay matrix of the claystone. The swelling observed at constant water content is then related, to a water transfer from inter-platelets pores to intra-platelets pores, resulting in crystalline swelling within the smectite fraction of the inter-layer illite-smectite minerals.

Very strong winters with temperatures under 0°C and hot summers with temperatures more than 30°C are observed in the South East part of Slovenia. Those big differences in temperature during the year and especially temperatures below freezing point have strong influence on asphalt layer and sub base of road pavement. The freeze/thaw cycles lead to formation of ice lenses in base course causing cracks in asphalt layers and degrade the pavement usually in a few years. For this reason one section of the national road in the South East part of Slovenia was rebuilt with cold in-situ pavement retreatment. A test field with inbuilt sensors for measuring water content, temperature and deformation in various depths and locations was constructed during the remediation works to study the mechanism of freeze-thaw degradation of pavements. The main goal of the test field is to determine water content in sub base, freezing depth, temperature distribution and deformations, which lead to cracks in asphalt layer after the remediation work in the road construction.

Extensive investigations have been and are being carried out on a stiff clay from an underground research laboratory located at Mol (Belgium) called Boom clay, in the context of research into deep nuclear waste disposal. Suction effects in deep Boom clay block samples were investigated through the characterisation of the water retention and of the swelling properties of the clay. The data obtained allowed an estimation of the sample initial suction that was reasonably compatible with the in-situ state of stress at a depth of 223 m. The relationship between suction and stress changes during loading and unloading sequences were also examined by running oedometer tests with suction measurements. A rather wide range of the ratio s/sigma 'v (being s the suction and sigma 'v the effective vertical stress) was obtained (0.61 - 1), different from that proposed by Bishop et al; (1974). Finally, the effect of suction release under an isotropic stress close to the estimated sample suction was investigated. A slight swelling (1.7%) was observed and further compression provided a satisfactory value of the overconsolidation ratio confirming the suggestions of taking some precautions before putting a swelling sample in contact with water as suggested by Graham et al. (1987). The various experimental data gathered in this study finally evidenced a relatively good state of conservation of the block sample used.

Lime treatment is a common technique of improving the workability and geotechnical properties of soils. In this study, the aggregate size effects on the water retention capacity and microstructure of lime-treated soil were investigated. Two soil powders with different maximum aggregate sizes (Dmax = 0·4 and 5 mm) were prepared and stabilised by 2% lime (by weight of dry soil). Soil samples were prepared by compaction at dry side of optimum water content (w = 17%) with a dry density of 1·65 Mg/m3. Suction and pore size distribution were determined after different curing periods. The results obtained show that: (a) the treated soil with smaller Dmax presents relatively smaller modal sizes and lower frequency of macropores (10–330 μm); (b) lime addition effectively improves the soil water retention capacity and decreases both the modal sizes of macro- and micropores gradually over time. Moreover, a higher air entry value and larger water retention capacity were also observed for a smaller Dmax value, in agreement with the pore size distributions.

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.

Problems related to unsaturated soils are frequently encountered in geotechnical or environmental engineering works. In most cases, for simplicity, one can study the problems by considering the suction effects on volume change or shear strength under isothermal conditions. Often, under these conditions, a temperature-independent water retention curve is considered in the analysis — obviously a simplification. When the temperature changes are too significant to be ignored, it is necessary to account for the thermal effects. This note presents a method for controlling suction with the vapour equilibrium technique at different temperatures. First, calibration of various saturated saline solutions was carried out at temperatures ranging from 20 °C to 60 °C. A mirror psychrometer was used to measure relative humidity generated by saturated saline solutions at different temperatures. The results obtained were in good agreement with the data from the literature. This information was then used to determine the water retention properties of MX80 clay, which showed that the retention curve shifts downward with increasing temperature.Key words: vapour equilibrium technique, hygrometer, water retention curve, temperature effects, compacted bentonite.

International audience; Boom Clay has been considered as a potential host-rock for the geological radioactive waste disposal in Belgium. In this context, it is important to well understand the thermo-hydro-mechanical behaviour of this clay around the disposal galleries. In this study, the effect of excavation damage on the thermo-hydro-mechanical properties of natural Boom Clay around the Connecting gallery (excavated in 2002) in the Mol underground Research Laboratory HADES (High-Activity Disposal Experimental Site) was investigated. Several samples taken from a horizontal borehole drilled in July 2012 were tested. The thermal conductivity in three different orientations (perpendicular, parallel, and 45° to the bedding plane) was measured using the needle probe method. The results show a cross-anisotropy of natural Boom Clay and an impact of the excavation damage on the thermal property of samples near the gallery. To further investigate the anisotropy behaviour, bender element tests were carried out under unconfined conditions to determine the small-strain shear modulus also in three different orientations. The obtained results confirm the anisotropic behaviour of Boom Clay. Moreover, the evolution of small-strain shear modulus with the distance from the gallery axis (r) was found to be similar to that of thermal conductivity: the values in the zone near the gallery are lower than those in the far field. From these experimental data, an extent of the excavation damaged zone (EDZ) of 4 m from the connecting gallery axis was determined. Further investigations on the microstructure of several samples taken at different distances r by mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) methods were carried out. Macro-pores of diameter ≥5 μm were identified in the samples near the gallery. The identified macro-pores were related to the effect of excavation damage, and a damage variable was thus defined, allowing a damage model to be developed. The values of the two model parameters have been determined from the observed relationship between macro-porosity and thermal conductivity. Comparisons between the predicted and experimental results in terms of small strain shear modulus and hydraulic conductivity show a reasonable agreement.

