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International audience; In energy geostructures, the soil-structure interface is subjected to thermo-mechanical loads. In this study, a non-isothermal soil-structure interface model based on critical state theory is developed from a granular soil-structure interface constitutive model under isothermal conditions. The model is capable of capturing the effect of temperature on sand/clay-structure interfaces under constant normal load and constant normal stiffness conditions. First, the developed model was verified for sand-structure interface in isothermal conditions. Then, it was calibrated for clay-structure interface under non-isothermal conditions. On one hand, a well-defined peak shear stress for the clay-structure interface and, on the other hand, the effect of temperature on the void ratio of the clay-structure interface were captured and reproduced by the model. The importance of interface thickness determination and some differences between the interface thicknesses of clay-structure and sand-structure interfaces are discussed in detail. The additional parameters have physical meanings and can be determined from laboratory tests. The modeling predictions are in good agreement with experimental results, and the main trends are properly reproduced.

Mechanical behaviour of the soil–structure interface plays a major role in the shear characteristics and bearing capacity of foundations. In thermoactive structures, due to nonisothermal conditions, the interface behaviour becomes more complex. The objective of this study is to investigate the effects of temperature variations on the mechanical behaviour of soils and the soil–structure interface. Constant normal load (CNL) and constant normal stiffness (CNS) tests were performed on the soil and soil–structure interface in a direct shear device at temperatures of 5, 22, and 60 °C. Fontainebleau sand and kaolin clay were used as proxies for sandy and clayey soils. The sandy soil was prepared in a dense state and the clayey soil was prepared in a normally consolidated state. Results show that the applied thermal variations have a negligible effect on the shear strength of the sand and sand–structure interface under CNL and CNS conditions, and the soil and soil–structure interface behaviour could be considered thermally independent. In clay samples, an increase in the temperature increased the cohesion and consequently the shear strength, due to thermal contraction during heating. The temperature rise had less impact on the shear strength in the case of the clay–structure interface than in the clay samples. The adhesion of the clay–structure interface is less than the cohesion of the clay samples.

A well-known backfill soil was considered to be used as the backfill substitutive material. The hydrothermal properties of the backfill material were estimated in laboratory and then injected in a numerical framework considering the atmosphere-soil-HGHE interaction. Numerical simulations were performed for a HGHE installed in the compacted backfill soil and the local materials. Two heat storage scenarios at three different installation depths were also investigated. The results show that an inlet fluid temperature of 50°C in summer increases highly the system performance (13.7% to 41.4%) while the improvement is less significant (0% to 4.8%) for the ambient inlet temperature scenario. A deeper installation depth increases also the system performance.

International audience

The good capacity of the numerical simulations makes possible to bring some further responses on the backfill soil selection and its installation depth in the Horizontal Ground Heat Exchanger (HGHE). Therefore, a well-known backfill soil was considered to be used as substitutive material. The hydrothermal properties of the backfill material were estimated in laboratory and then injected in a numerical framework considering the atmosphere-soil-HGHE interaction. Numerical simulations were performed for a HGHE installed in the compacted backfill soil and the local natural soil. The simulation results showed that the compacted backfill soil improves by 8.5 % the HGHE performance compared to local uncompacted soil. Two heat storage scenarios at three different installation depths were also investigated. The results showed that an inlet fluid temperature of 50 °C in summer increased highly the system performance by 13.7 % –41.4 % , while the improvement was less significant (0 % –4.8 % ) for the ambient inlet temperature scenario. A deeper installation depth of HGHE increased also the system performance, the more energy could be stored and extracted.

The compacted clay barrier of shallow depth repositories for wastes would be subjected to temperature variations. Consequently, the hydro-mechanical properties of compacted clays could be progressively modified, and thus affect the performance of repositories. The influence of effective stress and temperature on the creep behavior of a saturated compacted clayey soil was experimentally investigated by performing a series of incremental loading creep tests using a temperature-controlled oedometer. Applied effective vertical stress varied from 10 to 1300 kPa within a large temperature range of 5 ° C to 70 ° C. The results showed that the compression and swelling indices appear not to be affected by temperature, whereas the yield stress decreases as the temperature increases. The secondary compression is time-dependent; creep strains decrease with time till reaching a stable value corresponding to a period of 10 days. The creep coefficient C α e increases with the increase of the effective stress and temperature. Moreover, relationships between the creep coefficient C α e , incremental compression index C c ∗ , effective stress and temperature were further analyzed. A linear relationship between C α e and C c ∗ was observed in the considered stress range and the (C α e /C c ∗ ) ratio appears to be temperature dependent. Finally, the main results were discussed and interpreted in the light of a suitable constitutive framework.

International audience; In the context of the French Cigéo-project, a mixture composed of 70% processed Callovo-Oxfordian claystone spoil and 30% MX80-bentonite could be a potential backfill material, whose installation aims to stabilize the surrounding rock formation and to limit the propagation of the excavation damaged zone. The backfill material must sustain the overburden pressure, despite that it might be exposed to different hydraulic and mechanical paths. The reference concept considers employing conventional compaction techniques, although their employment involves spatial variations in the dry density after compaction. In general, as the initial dry density has a significant impact on the hydro-mechanical behavior of backfill materials, it is of major importance to relate the variations in the initial dry density to differences in the behavior. This experimental laboratory study aimed to analyze how variations in the initial dry density affect the swelling and compression behavior of the claystone/bentonite mixture, in particular in unsaturated state. Further, it evaluated whether those variations affected possible hydro-mechanical path dependences. The experimental program comprised suction-controlled oedometer and constant-volume swelling pressure experiments, in which samples characterized by different initial dry densities were exposed to different hydro-mechanical paths. The analysis of microstructural and water retention characteristics complemented the program. Major results indicated that the magnitude of swelling pressure at a given suction depends considerably on the initial dry density, but it is independent of the imposed hydro-mechanical path. Interestingly, the dependency of the yield behavior on the hydro-mechanical path appears to be more pronounced as the initial dry density increases.

