Search

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.

International audience

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.

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.

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.

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 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 ...

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.

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.

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 ...

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; 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; 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; 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.

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.

In the French model of deep nuclear wastes repositories, the galleries should be backfilled with excavated argillite after the site has been filled. After thousands of years, the degradation of the concrete lining of the galleries will generate an alkaline solute (pH > 12) that would circulate through the backfill. The goal of this paper is to describe the impact of such solute circulation on the properties of compacted argillite. Since additives (bentonite, sand or lime) are often introduced in the remoulded argillite for the backfill, such mixtures were also studied. Saturated-portlandite water was circulated through compacted samples for 3, 6 and 12 months at 60 °C. The shear strength behaviour of the samples was determined with triaxial tests. The microstructure of the samples was analysed via mercury intrusion porosimetry tests and scanning electron microscopy. The results showed that the influence of the alkaline fluid on the properties of the argillite is a function of the nature of the additive. In the case of the calcareous sand, no major changes were observed. The pure argillite underwent a slight decrease in its cohesion due to limited dissolution of its clayey particles. Conversely, intense alteration of the bentonite–argillite mixture was observed, and the shear strength behaviour was modified. Lime addition improved the mechanical characteristics of the argillite.

To further our knowledge of the coupling between the hydraulic and mechanical behaviours of the swelling soils, this paper presents an experimental study on a bentonite/silt mixture using an osmotic odometer. A loading/unloading cycle was applied to samples with different initial dry densities (1.27, 1.48, and 1.55 Mg m− 3) at different constant suctions (0, 2, and 8 MPa). We noted that the initial state of the soils after compaction significantly influenced the values of the apparent preconsolidation stress p0(s), the virgin compression index λ(s), and the elastic compression index κ. These experimental results provided a sufficient database to interpret the mechanical behaviour of the swelling soil and define three yielding surfaces: – the suction limit between micro- and macrostructure (sm/M) and the suction limit between nano- and microstructure (sn/m), which depend completely on the soil fabrics and the diameter separating the nano-, micro-, and macrostructure, – the Loading Collapse (LC) curve, representing the preconsolidation stress variation as a function of suction, – the Saturation Curve (SC), representing the variation of the saturation stress (Psat) as a function of suction. In general, we can state that the increase of compaction pressure unified the LC and SC surfaces and decreased the (sm/M) value without modifying the (sn/m) value.

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.

International audience; The main objective of the paper is to present a study on compacted expansive soil strains generated by complex hydric or mechanical loadings and their consequences on tested soil compressibility. In the first part of the paper, the suction controlled testing devices are described (osmotic and salt solutions methods). All the presented tests were performed in the range of suctions comprised between 0 and 40 MPa. The consequences of the strains induced by wetting as a function of the applied mechanical stress were analyzed. The results showed that, under these conditions, strain depends on the followed hydraulic path whereas the slope of the plastic compression line λ(s) was only affected by the suction applied during themechanical loading. The second test series demonstrated that the stress path influenced both deformation and slope λ(s) if suction higher than the initial sample suction was imposed during the test. As a conclusion, it could be stated that the slope λ(s) depends on the suction applied during a mechanical loading and on the maximum suction experienced by an expansive material during its past “history”.; L’objectif de cet article est de présenter une étude sur les déformations d’un sol gonflant provoquées par des sollicitations hydriques et/ou mécaniques complexes ainsi que leurs conséquences sur sa compressibilité notamment. Dans la première partie de l’article, les dispositifs expérimentaux à succion contrôlée employés (méthodes osmotique et solutions salines) pour conduire l’ensemble des essais sont décrits brièvement. Les essais ont été réalisés dans la gamme des succions comprises entre 0 et 40 MPa. Les conséquences d’une sollicitation hydrique monotone (humidification) conduite sous différentes contraintes ont d’abord été caractérisées. Il est apparu que dans ces conditions d’essai, les déformations dépendent du chemin de contrainte contrairement à la pente de compression plastique λ(s) qui ne dépend que de la succion imposée durant le chargement mécanique. Une deuxième série d’essais a permis de montrer l’influence du chemin de contrainte sur les déformations ainsique la pente λ(s) si une succion supérieure à la succion initiale des éprouvettes est imposée au cours de l’essai (dessiccation). En conclusion, la valeur de la pente λ(s) dépend de la succion sous laquelle est effectué le chargement mécanique mais aussi de la plus grande succion connue par le matériau au cours de son « histoire ».