International audience; An experimental characterisation on Ypresian clays that are considered as one of the potential geological host formations for the radioactive waste disposal in Belgium has been carried out. Physical, microstructural, hydraulic and mechanical properties were investigated on the samples taken from a cored borehole at Kallo in the Northern Belgium at 290-400 m depth. Identification tests, scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and oedometer tests were performed. The results obtained showed that Ypresian clays are highly plastic (large values of consistence limits, specific surface, blue ethylene value) and of relatively low density, in relation to their high swelling clay content. In addition, their physical and mineralogical properties present significant variations over depth. Two microstructural arrangements, matrix-type and aggregate-type respectively for the depths with higher and lower swelling content, were identified. The former exhibits bi-modal porosity while the latter shows a common deep sediment mono-modal porosity but with a larger dominant pore size. The permeability of the clays with matrix-type microstructure is significantly lower than that with aggregate-type despite their higher void ratio. Moreover, these microstructural features result in the initial unsaturated state of Ypresian clays sample. Typical hysteretic unloading-reloading loops are identified on both low- and high-pressure oedometer tests that can be explained by a microstructure-based swelling stress concept. The variation of this swelling stress with the corresponding initial void ratio over depth perfectly reflects the physical variations and is consistent with that of swelling stress determined from common swelling tests. The yield stress ratios are slightly higher than the over-consolidation ratio, suggesting negligible effects of diagenesis, cambering and valley bulging etc. The variations of compression index, compression and swelling slopes are consistent with the physical and microstructural variations.

International audience; This paper presents a kinematic hardening model for describing some important features of natural stiff clays under cyclic loading conditions, such as closed hysteretic loops, smooth transition from the elastic behavior to the elastoplastic one and changes of the compression slope with loading/unloading loops. The model includes two yield surfaces, an inner surface and a bounding surface. A non-associated flow rule and a kinematic hardening law are proposed for the inner surface. The adopted hardening law enables the plastic modulus to vary smoothly when the kinematic yield surface approaches the bounding surface and ensures at the same time the non-intersection of the two yield surfaces. Furthermore, the first loading, unloading, and reloading stages are treated differently by applying distinct hardening parameters. The main feature of the model is that its constitutive equations can be simply formulated based on the consistency condition for the inner yield surface based on the proposed kinematic hardening law; thereby, this model can be easily implemented in a finite element code using a classic stress integration scheme as for the modified Cam Clay model. The simulation results on the Boom Clay, natural stiff clay, have revealed the relevance of the model: a good agreement has been obtained between simulations and the experimental results from the tests with different stress paths under cyclic loading conditions. In particular, the model can satisfactorily describe the complex case of oedometric conditions where the deviator stress is positive upon loading (compression) but can become negative upon unloading (extension).

International audience; As opposed to the new ballasted railway tracks where a sub-ballast layer is often emplaced, the conventional railway tracks were constructed with ballast directly emplaced on the natural sub-soils. Thereby, a layer of mixed materials, namely interlayer, was formed over time mainly by interpenetration between ballast and sub-soils. As this layer plays an important role in transmitting load to the sub-soils, its behaviour under the effects of dynamic loading and climate changes is of primary importance for the stability of tracks. In order to understand the behaviour of the material in such interlayers, field monitoring was performed at a selected site in France, namely Moulin Blanc. Firstly, site investigation was done by borehole, allowing the interlayer location to be identified. Secondly, the interlayer was instrumented with suction probes, temperature sensors and accelerometers at different depths. Two piezometers were also installed for water table monitoring, and a weather station was installed for air data monitoring. The data recorded allowed the suction changes with water evaporation and the behaviour of tracks under the effect of temperature to be analysed. Moreover, the recorded data of acceleration allowed assessing the double-integration method for different kinds of train. It was observed that the site is out of the hazards related to freeze/thaw and the double-integration method can be applied to determine the particle velocity and displacement provided that appropriate filters are used. Note however that further study is needed to confirm this point with comparison between the measured particle displacement and the calculated one.

The mechanical behaviour of Boom Clay has been studied for many years in the context of geological disposal of radioactive waste in Belgium. The aim of this study is to investigate the anisotropic behaviour of Boom Clay in terms of compressibility and hydraulic conductivity. Oedometer tests (with effective vertical stress (\( \sigma_{v}^{{\prime }} \)) up to 32 MPa) were carried out on samples of various orientations: parallel, perpendicular and inclined 45° to the bedding plane. The compressibility index (C c ) and swelling index (C s ) were compared. Only a slight difference between these parameters was observed, suggesting that the anisotropic behaviour of Boom Clay cannot be revealed under the test conditions adopted. The hydraulic conductivity (k) was also determined by the Casagrande’s method for different values of vertical effective stress (\( \sigma_{v}^{{\prime }} \)). Unlike compressibility, the hydraulic conductivity, however, showed a clear anisotropic behaviour with k ver < k inc < k hor .