International audience; Recent technological advances in the field of heat pumps made it possible to propose new solutions for heating and cooling buildings. Energy geostructures consist in incorporating heat exchangers into buried elements of geotechnical structures. However, the heat exchange leads to a cyclic evolution of the adjacent ground temperature. Thus, many questions arise about the effect of these temperature variations on the hydromechanical parameters of the soil. These issues are important since energy geostructures combine the function of heat exchanger with the bearing or/and retaining functions. In this study, mini-pressuremeter tests were conducted in laboratory on a homogeneous material submitted to different thermal loadings (in the range of 1 to 40 °C). The tested material, an illitic soil, is compacted at its Proctor optimal water content 31.3% and to 90% of its maximal dry density (1.29 Mg/m 3 ) in a 0.6 m diameter and 0.8 m height thermo-regulated container. Six tests are performed in each container in different steps of temperature variations. A decrease in creep pressure and limit pressure with increasing temperature was observed. The first cycle induced more important parameter variations than the subsequent cycles especially for the creep pressure that tends towards a constant value. The limit pressure however, remains dependent to temperature variations beyond the first cycle.; Les récentes avancées technologiques dans le domaine des pompes à chaleurs ont permis de proposer des solutions nouvelles pour le chauffage et le refroidissement des ouvrages. Les géostructures énergétiques consistent à incorporer des échangeurs thermiques dans les éléments enterrés des ouvrages géotechniques. Cependant, l’échange de chaleur conduit à une évolution cyclique de la température du sol adjacent. Ainsi, de nombreuses questions se posent sur l’effet de ces variations de température sur les paramètres hydromécaniques des sols. Ces questions sont importantes puisque les géostructures énergétiques cumulent la fonction d’échangeur thermique et la fonction de portance ou de soutènement. Dans cette étude, quatre massifs de sol ont été compactés dans une cuve de 0,6 m de diamètre et 0,8 m de haut qui est thermo-régulée (1 à 40 °C). Le matériau testé est une argile (illite) compactée à sa teneur en eau optimale soit 31,3 % (essai Proctor normal), et à 90 % de sa masse volumique sèche maximale, soit 1,29 Mg/m 3 . Six essais mini-pressiométriques ont été réalisés dans chaque massif à différentes étapes des sollicitations thermiques appliquées. Les résultats montrent une diminution de la pression limite avec l’augmentation de la température. L’application de plusieurs cycles montre que le 1 er cycle a un impact prépondérant par rapport aux cycles suivants en particulier, pour la pression de fluage qui tend vers une valeur d’équilibre. En revanche, la pression limite conserve sa dépendance à la température au-delà du 1 er cycle.

This laboratory experimental program investigated the impact of variations in the expansive mineral content, the grain size distribution of employed bentonite, the initial dry density and the selected hydration path on the water retention characteristics and swelling properties of processed Callovo-Oxfordian claystone and its mixtures with MX80 bentonite. The French reference concept for the disposal of nuclear waste in deep sedimentary rock formations envisages the reemployment of excavated material as backfill material, which is installed in situ by means of conventional compaction techniques. The investigations were of special interest as the major issues involving in situ compacted backfill materials were portrayed. Experiments showed that the impact of variations in the dry density on the water retention characteristics of all materials vanished as suctions exceeded 100 MPa. The negligible impact of variations in the initial dry density on the collapse behavior of claystone/ bentonite mixtures remained questionable.

The positive effects of lime or cement treatment could be altered by weathering in the very long term. In this context, the main purpose of this study is to examine the impact of wetting/drying cycles on the strength and the hydraulic conductivity of a compacted soil treated with lime and cement. Compacted specimens were cured for 90 days before being exposed up to twelve wetting and drying cycles. A special concern of the study was the experimental method to impose the wetting and drying cycles. Two protocols were employed: one relied on relative humidity control to dry the samples, while the other was based on oven drying. The impact of the cycles was quantified by comparing the performance of the samples exposed to the cycles to the performance of the unsolicited samples. The results showed that the cycles induced a major alteration of the strength of the samples, with both methods. This degradation is associated to a significant increase of the hydraulic conductivity of the samples with the number of cycles.