Resume Cet article presente une etude menee sur un materiau gonflant compacte dans la gamme des succions comprises entre 8,5 et 287,9 MPa. Deux series d'essais œdometriques a succion controlee sont presentees. La premiere permet la caracterisation de l'evolution de la compressibilite avec la succion. Elle montre que la pression de preconsolidation apparente et la pente de compression plastique sont significativement influencees par la succion. La deuxieme serie etudie l'influence de sollicitations hydromecaniques complexes sur la compressibilite du materiau qui apparait tres dependante du chemin hydromecanique. Pour citer cet article : O. Cuisinier, F. Masrouri, C. R. Mecanique 331 (2003).

International audience; The main purpose of this paper is to examine the impact of a wetting and drying path on the swelling/shrinkage of a compacted lime-treated expansive clayey soil both at the macro- and micro-scales. At the macro-scale, the soil water characteristic curves (SWRCs) of the compacted lime-treated materials (0%, 2% and 5% of lime content) were determined for three curing times (0, 28 and 180 days). The modifications at the micro-scale were assessed with mercury intrusion porosimetry (MIP) tests. The results showed that lime treatment was efficient to prevent the volumetric swelling from the initial state upon wetting. Even if no significant macroscopic volumetric variation was observed, a reorganisation of the microstructure was evidenced. Upon drying, the lime addition led to an alteration of the hydro-mechanical soil behaviour from the initial state by increasing the compacted shrinkage limit suction. However, the volumetric shrinkage of the compacted lime-treated samples remained on the same order of magnitude of the untreated compacted soil, regardless of lime content and curing time. At the micro-scale, the \MIP\ tests showed that drying altered both the macro- and micro-porosity fabric of the lime-treated soils. This study showed that lime treatment had a limited effect on lime-treated compacted soil shrinkage whilst preventing swelling.

International audience

International audience

International audience; The positive effects of lime treatment are likely to be altered by weathering, and the sustainability of lime-treatment positive effects is in question. In this study, the effects of successive drying/wetting cycles on the hydromechanical properties of a lime-treated clayey soil are assessed. Osmotic suction-controlled oedometers were used to determine the swelling/shrinkage behavior of soils subjected to successive drying/wetting cycles of small amplitude. Significant degradation of the yield stress was observed while the efficiency of the treatment on volumetric behavior remained unchanged, this being related to the stabilization process. This study showed that the evaluation of the mechanical sustainability of a lime-treated material subjected to wetting and drying cycles requires tests in realistic conditions.

Sustainable development principles lead earthworks companies to use all natural extracted materials on the construction site. Most of the time, these materials have to be improved, lime and cement being the traditional additives. In this context, the use of organic by products from various industries has been proposed. But the effects of these non-traditional additives on soil behaviour and stabilizing mechanisms are not well understood. The aim of this paper is to characterize the modification of compaction and bearing capacity of a low plastic soil (Plasticity index PI = 14) and to propose a stabilization mechanism. In this study, three non-traditional products were tested: an acid solution, an enzymatic solution and a calcium lignosulfonate powder. For the enzymatic, and lignosulfonate treatments, the experimental results showed an increase in the compaction ability of the selected soil with a reduction in optimum Proctor water content. These effects induced potential water and energy savings for the compaction of dry soils on a construction site. Regarding the stabilisation mechanisms, the obtained results showed that the effects of non-traditional products are mainly related to their surfactant features.