A new constant volume cell was developed, allowing the measurement of swelling pressure without any strain adjustment and any effect of the stiffness of the testing device. By employing this new cell, the swelling behavior of compacted soil specimen under different suctions (57, 38, 9 and 0 MPa) was investigated. The results show good repeatability, indicating the reliability of the new cell and validating the test procedures adopted. Moreover, the developed cell is quite convenient to study the long-term swelling behavior of soil since no load adjustment is necessary. The obtained results show that, during the progressive wetting by applying successively the suctions of 57, 39, 9 and 0 MPa, the swelling pressure increases to 0.17, 0.31, 0.46 and 0.89 MPa, respectively. The swelling pressure and the time required to reach equilibrium are function of suction. Vapour-wetting and water-wetting show different hydration mechanisms and result in different swelling behavior: the swelling pressure develops slowly and gradually to finally reach stabilization upon vapour-wetting, while it increases quickly to a peak value and followed by a small decrease upon water-wetting.

International audience; A possible option for the disposal of high level long lived radioactive waste is to store the waste in galleries excavated at great depth in clays or claystones. In such conditions, the walls of the galleries will be successively subjected to desaturation induced by ventilation followed by resaturation once the galleries are closed. Partial saturation also affects laboratory samples, due to the added effects of coring, transportation, storage and trimming. In this study, a series of tests was conducted to investigate the water retention properties of the Callovo-Oxfordian claystone (in which the ANDRA Underground research laboratory in France has been excavated) under drying/wetting cycles so as to provide a complete description of the changes in water content, volume and degree of saturation from the as-provided state. The results show that water content changes between 3% (at 150 MPa of suction) and 12% at zero suction, with a shrinkage of 1.4% (at 150 MPa of suction) and a 7% swelling at zero suction. The data obtained complete existing data by defining the initial state with respect to the main drying and wetting paths and by providing volume changes along the suction cycles. Hysteresis effects are also evidenced. Irreversible volume changes are assumed to be due to micro-crack generation and incomplete recovery. The changes in degree of saturation with respect to water content appeared to be reversible, an important fact to consider when simulating volume changes in COx.

International audience; Boom Clay is considered to be a potential host formation for a geological repository of radioactive waste in Belgium. In this context, a thorough understanding of its mechanical behaviour is thus important. In this study, the anisotropy of the shear modulus of natural Boom Clay samples was investigated using the bender element method. Seven samples located at different distances from the connecting gallery of the HADES underground research laboratory were tested for shearing in three different directions with respect to the bedding plane. The results obtained confirm the mechanical anisotropy of Boom Clay. Furthermore, the values of small-strain shear modulus in the near field of the gallery were found to be lower than those in the far field, suggesting that the mechanical properties of Boom Clay remain affected by gallery excavation damage that occurred more than 10 years ago.

International audience; This paper presents the driving factors for the interlayer creation and mud pumping phenomena in railway sub-structure. Physical model tests on a ballast layer overlying a sub-soil layer were carried out under different conditions in terms of water content, loading and sub-soil dry unit mass. The physical model was equipped with various sensors and devices allowing water content, pore water pressure, and axial displacement to be monitored. Visual observations were also made using a digital camera. It was observed that the ballast behavior depends on the sub-soil state. Both the interlayer creation and mud pumping are related to the migration of fine particles, and the water content of the sub-soil is the most important factor for this migration. Under the unsaturated conditions, the ballast/sub-soil interface did not change. On the contrary, under the near saturated conditions, significant migration of fine particles occurred. In case of low dry unit mass, the dissipation of high pore water pressure in the sub-soil gave rise to mud pumping. In case of higher dry unit mass, the excess pore water pressure was lower due to the smaller volume change of sub-soil. As a result, the effect of water pressure dissipation was limited and the upward migration of fine particles was only due to the penetration of ballast into the sub-soil, thereby, forming the interlayer.

International audience; There are quite few data available about the thermal behaviour of claystones while their knowledge becomes necessary to better understand the behaviour of exothermic high-level radioactive waste confining systems during the thermal phase. In this paper, some aspects of the thermomechanical response of the Callovo-Oxfordian claystone, a possible host rock for radioactive waste in France, were investigated by using a new isotropic hollow cylinder isotropic cell. The short drainage path permitted in this device (half the thickness of the hollow cylinder) allows in low permeability materials a good saturation (conducted here under stress conditions close to in-situ) in a reasonably short time. It also ensures enhanced drainage conditions and reasonable homogeneity in pore pressure during drained isotropic compression and heating tests. The thermomechanical experimental programme conducted here evidenced a plastic contraction of the claystone during drained heating under in-situ stress conditions, like in normally consolidated soils. Previous loading induced a dilating-contracting response comparable to that of overconsolidated clays. Finally, whereas the elastic response appeared to be temperature independent, the plastic compressibility investigated through a drained isothermal isotropic compression test at 80°C increased compared to that at 25°C. These first results should be confirmed by further thermomechanical investigation on claystones.