L’incorporation d’échangeurs de chaleur aux géostructures pose la question de l'impact des variations de température sur les paramètres géotechniques des sols. Dans cette étude, des essais mini-pressiométriques ont été réalisés sur des massifs de sol compactés dans un modèle réduit thermo-régulé. Un matériau limoneux compacté est soumis à trois cycles thermiques (20-50-20°C). Une diminution nette de la pression limite (Pl) et la pression de fluage (Pf) est mesurée tandis que l’évolution du module pressiométrique (EM) est beaucoup plus faible. Dans cet interval de température, l’effet de trois cycles de chauffage peut être considéré comme réversible., Academic Journal of Civil Engineering, Vol 35 No 1 (2017): Special Issue - RUGC 2017 Nantes

The incorporation of heat exchangers into geostructures leads to changes in the temperature of the adjacent soil, which may affect its hydro-mechanical properties. In this study, mini-pressiometer tests were carried out in the vicinity of three experimental energy piles of 12 meters length and 0.52-meter diameter installed in saturated sandy soil. Tests were carried out in three locations and in two different depths (namely 3 and 4 meters in depth) before and after cyclic variations of their temperature. The pressuremeter parameters are the pressuremeter modulus EM, the limit pressure PL and the creep-pressure Pf. These parameters characterize the properties of the soils; some measurements were done close to the energy piles (1.25 meters from the center of the pile) using a mini-pressuremeter cell (380 mm in height and 28 mm in diameter). The comparison of the results before and after the four warming-cooling cycles (8° to 19° C) showed a thin thickening of the material at 3 meters depth. These results are coherent with in-lab measurements and with the results of the pile loading tests carried out later on the same site.

Thermal energy storage in embankments can be considered a new economically efficient and environmentally friendly technology in geotechnical engineering. In these structures, horizontal heat exchanger loops can be installed inside different layers of compacted soil to store heat in the medium during the summer to be extracted during the winter. Compacted soils are usually unsaturated; therefore, reliable estimates and measurements of unsaturated compacted soil thermal properties, such as the volumetric heat capacity, thermal conductivity, and thermal diffusivity, are important in the efficiency analysis of these structures. However, there is no available method to characterize the evolution of these parameters over time in compacted soil. In this study, several temperature sensors were placed inside different layers of unsaturated compacted soil in a cylindrical container (height of 0.8 m and diameter of 0.6 m ) to monitor imposed temperature cycle variations. An inverse analytical model based on the one-dimensional radial heat conduction equation is proposed to estimate the thermal diffusivity using the temperature variation between two temperature sensors. The volumetric heat capacity was measured with a calorimeter in the laboratory, enabling estimation of the thermal conductivity of the compacted soil. Then, this estimated thermal conductivity was compared with the thermal conductivity values measured with two different methods (one steady-state and one transient-state method). The estimated thermal conductivity was close to the value measured with the transient-state method. It was demonstrated that steady-state methods are not suitable for the measurement of thermal conductivities as high as 2.5 W . m - 1 . K - 1 since thermal contact resistances are no longer negligible.

International audience; The determination of the shear strength of coarse-grained soils in standard shear devices is a challenging task due to the presence of large grains. Three approaches could be used to reduce the grain size of the tested specimen. This study aimed to determine whether the shear strength of a coarse-grained soil with fines (particles smaller than 0.08 mm) can be estimated by the parallel gradation approach. Direct shear tests were conducted on parallel graded specimens of two soil types with different low plastic fines content. Two criteria can be used to select the dry density of the reconstituted specimen: the same dry density or the same relative density as the initial soil. The results showed that when the specimens had a low fines content, both criteria resulted in a similar dry density of the reconstituted soil, and the shear strength of the initial soil was correctly estimated. However, for the specimens with a high fines content, the friction angle of the initial soil was correctly estimated, and its cohesion was overestimated when the reconstituted soil was tested at the same dry density as the initial soil. A cohesion-fines content relationship was proposed to predict the initial soil cohesion.

The shaft capacity of foundations highly depends on the monotonic and cyclic loads applied to the soil–structure interface. In energy geostructures that exploit the heat of soil using earth...

This study intends to examine the impact of successive wetting and drying cycles on the strength and hydraulic conductivity of lime- and cement-treated soil, with a special emphasis on the ...

Settlement and slope stability of an embankment are mechanical aspects that play a major role in structural safety. Due to the tendency towards renewable energy, the embankments can be considered as a suitable medium for thermal energy storage. Using horizontal heat exchanger tubes, the thermal energy will be stored in different compacted soil layers in an embankment. Therefore, this structure can be subjected to daily and seasonally temperature variations due to heat extraction. Cyclic temperature variation can modify the mechanical behavior of compacted soils. Thus this study aims to investigate the effect of monotonic and temperature cycles in the range of 5 to 50 oC, on consolidation parameters and shear characteristics of a compacted sandy lean clay. To achieve this, temperature-controlled oedometric and direct shear tests were performed. Results showed that the effect of heating and cooling on mechanical properties is more pronounced under vertical stresses higher than the preconsolidation pressure. By heating, the normal consolidation line shifted to the left and consequently, the apparent preconsolidation decreased. Compression and swelling indexes could be considered independent of temperature variation. Due to the temperature cycles, the volumetric response of the compacted soil in oedometric and shear tests was stress history-dependent. Results of the direct shear tests showed that by temperature variation (heating/cooling and temperature cycles) the cohesion increased and the friction angle remained unchanged.

International audience; Soil thermal properties are needed in specific engineering applications, such as the design of high-level radioactive waste disposal, buried power transmission cables and geothermal systems. The propagation of the heat flow in a soil exposed to a temperature gradient is based on three main soil thermal properties: the thermal conductivity (λ), the thermal capacity (C) and the thermal diffusivity (α = λ/C). The temperature variations affect the physical properties of soil, inducing changes in its thermal properties. In this study, the evolution of thermal properties of different compacted soils exposed to cyclic temperature variations were investigated through laboratory tests. The results showed that at 20 °C, the thermal conductivity of the compacted soils increased on the dry side of the compaction curve until it reached a maximum near the optimum water content. Variation of λ in a temperature range of 1–20 °C was not shown, but a slight increase of λ was measured in the range of 20–40 °C, which was clearly confirmed in the range of 40–70 °C. The application of cyclic temperature variations to the samples showed partially reversible evolution of thermal conductivity and totally reversible evolution of volumetric heat capacity.