International audience; When subjected to a very high-pH, most of the soil minerals undergo physico-chemical transformation. This could induce strong modifications of the shear strength behaviour of the soil. This issue is of high interest in the framework of the design of deep nuclear wastes repositories, since the degradation of the concrete lining of deep galleries after thousands of years will generate an alkaline solute (pH > 12) that would circulate through the backfill, and alter its hydromechanical characteristics. A study was undertaken to assess the impact of high-pH fluid circulation on the shear strength behaviour of a backfill material. Because of the complexity of the existing constitutive theories, a new approach was used, based on evolutionary polynomial regression (EPR), for modelling of these processes. \EPR\ is an evolutionary data mining technique that generates a transparent and structured representation of the behaviour of a system directly from data. An \EPR\ model was developed and validated using results from a comprehensive set of triaxial tests. Through a sensitivity analysis, the \EPR\ model permitted to identify the specific surface, and to a lesser extent the micropore void ratio, as coupling parameters between hydromechanical behaviour alteration during alkaline fluid circulation and a physical process.

International audience; To better understand the hydro-mechanical behavior of dual porosity expansive clays, three groups of tests were carried out. First, isotropic free drying and free wetting tests were performed to determine both branches of the soil water retention curve of a silt–bentonite mixture and its parameters. Next, constant suction oedometer tests at different matric suctions were employed using osmotic method. Finally, wetting–drying cycles were imposed on the samples in oedometric condition. At the end of wetting–drying tests, different suction values were enforced on the specimens and consolidation tests were performed, while the suction values were regulated through the samples using the osmotic method. Cross-examination of results indicates good consistency between the observed behaviors from different tests: Results of constant suction oedometer tests confirm the hardening phenomenon at low suctions indicated by the free drying test. Besides, all tests show that the amount of the suction hardening is negligible at suctions between the air entry of macropores and that of the micropores. However, the results contradict the physical models of hydro-mechanical behavior of expansive clays which do not take into account the suction induced hardening in the range of suctions less than the air entry value. Moreover, while most of the models assume that the shrinkage limit and air entry suction are coincident; results of the free drying test show the coincidence of the shrinkage limit and the air entry value of micropores

International audience; In the French concept of deep nuclear waste repositories, the galleries should be backfilled with excavated argillite after the site has been filled. Some additives like lime could be used to improve the mechanical characteristics of the argillite. After thousands of years, the degradation of the concrete lining of the galleries will generate an alkaline solution (pH value > 12) that will diffuse through the backfill. This study presents the effect of a saturated Ca(OH) 2 solution circulation through lime-treated sample at 60 °C for 3, 6 and 12 months, respectively. The effect of such circulation on the lime-treated Manois argillite (MA) was assessed by petrographical examination coupled to image analysis and scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) analyser of soil pieces. The objective of this study is to make the link among the mineralogical transformations, the textural and mechanical changes produced in the compacted clayey soil as a consequence of the alkaline solution circulation.

International audience; Cet article rapporte un ensemble de résultats d'études expérimentales effectuées sur un matériau gonflant naturel. Dans un premier temps, un essai de porosimétrie au mercure a mis en évidence la double structure du sol étudié: la micro- et la macrostructure. Afin d'obtenir la courbe de rétention d'eau, deux méthodes d'imposition de succion ont été appliquées: la méthode osmotique pour les succions inférieures à 8,5 MPa et la méthode de solutions salines pour les succions supérieures à 8,5 MPa. Des essais œdomètriques ont également été effectués avec imposition de succion par la méthode osmotique. Plusieurs cycles de séchage―humidification entre 0 et 2 MPa ont été appliqués sur des éprouvettes sous trois faibles charges mécaniques constantes (20, 40 et 60 kPa). Pendant ces cycles, ces éprouvettes présentent un gonflement cumulé. Les résultats montrent que les déformations volumiques convergeant vers un état d'équilibre où le sol présente un comportement réversible. L'ensemble de ces résultats a permis d'obtenir les paramètres du modèle élastoplastique (BExM). Ce modèle implanté dans un code de calcul aux éléments finis (Code_Bright) est utilisé pour l'étude du comportement hydromécanique de ce sol naturel chargé par une semelle filante. L'objectif de cette étude est d'analyser l'influence des sollicitations hydriques sur les déplacements de cette fondation. Le modèle est capable d'aboutir à des résultats représentatifs de ce type de chargement hydromécanique complexe