International audience; This study aims at evidencing the effects of lime treatment on the microstructure and hydraulic conductivity of a compacted expansive clay, with emphasis put on the effect of lime hydration and modification. For this purpose, evolutions of hydraulic conductivity were investigated for both lime-treated and untreated soil specimens over 7 d after full saturation of the specimens and their microstructures were observed at the end. Note that for the treated specimen, dry clay powder was mixed with quicklime prior to compaction in order to study the effect of lime hydration. It is observed that lime hydration and modification did not affect the intra-aggregate pores but increased the inter-aggregates pores size. This increase gave rise to an increase of hydraulic conductivity. More precisely, the hydraulic conductivity of lime-treated specimen increased progressively during the first 3 d of modification phase and stabilised during the next 4 d which correspond to a short period prior to the stabilisation phase. The microstructure observation showed that stabilisation reactions took place after 7 d. Under the effect of stabilisation, a decreasing hydraulic conductivity can be expected in longer time due to the formation of cementitious compounds.

International audience; The thermal conductivity of host rocks is an important parameter in the design of deep geological disposal of heat-emitting radioactive waste. Due to bedding, heat transfer in sedimentary rocks is affected by their transversally isotropic structure. In this work, an experimental program is run to measure the thermal conductivities of Boom Clay along various orientations with respect to the bedding plane by using the needle thermal probe technique. Measurements were performed on specimens obtained from cores drilled from the HADES Underground Research Laboratory (URL) at Mol, Belgium, at a depth of 223 m. The thermal conductivity values obtained are in good agreement with those previously published, confirming the thermal anisotropy of Boom Clay. Moreover, the observed changes in thermal conductivity with respect to the distance to the gallery provide further evidence on the extent of the Excavation Damaged Zone around the gallery.

International audience; To study the interlayer creation and the mud-pumping phenomena in the conventional French railway substructure, a physical model was developed with a 160-mm-thick ballast layer overlaying a 220-mm-thick artificial silt layer (mixture of crushed sand and kaolin), both layers being compacted in a transparent cylinder of a 550-mm-inner diameter. One positive pore water pressure sensor, three tensiometers, and three time domain reflectometer (TDR) sensors were installed around the ballast/silt interface allowing the evolution of pore water pressure (negative or positive) and volumetric water content to be monitored, respectively. A digital camera was installed allowing direct monitoring of different movements (ballast, ballast/subsoil interface, etc.). The effects of loading (monotonic and cyclic loadings) and degree of saturation of subsoil (w = 16 %, Sr = 55 %, and near-saturation state) were investigated. It was found that the development of pore water pressure in the subsoil is the key factor causing the migration of fine particles, hence, resulting in the creation of interlayer, as well as mud pumping. In particular, the camera exposed the pumping up level of fine particles during the test, showing that the migration of fine particles was not only a result of the interpenetration of ballast particles and subsoil, but also of the pore water pressure that pushed the fine particle upward. The quality of the recorded data showed that the physical model developed worked well and the test protocol adopted for studying the mechanisms of intermixing of ballast and subsoil and mud pumping was appropriate.

International audience; Compacted bricks of bentonite/sand mixture are planned to be used as sealing plugs in deep radioactive waste disposal repositories because of their low permeability, high swelling capacity and favourable properties with respect to radionuclide retention. The isolating capacity of compacted bentonite/sand mixture is closely related to microstructure features that have been often investigated, in particular by using scanning electron microscope (SEM or ESEM) and mercury intrusion porosimetry (MIP). In this work, microfocus X-ray computed tomography (µCT) observations were used in parallel with MIP measurements to further investigate at larger scale the microstructure of a laboratory compacted bentonite/sand disk (65/35% in mass). Qualitative observation of µCT images showed that sand grains were inter-connected with some large pores between them that were clearly identified in the bimodal pore distribution obtained from MIP measurements. Due to gravitational and to frictional effects along the specimen periphery, a higher density was observed in the centre of the specimen with bentonite grains more closely compacted together. This porosity heterogeneity was qualitatively estimated by means of image analysis that also allowed the definition of the representative elementary volume. Image analysis also provided an estimation of the large porosity in good agreement with MIP measurements.

International audience; In the ancient railway sub-structure in France, after years of operation, the inter-penetration of fine particles of sub-grade and ballast has created a new layer referred to as the interlayer. As it was naturally formed, the fines content and water content of the interlayer vary considerably. In this study, the effects of the fines and water contents on the mechanical behavior of interlayer soil were investigated by carrying out large-scale monotonic and cyclic triaxial tests. The results of the monotonic triaxial tests show that adding more fines in the interlayer soil does not significantly change the shear strength in the dry condition (water content w=4% and 6%), but drastically decreases the shear strength parameters (friction angle and cohesion) in the nearly saturated condition (w=12%). The cyclic triaxial tests were performed at various deviator stress levels. By considering the permanent axial strain at the end of application of each stress level, it was found that the higher the fines content in the nearly saturated condition (w=12%), the larger the permanent axial strain. In the case of lower water content (w=4% and 6%), the opposite trend was identified: adding fines decreases the permanent axial strain.