The incorporation of heat exchangers in geostructures changes the temperature of the adjacent soil, raising important issues concerning the effect of temperature variations on hydro-mechanical soil behaviour. The objective of this paper is to improve the understanding and quantification of the impact of temperature variation on the bearing capacity of thermo-active piles. Currently, the design of deep foundations is based on the results of in situ penetrometer or pressuremeter tests. However, there are no published data on the effect of temperature on in situ soil parameters, preventing the specific assessment of the behaviour of thermo-active piles. In this study, an experimental device is developed to perform mini-pressuremeter tests under controlled laboratory conditions. Mini-pressuremeter tests are performed on an illitic soil in a thermo-regulated metre-scale container subjected to temperatures from 1 to 40 °C. The results reveal a slight decrease in the pressuremeter modulus (E p) and a significant decrease in the creep pressure (p f) and limit pressure (p l) with increasing temperature. The results also reveal the reversibility of this effect during a heating–cooling cycle throughout the investigated temperature range, whereas the effect of a cooling–heating cycle was only partially reversible. In the case of several thermal cycles, the effect of the first cycle on the soil parameters is decisive.

International audience; The determination of the mechanical properties of soils containing particles larger than the allowable size of standard laboratory equipments is complex. It is indeed necessary to remove the coarsest fraction to carry out the tests. This scalping poses a problem of reliability of the results at the scale of the structure. Parallel gradation is the method commonly used for estimating the shear strength of heterogeneous granular soils from tests on their finer fraction. However, the effect of high fines content on the estimation of shear strength by this method is not well understood. The results of this study showed that the parallel gradation method could predict the friction angle of the initial soil with high fines content when the modelled soil had a similar skeleton as the initial soil. However, the cohesion of the initial soil was overestimated.

Mixtures composed of 70% crushed Callovo-Oxfordian claystone and 30% MX80-bentonite are considered as materials, that could be used for backfilling a future radioactive waste repository in deep sedimentary rock formations. Their characterization is of interest, as the replacement of fractions of crushed claystone by bentonite enhances the chemo-hydro-mechanical performance of backfill. The materials are envisaged to be installed directly in the drifts and shafts by means of conventional compaction techniques. The hydro-mechanical behavior of materials containing expansive mineral phases, and especially their swelling behavior, is known to be significantly affected by the initial material properties and environmental and stress conditions. The present study aimed to assess the combined impact of variations in the material properties and environmental conditions, particularly the grain size distribution, dry density and saturating solution chemistry, on the swelling pressure of the mixtures, by conducting a comprehensive laboratory experimental program. The results revealed that the adjustment of the grain size distribution of employed bentonite enhanced the compaction behavior and, in turn, the swelling behavior of the mixtures. Generally, swelling pressures of mixtures were less affected by the employed saline and alkaline solutions than those of crushed claystone. The measured swelling pressures were exponentially related to the initial dry density of the expansive mineral phase, regardless of the grain size distribution. Based upon the finding that the expansive mineral phase being present in crushed claystone contributed to measured swelling pressures, a new approach was introduced to calculate the dry density of the expansive mineral phase in bentonites and their mixtures with non-expansive or less-expansive materials.

International audience; The mechanical behaviour of the soil-structure interface plays a major role in the shear characteristics and bearing capacity of foundations. In thermo-active structures, due to non-isothermal conditions, the interface behaviour becomes more complex. The objective of this study is to investigate the effects of temperature variations on the mechanical behaviour of soils and soil-structure interface. Constant normal load (CNL) and constant normal stiffness (CNS) tests were performed on soil and soil-structure interface in a direct shear device at temperatures of 5, 22 and 60 o C. Kaolin clay was used as proxy for clayey soils. The results showed that, in clay samples the temperature increase, increased the cohesion and consequently the shear strength, due to thermal contraction during heating. The temperature rise had less impact on the shear strength in the case of the clay-structure interface than in the clay samples. The adhesion of the clay-structure interface, is less than the cohesion of the clay samples.

In energy geostructures, which exploit the heat in soil using earth contact elements, the interface is subjected to cyclic thermo-mechanical loads. Monotonic and cyclic constant-volume equivalent-undrained (CVEU) direct shear tests were performed on clay-clay and clay-structure interface at different temperatures (22 and 60 °C). Different cyclic and average stress ratios (CSR and ASR) were applied to the kaolin clay-structure interface under 300 kPa of normal stress. The results showed that, the number of cycles to failure for the clay-structure interface test was lower than that for the clay-clay case in the same range of cyclic and average shear stress ratios. In cyclic clay-structure tests, decreasing the cyclic stress ratio, increased the number of cycles to failure; however, decreasing the average shear stress ratio decreased the number of cycles to failure. Increasing the temperature, decreased the rate of strain accumulation and the number of cycles to failure increased by 2-3 times. The rate of degradation (degradation parameter, t) decreased by 16% with heating from 22 to 60 °C for the different cyclic stress ratios tested.