International audience; In the French concept of deep nuclear wastes repository, the galleries should be backfilled with excavated argillite after the site exploitation period. After several thousands of years, the degradation of the concrete lining of the galleries will generate alkaline solute (pH N 12) that will diffuse through the backfill. The object of the paper is to describe the influence of such solute diffusion on the properties of compacted argillite. Since it is planned to introduce additives (bentonite, calcareous sand or lime) in the remoulded argillite to backfill the deep galleries, such mixtures were also studied. Saturated-portlandite water was circulated through compacted samples for 3, 6 and 12 months at 20 or 60 °C. The microstructure before and after fluid circulation was determined with mercury intrusion porosimetry. The results showed that the 50% sand-50% argillite mixture microstructure was slightly affected by the alkaline fluid circulation. Conversely, the microstructure of the other tested materials changed dramatically. The fluid circulation provoked an increase of the macropore void ratio by almost 3 times for the pure argillite, almost 1.5 for the MX-80 and argillite mixture and 2 for the lime-treated argillite. This particular behaviour could be attributed to the dissolution of soil minerals in such a high pH environment. Hence, it is likely that hydraulic and mechanical properties of these mixtures would be dramatically affected by alkaline solute diffusion over a long period of time.

A study of the hydromechanical behaviour of a compacted swelling material in the range of suctions comprised between 0 and 40 MPa was performed. This study has required the development of two kinds of suction controlled oedometer devices based on two different suction control techniques. In the range of suctions higher than 8.5 MPa, the saturated salt solutions method was used and a new oedometer using this suction control technique was developed. For suctions lower than 8.5 MPa an osmotic oedometer was used. Despite the differences between the applied suction components (matric and total), the correlation between the two methods was verified for the tested material. The second part of the paper presents a set of oedometer tests conducted under various suctions. The effect of the applied suction on the hydromechanical parameters was studied. First, two swelling phases were highlighted: a low swelling phase above a suction of 4 MPa and a high swelling one below this value. These phases were considered as being related to the microstructure of compacted swelling clays. Secondly, it was shown that the slopes of the elastic part and of the plastic part of the consolidation curves were not influenced significantly by the applied suctions. In opposition, the preconsolidation pressure is affected by the decrease of the applied suctions even in the low swelling phase. Such a behaviour could be explained by the effects of wetting on the microstructure.

International audience; The soilfabric plays an important role in complex hydromechanical behaviour of the expansive soils. This article addresses the influence of the wetting and drying paths on the soilfabric of compacted bentonite and silt mixtures at two different initial dry densities corresponding to loose and dense states. To obtain the hydric response of the soil, two suction imposition techniques were used: osmotic technique for the suction range less than 8.5 MPa and the vapour equilibrium or the salt solution technique for the suction range between 8.5 and 287.9 MPa. Additionally, the soilfabric analysis was performed using mercury intrusion porosimetry (MIP) and nitrogen gas adsorption (BET) techniques. The dense samples produced cumulative swelling strains during the suctioncycles, while shrinkage was observed for the loose samples. The suctioncycles induced an equilibrium state indicative of the elastic behaviour of the samples. The soilfabric analysis showed that regardless of the soil's initial state (loose or dense), the samples obtained the same soilfabric at the equilibrium state. The experimental results illustrated also the existence of an elastic void ratio (e0el) where the compactedsoils at this state present an elastic hydric behaviour during the successive suctioncycles.