A large-scale environmental chamber was developed to study soil water evaporation mechanisms. A large soil specimen (300 mm high, 800 mm wide, and 1000 mm long) was used, allowing sensors to be installed with minimal effect on the soil’s hydraulic properties. Sensors for measuring soil suction, temperature, and volumetric water content were either buried inside the soil specimen or installed on the chamber’s wall at various locations. Other sensors for monitoring air temperature, relative humidity, air flow rate, and soil surface temperature were installed at different locations above the soil surface. Various atmospheric conditions were controlled by an air supply system, and a steady water table at the bottom of the soil was set through a big water tank. Fontainebleau sand was studied, and it was compacted in the chamber in layers. After saturation, an 11.5-day evaporation test was performed. The results obtained are presented in terms of evolutions of suction, volumetric water content, air relative humidity, and soil/air temperature. The data on air relative humidity and air temperature were used further for determining the actual evaporation rate; the data on soil volumetric water content and soil suction were used for determining the soil water retention curve. The quality of the results obtained speaks to the performance of the environmental chamber developed. In addition, these results can be further analyzed for theoretical and numerical developments involving soil water evaporation.

Large-scale evaporation experiments were conducted on bare sand using an environmental chamber. Four different atmospheric conditions and various drying durations were imposed on the soil sample. Atmospheric parameters (air flow rate, relative humidity, and temperature) and response of the soil (volumetric water content, temperature, and soil suction) were both monitored simultaneously. Notably, the temperature and matric suction at the soil surface were monitored using an infrared thermometer and high-capacity tensiometer, respectively. The results show that the air and soil temperatures depend on the evaporation process and atmospheric conditions. In addition, volumetric water content in the near-surface zone is strongly affected by the evaporation process and changes linearly over depth. The evaporation rate is strongly dependent on the air conditions.

Compacted bentonite-based materials are often considered as sealing/backfill materials in deep geological repository for high level radioactive waste. A good understanding of their hydration process is essential as this process is directly related to over-pack corrosion and nuclide migration. In this study, the unsaturated hydraulic properties of MX80 bentonite/sand mixture were characterized by carrying out a series of experiments including water retention test, infiltration test as well as microstructure observation. It was found that with suction decrease under constant volume condition, the hydraulic conductivity decreased followed by an increase after a suction threshold. At suctions higher than 12.6 MPa, hydration led to progressive large-pore clogging by exfoliation of clay particles. On the contrary, when saturation was approached (suction lower than 4.2 MPa), the large-pore quantity increased due to the creation of two-dimensional pores. It was also observed that the soil hydraulic conductivity changed following the same tendency as the large-pore quantity during hydration. In other words, water transfer was primarily governed by the network of large-pores.

International audience; In this study, a fouled ballast taken from the site of Sénissiat, France, was investigated. For the hydraulic behaviour, a large-scale cell was developed allowing drainage and evaporation tests to be carried out with monitoring of both suction and volumetric water content at various positions of the sample. It was observed that the hydraulic conductivity of fouled ballast is decreasing with suction increase, as for common unsaturated soils. The effect of fines content was found to be negligible. For the mechanical behaviour, both monotonic and cyclic triaxial tests were carried out using a large-scale triaxial cell. Various water contents were considered. The results were interpreted in terms of shear strength and permanent axial strain. It appeared that the water content is an important factor to be accounted for since any increase of water content or degree of saturation significantly decreases the shear strength and increases the permanent strain. Constitutive modelling has been attempted based on the experimental results. The model in its current state is capable of describing the effects of stress level, cycle number and water content.

International audience; Compacted bentonite-based materials are often used as buffer materials in radioactive waste disposal. A good understanding of their hydro-mechanical behaviour is essential to ensure the disposal safety. In this study, a mixture of MX80 bentonite and sand was characterized in the laboratory in terms of water retention property, swelling pressure, compressibility and hydraulic conductivity. The effects of the technological voids or the voids inside the soil were investigated. The technological voids are referred to as the macro-pores related to different interfaces involving the buffer material, whereas the voids inside the soil is referred to as the common macro-pores within the compacted bentonite/sand mixture. The results obtained show that at high suctions, the amount of water absorbed in the soil depends solely on suction, whereas at low suctions it depends on both suction and bentonite void ratio. There is a unique relationship between the swelling pressure and the bentonite void ratio, regardless of the sample nature (homogeneous or not) and sand fraction. However, at the same bentonite void ratio, a higher hydraulic conductivity was obtained on the samples with technological voids. The effect of sand fraction was evidenced in the mechanical yield behaviour: at the same bentonite void ratio, the bentonite-sand mixture yielded at a higher pre-consolidation stress.