International audience; Thermal energy storage in compacted soils can be considered as a new economically efficient and environmentally friendly technology in geotechnical engineering. Compacted soils are usually unsaturated; therefore, reliable estimates and measurements of their thermal properties are important in the efficiency analysis of these structures. In this study, a method is used to estimate the thermal properties of an unsaturated compacted soil. Several temperature sensors were placed in a thermo-regulated metric scale container to monitor the imposed temperature variation in the range of the 20 to 50 °C. This imposed temperature variation reproduced the temperature variation in the thermal energy storages. An inverse analytical model based on a one-dimensional radial heat conduction equation is used to estimate the thermal diffusivity using the temperature variation between two temperature sensors. The volumetric heat capacity was measured using a calorimeter in the laboratory, enabling the estimation of the thermal conductivity of the compacted soil. Then, this estimated thermal conductivity was compared with the thermal conductivity values measured with two other methods (steady-state and transient-state method). The difference between them are discussed in terms of the sample heterogeneity, sample size, and measurement method.

International audience

L’objectif principal de ce travail est d’étudier l’impact des traitements au ciment, à la chaux ou avec des argiles, des sols compactés sur leurs caractéristiques d’érosion interne. Un nouveau dispositif d’étude de l’érosion interne modifié et amélioré a été conçu pour quantifier les effets de différents traitements sur l’évolution de la résistance à l’érosion interne du limon de Saint-Quentin compacté. Les résultats montrent que l’impact des différents traitements sur les caractéristiques d’érosion de ce limon se traduit principalement par une augmentation de la contrainte critique. Cette augmentation est plus grande avec l’augmentation du dosage, et dans le cas du traitement au ciment. De plus, pour les traitements avec la chaux et le ciment, la contrainte critique tend à augmenter avec le temps de cure. Par ailleurs, le coefficient d’érosion est affecté uniquement par le traitement à la bentonite dans le cas d’utilisation d’un grand dosage de traitement.

Clayey soils could be exposed to thermal cycles in some engineering applications such as geothermal piles and nuclear waste storages. These temperature changes could affect the consolidation behaviour of these soils. In this context, this paper investigates the impact of temperature on the consolidation behaviour of saturated compacted clays. A temperature-controlled oedometric cell has been developed to perform constant rate of strain (CRS) consolidation tests for different strain rates (0.002 – 0.02%/min) within a temperature range of 5 °C – 70 °C. Two different compacted saturated clays were used in these tests. The results indicated that the compression and swelling indices for both materials changed with temperature and strain rate. The preconsolidation pressure of both clays decreased as the temperature increased, while it decreased as the strain rate decreased. The creep index increased as the temperature increased for both clays. These variations were completely dependent on the nature of ...

International audience; The impact of temperature on the creep deformation of compacted clayey soils is a major issue in the prediction of thevery long term settlement of soils used as engineered barriers for the disposal of high and long-life radioactive wastes at great depth.This paper is focused on the creep behavior of two compacted materials, illite and a swelling clayey soil, as a function of temperatureand suction. Specific temperature controlled oedometric cells have been developed to perform constant rate of strain (CRS)consolidation tests under different strain rates (0.002%/min to 0.02%/min) within a temperature range of 5 to 70° C. Results indicatedthat the compression and swelling indices could be considered independent of temperature and strain rate with both materials. Withtemperature increase, the preconsolidation pressure decreased, and creep index increased. This can be related to a soil softeningbehavior under high temperatures.; Influence de la température sur le fluage d'une argile compactée Temperature effect on the creep behaviour of a compacted clay RÉSUMÉ : L'étude de l'influence des variations de température sur le comportement au fluage des sols compactés argileux est indispensable pour la prédiction des tassements et des déformations des dispositifs d'enfouissement des déchets radioactifs à très long terme. Ce travail est axé sur la compressibilité et le fluage de deux sols compactés : une illite et un sol argileux gonflant. L'impact de la température et de la succion sur les caractéristiques de compactage a été évalué. Cet article présente les résultats d'une campagne d'essais de compression à vitesse de déformation constant (CRS) effectuée dans des cellules oedométriques à température contrôlée. Différentes valeurs de vitesse de déformation allant de 0,002%/min à 0,02%/min ont été appliquées, dans une gamme de température comprise entre 5 et 70°C. Les résultats ont montré que les indices de compression et de gonflement sont peu sensibles à une variation de la température, cette observation étant valable pour les deux sols compactés utilisés. L'augmentation de température entraîne en revanche une baisse de la pression de préconsolidation et une augmentation significative de l'indice de fluage.

International audience; This study is focused on the hydromechanical behaviour of a compacted swelling clay upon wetting. More precisely, a multi-scale investigation of the swelling behaviour of a purified bentonite under constrained conditions was carried out by a monitoring of the swelling pressure and the porosity evolution at various scales (inter-aggregate, inter-particle, interlayer). A specific oedo-tomometer, has been designed to monitor both the swelling pressure and the inter-aggregate porosity evolution deduced from microfocus X-ray computed tomography measurements (size higher than 5 µm). The results showed the influence of hydration on specimens with an increase of the swelling pressure and a decrease of the inter-aggregate pore size during hydration. Therefore, a coupling between swelling pressure and textural reorganisation at inter-aggregate pore scale was highlighted. This coupling was also based on hydration process at interlayer scale.; L'hydratation d'une argile gonflante provoque une augmentation de sa pression de gonflement associée à une réorganisation profonde de sa microstructure à toutes les échelles. L'objectif de ce travail est de caractériser et de comprendre les processus couplés chémo-hydromécaniques multi-échelles dans les argiles soumises à la circulation d'un fluide. Un dispositif spécifique, oedo-tomomètre, a été conçu pour assurer le suivi couplé de la pression de gonflement en corrélation avec l'évolution de la porosité inter-agrégat identifiable par microtomographie de rayons X (taille > 5 µm). Les résultats ont permis d'établir une corrélation entre le développement de la pression de gonflement et la réorganisation progressive de la porosité d'une argile gonflante compactée. L'utilisation de différents fluides a permis de montrer le couplage entre l'évolution de la pression de gonflement et la réorganisation de la porosité inter-agrégats, couplage lui-même dépendant des processus d'hydratation induits à l'échelle interfoliaire (gonflement cristallin ou osmotique).

International audience; In geotechnical engineering, the proper design of thermo-active geostructures (piles, foundations, etc.) and deep waste storage disposals requires a better understanding of the thermo-hydro-mechanical behaviour of natural and compacted soils. Important mechanical parameters of the soils such as the cone resistance (qc) and the friction sleeve resistance (fs) are usually obtained by penetration in-situ tests. In the present study, penetration tests adapted from the static penetration method were conducted in laboratory, using a mini-penetrometer, to characterise compacted samples. The objective was to examine and quantify the influence of temperature changes on penetration parameters of two different compacted soils. Samples of an illitic material and the kaolinite–sand mixture compacted at different initial water contents and dry densities were subjected to a range of temperatures from 1 to 70 °C. The cone resistance (qc) and the friction sleeve resistance (fs) were measured. Obtained results showed that the particle size, the density and the nature of compacted soils have an important effect on the penetration test parameters. For illitic samples, we obtained significant variation of the cone resistance and friction sleeve resistance with temperature on the dry side of the compaction curve while limited changes were observed on the wet side. For the kaolinite–sand mixture, the temperature effect on these two parameters was negligible.

The main object of the study was to depict couplings between the hydromechanical behaviour of compacted clayey soil and the mineralogical and microstructural transformations induced by high-pH water. This high-pH water could be generated by the degradation of concrete lining of deep galleries in the context of the storage of nuclear wastes in deep facilities. The tested material is the clay that comes from the laboratory of the Meuse-Haute Marne laboratory (MHM-clay). Compacted MHM-clay samples were subjected to the circulation of high-pH water or natural sitewater over an 18-month period. The hydromechanical properties, microstructural characteristics and somephysicochemical properties of the sampleswere then determined (i.e., shear strength, swelling properties, retention curve, mineralogy and microstructure). After the circulation of high-pH water, a strong reduction of the swelling properties associated to an increase in friction angle was evidenced, whereas the hydrodynamic properties remained stable. These modifications were associated with an alteration of the fabric of the samples, i.e., the dissolution of the initial clay minerals and the precipitation of neoformed illite, which is a non-swelling mineral. The results demonstrated that chemohydromechanical couplings occurred during the circulation of high-pH water and resulted in the modification of the material's mineralogy. Thus, these processes are likely to alter the sealing characteristics of a backfill constructed from a compacted MHM-clay in the very long term.

International audience; Soil cracking has been studied for many decades; however, it still deserves further investigation. This paper describes a new testing device that uses image processing to perform simultaneous and continuous quantification of the vertical strain and radial strain, crack area, and water content of a soil specimen subjected to suction cycles imposed by the vapour equilibrium method. An image processing method was designed to determine the crack area and the soil radial strain separately. The evolution of each measured parameter over time in a clayey soil specimen subjected to 3 suction cycles is presented and discussed. The first suction cycle has the greatest impact on the crack area, the mass water content, the vertical strains and the radial strains. Hysteresis loops were observed for crack area and radial strain evolution.

International audience; Lightly loaded structures constructed on expansive soils may develop structural damage as a result of changes in the soil’s moisture content. This study investigated an analytical model of soil–structure interaction to assess the settlement of dwellings built on swelling soils when droughts occur. The building behavior was investigated with the Euler–Bernoulli beam theory, and the ground behavior was investigated with a Winkler-derived model based on the state surface approach. The analytical model results were compared to those of a finite element analysis using the Barcelona Expansive Model (BExM) performed with Code_Bright. The analytical model was then used to assess the settlement transmission ratio for a typology of clayey soils and different parameters of building. The results indicated that the final deflection of the building increased with the building length and soil suction. The building deflection due to the suction variations was inversely proportional to the load, the rigidity of the building and the embedding depth of the foundation. Increasing these parameters made the building less vulnerable to shrinkage and swelling action.

International audience; The aim of this paper is to study the impact of certain soil treatments on the internal erosion characteristics of treated compactedsilt. The experiments measured the internal erosion using the hole erosion test (HET). This study aims to describe the effects of clay, lime andcement soil treatments on the internal erosion, specifically with regard to the amount of treatment used and the curing time. A new enhancedHET was developed to apply a high inlet pressure up to 650 kPa and thus generate a hydraulic shear stress up to 10,000 Pa. The internalerosion resistance was quantified by the coefficient of soil erosion and the critical shear stress. The results demonstrated that clay treatmentcould reduce the coefficient of soil erosion depending on the nature and percentage of clay added to the soil. The results also showed that limeand cement treatment primarily increased the critical shear stress of the tested silt. This increase was higher with cement treatment and wasdependent on the amount of the added product. The impact of the curing time (up to 30 days) on the evolution of the erosion characteristicswas not relevant for the lime-treated silt, whereas that of the cement-treated silt was dependent on the amount of cement added to the soil.