International audience; To study the hydraulic behavior of fouled ballast, an infiltration column 600 mm high and 300 mm in diameter was developed. Five time domain reflectometer (TDR) sensors and five tensiometers were installed at various levels, allowing the measurement of volumetric water content and matric suction, respectively. The material studied was fouled ballast that was formed in the railway track-bed by penetration of fine-grained soil into the ballast. This material is characterized by a high contrast of size between the largest and the smallest particles. During the test, three stages were followed: saturation, drainage, and evaporation. Based on the test results, the water-retention curve and the unsaturated hydraulic conductivity were determined. The quality of the results shows the capacity of this large-scale infiltration column in studying the unsaturated hydraulic properties of such fouled ballast.

Nous presentons un critere de resistance des roches en enrichissant le critere de Drucker-Prager par un parametre supplementaire. Ce critere a une forme de surface de resistance hyperbolique dans l'espace des contraintes principales. Nous montrons qu'il resout certains defauts de criteres classiques ou largement utilises pour les roches qui sont les criteres de Mohr-Coulomb, Drucker-Prager et Hoek et Brown. Il permet de modeliser un angle de frottement decroissant avec la contrainte moyenne, un rapport de resistance de traction sur compression qui peut mieux s'ajuster aux donnees experimentales. Compare au critere de Hoek et Brown qui a ete propose pour ameliorer les memes defauts des criteres de Mohr-Coulomb et Drucker-Prager, ce critere a l'avantage de s'exprimer en fonction des invariants des contraintes et faciliter donc la mise en place de modeles numeriques. Par ailleurs, du fait de sa forme hyperbolique, il permet de definir un comportement asymptotique pour les fortes contraintes de confinement contrairement au critere parabolique de Hoek et Brown. Une comparaison de ce critere avec une serie de donnees experimentales de resistance des roches a montre qu'il permet de representer ces donnees de maniere satisfaisante et comparable a d'autres criteres.

We present an investigation into macroscopic and microscopic behaviors of wet granular soils using the discrete element method (DEM) and the X-ray Computed Tomography (XRCT) observations. The specimens are first prepared in very loose states, with frictional spherical grains in the presence of a small amount of an interstitial liquid. Experimental oedometric tests are carried out with small glass beads, while DEM simulations implement a model of spherical grains joined by menisci. Both in experiments and in simulations, loose configurations with solid fraction as low as 0.30 are prepared under low stress, and undergo a gradual collapse in compression, until the solid fraction of cohesionless bead packs (0.58 to 0.6) is obtained. In the XRCT tests, four 3D tomography images corresponding to different typical stages of the compression curve are used to characterize the microstructure.

The compression behavior of Canadian oil sands was investigated by running high-stress oedometer compression tests up to 65 MPa. An X-ray microtomography investigation confirmed the significant disturbance affecting the core samples, showing large pores located between dense intact clusters made up of angular sand grains. Compression tests confirmed the significant effects of disturbance on the com- pression characteristics, providing a much softer response without any memory of the significant load applied by the 3- to 4-km-thick ice sheet during the Quaternary period. High-stress compression allowed us to bring back the samples close to the in situ estimated porosity, providing compression characteristics (estimated fromaloading cycle)close totheintact ones,despiteobviousdifferencesinfabric (includingtheeffects attributable to grain crushing at high stress). Creep behavior was also observed, and an estimate of the order of magnitude of the creep strain corresponding to the Quaternary period is provided. DOI: 10.1061/(ASCE)GT.1943-5606.0000808. © 2013 American Society of Civil Engineers. CE Database subject headings: Oils; Compression; Sand (soil type); X rays; Canada. Author keywords: Heavy oil; Oil sand: High-stress compression; X-ray microtomography; Core disturbance.

International audience; This study focuses on the hydro-mechanical behaviour of a MX80 bentonite/sand mixture. Barcelona Expansive Model (BExM) was used to describe this behaviour. For this purpose, experimental data obtained in the laboratory were analysed, allowing determination of the parameters used in the model. These parameters were then used to simulate the hydro-mechanical (H-M) responses of the material from other tests with different stress paths. It was observed that the model reproduced well the main feature of swelling pressure development and micro-structural volume change during suction decrease. On the other hand, the simulations also showed some limitations of the model which is based on the consideration of two levels of soil structure or double structure: the pore size distribution curve determined by the technique of mercury intrusion porosimetry (MIP) in the range of low suctions (4.2 and 1 MPa) being revealed to be tri-modal, the BExM model cannot capture this aspect to correctly describe the macro-structural and micro-structural void ratio changes. Moreover, because the BExM model lumps clay intra-particle spaces and inter-particles voids together as micro-pores, it leads to overestimation of the micro-structural water content compared to the experimental value.