International audience; The incorporation of heat exchangers in geostructures produces a cyclic variation of the temperature in the adjacent soil. Therefore, there are important questions about the effect of temperature variations on hydro-mechanical soil parameters. In this study, mini-pressuremeter tests were conducted in laboratory on a homogeneous material submitted to different thermal loading (1 to 40 °C). The tested material, an illitic soil, is compacted in a thermo-regulated container with 0.6 m diameter and 0.8 m height, at optimal water content (31.3 %) and 90% of maximum dry density (1.29 Mg/m 3). Only six tests are performed in each container to prevent edge effects and the influence between the tests. The impact of temperature variation on creep pressure (pf), limit pressure (pl) and pressuremeter modulus (Ep) were determined. A decrease in creep pressure and limit pressure with increasing temperature was observed, while the variation of pressuremeter modulus was less pronounced. The first cycle induced more important parameter variations than the subsequent cycles.

International audience; Sustainable development principles are leading earthwork companies to use all-natural materials extracted from the construction site to build the infrastructure. Natural materials with low characteristics must be improved. For dry soils, the common solution is to increase the compaction energy or add important quantities of water to reach the target dry density and bearing capacity. To reduce the environmental impact of their activities, the use of industrial organic products has been proposed. The aim of this study was to assess the potential benefits that could be expected from the use of these non-traditional treatments in earthworks with a well-recognised environmental impact assessment methodology.

La mise en place de méthodes permettant d’emmagasiner la chaleur dans le sol engendre des questionnements sur l’impact des variations de températures sur les propriétés thermiques des matériaux et sur la possibilité d’optimiser l’efficacité de ces procédés dans le cas où le massif de stockage est anthropique (sols compactés). En effet, à proximité des échangeurs, la modification de la température pourrait provoquer une évolution du volume des constituants des sols et des propriétés d’écoulement de l’eau, ce qui pourrait engendrer une variation des propriétés thermiques du matériau. Dans cette étude, des essais sont réalisés afin de quantifier l’effet couplé de la teneur en eau (w), de la densité sèche et de latempérature (T) sur les paramètres thermiques de différents sols compactés argileux ou limoneux. Pour les 5 matériaux testés, les résultats montrent une augmentation de la conductivité thermique du côté sec de la courbe de compactage jusqu'à atteindre un maximum aux alentours de l’optimum Proctor. Les capacités thermiques calorifiquesmesurées (C) évoluent dans le même intervalle de variation pour les 5 matériaux malgré leur différence de minéralogie et de texture. Enfin, la diffusivité thermique suit la même évolution que lambda .Les résultats montrent une augmentation des propriétés thermiques avec l’augmentation de la température, plus particulièrement marqué dans l’intervalle 40-70 °C. Cette augmentation est plus importante pour les éprouvettes compactées du côté sec de la courbe de compactage., Academic Journal of Civil Engineering, Vol 34 No 1 (2016): Special Issue - RUGC 2016 Liège

International audience; A specific oedometer cell has been set up to measure the swelling pressure of compacted montmorillonites at constant volume and to concomitantly visualise the evolution, upon wetting, of how the microstructure is organised through X-ray microtomography. The swelling pressure experiments were conducted with solvents of various natures. In addition to conventionally used water and saline solutions, we used an organic solvent (methyl methacrylate – MMA). We chose this to explore the effect of its different physical and chemical properties, and to differentiate the respective roles of crystalline and osmotic pressures on macroscopic swelling behaviour.The results, which combined both swelling pressure measurements and quantification of microstructure evolution upon hydration for the two different solutes, give sound understanding on the development of osmotic and/or crystalline swelling and their relative impact both on the microstructure and on the magnitude of the macroscopic swelling pressure of compacted montmorillonites.

International audience; The paper reports results of an experimental study focused on the investigation of the development of swelling pressure in compacted bentonites during wetting under confined conditions. The main swelling pressure testing program has been performed using a new constant-volume oedometer especially designed to apply suction using the osmotic method. The device allows to continue to determine swelling pressure under constant-volume conditions during wetting close to saturation (suction below 8.5 MPa). Constant-volume swelling pressure tests were performed by decreasing the suction (or increasing the humidity) of the specimens from almost 100 MPa to 8.5 MPa and then in stepwise manner toward a zero value. In addition, a series of swelling pressure tests were performed using different experimental devices to compare test results obtained using the new device and to investigate the effect of hydration mode on the swelling pressure of compacted bentonites.Different wetting modes, water or vapor wetting, resulted different equilibrium swelling pressure values. Three stages of swelling pressure development are identified during suction reduction. In the first stage, following large suction reduction, swelling pressure increases up to a peak value; then, in second stage, in suction range comprised between 8.5 and 3.5 MPa, swelling pressure tends to decrease to a minimum value; and finally, upon further suction reduction to low values, swelling pressure increases again and eventually stabilizes. A possible interpretation of this pattern is given from micro-structural perspective.