Boom Clay and Ypresian Clays have been considered as potential geological host formations for radioactive waste disposal in Belgium. Considering the significant differences in pore water chemical composition between several sites involving these two formations as well as the possible evolution of the chemical composition during the large lifespan of a radioactive waste disposal, it appeared important to investigate the effects of pore water chemical composition on the hydro-mechanical behaviors of these two potential host formations. In this study, these effects were investigated by carrying out specific œdometer tests. Different compositions were considered for this purpose: distilled water, synthetic site water, and Sodium Chloride solutions at concentrations of 15 and 30 g/L. Clear effects of pore water chemical composition on the hydro-mechanical behavior were observed: increasing pore water salt concentration gave rise to (i) increase of œdometric modulus E œd , permeability K and consolidation coefficient C v ; and (ii) decrease of compression slope c ′ c , swelling slope c ′ s and secondary compression coefficient C αe , which is in agreement with the diffuse double layer theory and the clay particle aggregation as identified by microstructural analyses. Furthermore, the pore water chemical composition effects were found to be mineralogy, stress state and salt concentration dependent: (i) Ypresian Clays with higher smectite content showed clearer chemical effect on the coefficient of consolidation C v ; (ii) the chemical effects on compressibility and swelling capacity parameters were found to be attenuated with increasing vertical stress; and (iii) the increase of chemical effect with increasing pore water salinity was limited to a certain salt concentration. The competition between the physico-chemical and mechanical effects was identified: the pore water chemical composition effect is clear only in the low-stress range where the hydro-mechanical behavior is dominated by the physico-chemical effect.

International audience; This paper considers the numerical implementation of an existing constitutive model that was developed to simulate the thermo-mechanical behavior of clays. The model, which combines two plastic mechanisms, has been reformulated using generalized stress and strain variables accounting for the presence of temperature as an additional variable. Mixed control conditions are introduced to simulate standard thermo-mechanical laboratory tests. An explicit integration scheme is used with an adaptive time-stepping scheme specifically designed for the considered thermo-mechanical model. This sub-stepping procedure enables the constitutive model to be integrated accurately and efficiently by automatically adjusting the size of sub-steps to maintain the local integration error below a specified tolerance. Special attention is paid to the cases where the two yield surfaces are activated simultaneously. The correction of the drift of the stress point from the yield surface is also accounted for. This thermo-elastoplastic model has been implemented in a constitutive driver aiming at simulating thermo-mechanical tests in the triaxial stress space. The efficiency of this integration algorithm is discussed and the clear advantage of using the adaptive sub-stepping procedure is shown. The constitutive model is finally used to simulate thermo-mechanical tests. The results are compared to available experimental data and show the good capabilities of the constitutive model.

International audience; In the framework of research into radioactive waste disposal, it was decided to investigate the thermally induce pore pressure occurring in the Callovo-Oxfordian claystone, a possible host rock in which the ANDRA underground laboratory of Bure (East of France) has been excavated. Thermal pore pressures appear in low permeability soils and rocks because the thermal expansion coefficient of water is significantly higher than that of the solid grains (Campanella and Mitchell; 1968 [1], Ghabezloo and Sulem; 2009 [2]). This phenomenon has clearly been observed in various in-situ heating tests conducted in Opalinus claystone in the Mont-Terri Underground Research Laboratory (URL) in Switzerland (HE-D test) and in Callovo-Oxfordian (COx) claystone in the Bure URL in France (TER test, Wileveau and Su; 2007 [3]) The processes of coring, transportation, storage and specimen trimming induce some desaturation in the sample. Due to the very low permeability (10-20 m2) of the COx claystone, a long period of time is necessary to properly resaturate the sample, a mandatory condition to satisfactorily investigate thermal pressurisation. Particular emphasis was hence put on the previous saturation procedure that was carried out under in-situ effective stress condition. Thermal pressurization has been investigated by performing undrained heating tests while measuring pore pressures changes in a specially adapted thermal isotropic compression cell. Special care was devoted to calibration procedures to account for the effects of the system on the pore pressure measurements. The thermal pressurization coefficient measured appeared to change with temperature, mainly because of the changes with temperature of both the water thermal expansion coefficient of water and the drained compression coefficient of the claystone.

International audience; The effects of temperature changes on the mechanical behaviour of heat exchanger piles are necessary to be taken into account in geotechnical design. In this preliminary study, the behaviour of an axially loaded pile under thermal cycles was investigated using a physical model. After applying the axial load by dead weights on the pile's head, the pile was heated from 25 °C to 50 °C and subsequently cooled to 25 °C. Four tests (corresponding to four values of axial load) were performed and two temperature cycles were undertaken in each test. The pile's temperature and the pile head displacement were monitored. When low axial loads were applied, the heating induced heave and cooling induced settlement of the pile's head. In this case, the displacement-temperature curve was found to be reversible and similar to the thermal expansion curve of the pile. However, in the case of higher axial loads, the heave of the pile's head, obtained during heating, was lower than the thermal expansion of the pile demonstrating the settlement of the pile's toe. Reversible settlement of the pile's head was also observed after these thermal cycles.