The admissible current within a buried electrical power cable is limited by the maximum allowed temperature of the cable (Joule effect). The thermal properties of the surrounding soil controls heat dissipation around the cable. The main focus of the study was to evaluate the coupled heat and moisture flow around such buried electrical cables. The heat dissipation of a buried power cable was simulated in the surrounding soil at unsteady conditions. The hydro-thermal coupling was modelled by taking into account the moisture flow of liquid water and vapour, and the heat flow in the soil by convection and advection. As the thermal vapour diffusion enhancement factor (η) appears to be a key parameter, the sensitivity study of the coupled heat and moisture flow in the ground regarding this parameter was performed. The variations of the degree of saturation and the temperature of the surrounding soil were studied over 180 days of heating. The results showed that the moisture flow was mainly caused by the vapour transport under temperature gradients. These results emphasized the significant effect of the hydrothermal characteristics of surrounding soil. The radius of influence of the power cable was also evaluated.

Energetic geostructures consist in establishing a thermal exchange with the ground directly through systems integrated into the building foundations or structures. Incorporation of heat exchangers into geostructures brings questions on the impact of the temperature variations on the geotechnical parameters of the sensitive soils (illite, smectite...). The design of the bearing capacities of deep foundations is actually based on pressuremeter or penetrometer test results. In this study, a mini-pressuremeter test was developed to carry out laboratory tests on compacted samples submitted to different thermal variations. Mini-pressuremeter tests were conducted on an illitic soil compacted in a cylindrical container, submitted to temperature equal to 1, 20 and 40 °C. The impact of the temperature variation on the limit pressure (pl) and the creep pressure (pf) is presented.

Despite the scant quantitative data available in the literature, it has been hypothesized that some chemical compounds can have deleterious effects on soil stabilization with lime and cements (e.g., nitrates, phosphates and chlorides). This study intends to assess their influence on soil stabilization quantitatively. An original experimental procedure was followed. Selected soils were mixed with a potential deleterious compound at a concentration representative of what can be found in the field. The performance of the different mixtures in terms of soil stabilization was then assessed by performing mechanical tests on samples submitted to several curing conditions (temperature and humidity). The results showed that the tested compounds are likely to alter the soil stabilization processes and thus lower the mechanical performance of the stabilized soil. The results also showed that it is not possible to determine a single threshold value for the compounds considered because their influence on soil stabilization is also a function of the nature of the soil (silt or fine sand), the type of cement (CEM I or CEM II) and the curing conditions.

This paper presents results from an experimental study performed on natural and compacted expansive soils in different loose and dense states using principally osmotic oedometers. The loose soils showed significant shrinkage, while the dense soils produced a swelling accumulation during the suction cycles. These cycles induced an equilibrium stage, which indicates an elastic behaviour of the samples under suction cycles. At the end of suction cycles, a loading/unloading test was performed on all samples at different constant suctions to define LC (loading collapse) yield curve as well as the newly defined SC (saturation curve) yield curve. Generally, the suction cycles for the looser soils approached the LC and SC yield curves while they became the same at the equilibrium state. For the denser soils producing the swelling strain accumulation, the unique LC and SC curves were activated together. Additionally, we found that at the equilibrium stage, both initially loose and dense samples presented the same ...

This study presents the influence of suction variation on the fabric of two clayey soils, using the mercury intrusion porosimetry (MIP) technique on dense and loose remoulded samples. The experimental study focussed on the evolution of the compacted soil fabric, including the macropores, mesopores, and micropores under various hydromechanical conditions. The variation of void ratio of both samples was initially studied during a single wetting and drying cycle. The experimental results confirm the existence of a suction limit between the meso- and macropores (sm/M) as well as the soil “shrinkage limit” suction (sSL). The suction increase between (sm/M) and (sSL) completely eliminated the macropores and produced mesostructural rearrangement. The higher suctions modified the meso- and micropores. The measured soil water retention curves (SWRC) of these two soils compared with the MIP calculations showed a more reasonable agreement for the dense soil with a fewer macropores. Moreover, several wetting and drying cycles were applied to the remoulded samples. The suction cycles produced a cumulative swelling strain for the dense soil while a shrinkage accumulation was observed for the loose samples. The wetting/drying cycles induced an equilibrium stage in which the samples behaved in an elastic way. The analysis of the soil fabric at this elastic equilibrium stage showed that the suction cycles created significant macrostructural modifications and reorganization of the mesopores towards smaller sizes.

This study addresses firstly the soil fabric variations of loose and dense compacted soil samples during a single wetting/drying cycle at suctions between 0 and 287.9MPa using mainly the mercury intrusion porosimetry (MIP) tests. Two suction techniques were employed to apply this wide suction range: the osmotic technique for suctions less than 8.5MPa, and the vapor equilibrium or salt solution technique for suctions higher than 8.5MPa. Secondly, the soil water retention curves (SWRCs) were predicted by the MIP test results for both loose and dense soil samples. A reasonable correspondence between MIP results and SWRCs was found on the wetting path at lower suctions close to saturation and on drying path at higher suctions.

To further our knowledge of the coupling between the hydraulic cycles and mechanical behavior of treated swelling soils, this Note presents an experimental study on a bentonite/silt mixture treated with lime using a suction controlled osmotic oedometer. Successive wetting and drying cycles were applied at different suction ranges between 0 and 8 MPa, followed by a loading/unloading cycle at different suctions (2, 4 and 8 MPa). The compression curves of the aforementioned suctions provide the necessary information to analyze the influence of hydraulic cycles on the hydromechanical parameters of the treated swelling soils, such as the apparent preconsolidation stress P 0 ( s ) , the virgin compression index λ ( s ) and the unloading elastic index value κ. The suction ranges as well as the number of suction cycles influence the mechanical parameters of the treated swelling soil greatly.