Boom clay formation, a deposit of slightly over-consolidated marine clay that belongs to the Oligocene series in the north east of Belgium, has been studied as a possible host material of nuclear waste disposal. In this context, the long-term deformation behavior of Boom clay is of crucial importance in the performance assessment of the whole storage system. In this study, low and high pressure oedometer tests are carried out; the e-log σ′ v (void ratio–logarithm of vertical effective stress) and e-log t (void ratio–logarithm of time) curves obtained are used to determine the compression index C c *, swelling index C s * and secondary deformation coefficient C α during both loading and unloading. The relationship between C α and the effective stress ratio (σ′ v /σ′ c , vertical effective stress to pre-consolidation stress) is analyzed, and it is observed that C α increases linearly with log σ′ v /σ′ c . Examination of the ratio of C α / C c * for various soils shows that the secondary deformation behavior of Boom clay is similar to that of shale and mudstone. The relation between C α and C c * is linear; but the relation between C α and C s * is bi-linear. The bi-linearity observed is related to two different mechanisms: the mechanically dominated rebounding and the physico-chemically dominated swelling.

Crushed and compacted argillite extracted from Bure site (France) has been proposed as a possible sealing and backfilling material in the geological high-level radioactive waste disposal. In order to better understand its coupled hydro-mechanical behaviour in the real storage situation, changes in microstructure of two crushed argillites of different grain size distributions with decreasing suction under confined condition were investigated using mercury intrusion porosimetry (MIP). The results show that wetting under constant-volume condition reduces the size of macro-pores, without significantly affecting the micro-pores size. A clear effect of grain size distribution on the microstructure was also observed: two pore families were identified for the coarse soil; however, there was only one maro-pore family for fine soil (inter-aggregates pores).

In the radioactive waste disposal concepts, highly pre-compacted bentonite-sand mixtures are considered as appropriate sealing materials to fill the galleries. Compacted elements of bentonite-sand mixtures are placed adjacent to each others, forming a plug that should limit the transfer of radionuclide. The bentonite has proven to have the capability of retaining radionuclides and the capability of swelling when getting in contact with water and then closing all the voids in the system especially the radial technological void being the gap between the bentonite-sand elements and the host rock. Indeed, when the bentonite-sand elements is emplaced in the gallery, they are first in an unsaturated state and they will start being saturated by the host-rock pore water. They will then start swelling, forming a gel that will close the gap. In this work, such hydration process is experimentally analysed on a small scale. The swelling is analysed by time-lapse photography, its kinetic is also studied by image processing. We can see that the swelling of this material is fast and large, and we can distinguish different states of the material during swelling, starting from the dry centre to the outer boundary. On the other hand, the presence and behaviour of sand grains in the gel formation is evidenced by images as well. The sand grains are found to be covered by bentonite particles and being carried with the bentonite swelling.

The hydration of unsaturated Callovo-Oxfordian (COx) claystone specimens from the 490 m deep URL of Bure in France has been investigated in high stress oedometers by conducting hydration tests on unsaturated specimens under stress values that were lower, equal and higher than the in-situ stress (resaturation of swelling clays should be done under in-situ stress to avoid any swelling that would affect their structure). Hydration tests always exhibited a swelling behaviour, even under stresses higher than in-situ. Based on other published experimental evidence, it is suggested that this swelling behaviour is due to damage, since the presence of cracks within claystones are known to enhance the swelling properties of claystones. A procedure of saturating claystone samples in the oedometer with limited swelling is then proposed.

The ancient railways in France, which was initially built by emplacing ballasts directly on the subgrade soils without a separation layer as that for new railway lines, are facing to various problems that affect the stability of trackbed and thus the proper circulation of trains. One of these problems is related to the loss of efficiency in water drainage of the track. Actually, fouled ballast has been formed progressively along the life time of the track mainly due to the infiltration of fine particles of subgrade layer into the ballast layer. In this study, the hydraulic properties of these fine particles investigated. Soil taken from a fouled ballast layer of a rail track was first sieved at 2 mm and then re-compacted to its initial in-situ dry density. The water retention curve and the unsaturated hydraulic conductivity were finally analyzed.

International audience; This paper presents an experimental study on the swelling pressure of heavily compacted crushed Callovo-Oxfordian (Cox) claystone at a dry unit mass d = 2.0 Mg/m3 using four different methods: constant-volume, swell-reload, zero-swell and adjusted constant-volume method. Results show that the swelling pressure varies in the range of 1-5 MPa and depends significantly on the test method. From the constant-volume tests, it is observed that the swelling behaviour during wetting is a function of the suction and depends on both the hydration paths and wetting conditions (e.g. vapour-wetting or liquid-wetting). The swelling pressure decreases significantly with saturation time. To identify the microstructure changes of specimens before and after wetting, mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) tests were performed. It is observed that, after wetting, the large inter-aggregate pores observed in the as-compacted specimen are no longer apparent; the whole pattern is characterized by a general swell of hydrated clay particles, rendering the soil more homogeneous. Results from MIP indicated that wetting caused a significant reduction of the entrance diameter of the dominant inter-aggregate pores from 2.1 to 0.5 m whereas intra-aggregate pores were not significantly influenced.