Granular materials are generally used in unbound layers of road pavement structures. The mechanical behavior of these materials is widely studied with repeated load triaxial tests (RLTT) in which the elastic response is defined as the resilient behavior. Usually observed under total stress conditions, the effect of pore pressure changes during loading are not usually included in design. Further, the unbound layers frequently exist under partially saturated conditions. The influence of the unsaturated state, i.e., the suction, on the mechanical behavior, of unbound granular materials for roads has not been sufficiently studied and is generally not taken into account in models used for these materials. This article presents an experimental study of the repeated load response of a compacted clayey natural sand, and describes a model for the response which includes the effects of soil suction. The response of the proposed model formulated in terms of effective stress is compared with a similar model formulated in terms of total stress. The results from both the effective stress model and the total stress model are compared with the measured volumetric and deviatoric response. It is suggested that since the model parameters for the effective stress formulation are relatively constant for all values of suction (water content), the resilient response can be best captured by an effective stress model.

International audience; This paper presents a simple approach to model the hydro-mechanical behaviour of swelling soils at different initial states during successive wetting and drying cycles. This model was developed based on the experimental results obtained for an expansive bentonite/silt mixture compacted at three different initial dry densities representing loose, intermediate and dense states. The parameters of the simplified model were initially fitted based on the results obtained for the loose and dense samples and then used to estimate the hydraulic and mechanical behaviour of the intermediate samples. Generally, the model prediction produced a satisfactory correspondence with the experimental results.

The use of compacted swelling soils in engineering practice is very widely spread, especially in geotechnical and environmental engineering. After their setup, these materials are likely to be subject to complex suction/stress paths involving significant variations of their hydro-mechanical properties which can affect their initial behaviour. It is important to be able to predict the hydromechanical behaviour of these materials taking into account the significant applications for which they are intended.Barcelona team developed a finite-element code (Code_Bright) for the thermo-hydro-mechanical coupling (THM) integrating the BBM elastoplastic model for unsaturated soils based on the independent variables approach. This model is recognized to correctly describe the hydromechanical behaviour of unsaturated soils but fails to take into account some particular observed aspects on swelling soils. A second model BExM was then proposed to address these aspects. The objectif of this study is: (i) to implement the elastoplastic model BExM for the unsaturated swelling soils in the finite-element code (Code_Bright); (ii) to check the numerical model validity through the numerical simulation of laboratory tests made on swelling soils; and (iii) to apply this model to some practical problems. For this purpose, a new family of numerical procedures adapted to the BExM model was introduced into the code. The equation of the yield surface of this model for a given deviatoric stress states was given in a manner to facilitate calculations of its derivatives. The model was checked by the numerical simulation of suction-controlled œdometric tests made on three different swelling soils. The simulation results showed that the numerical model is able to correctly reproduce the experimental data. Lastly, the model was applied to two practical problems: radioactive waste repository in deep geological layers and a shallow footing under the action of a swelling soil. The results obtained showed the ability of the numerical model to modeling hydromechanical coupled problems., Les sols gonflants compactés sont largement utilisés dans de nombreuses applications, notamment en géotechnique et en géotechnique de l'environnement. Après leur mise en place, ces matériaux risquent d'être soumis à des sollicitations hydriques et/ou mécaniques complexes qui peuvent entraîner de fortes modifications de leurs propriétés hydromécaniques susceptibles de mettre en cause leur fonctionnement initial. Il est donc important de prévoir le comportement hydromécanique de ces matériaux compte tenu des applications sensibles auxquelles ils sont destinés.L'équipe de Barcelone a développé un code de calcul aux éléments finis (Code_Bright) pour l'analyse des problèmes couplés (THM) dans des géomatériaux, avec l'intégration du modèle élastoplastique (BBM) pour les sols non saturés non gonflants. Ce modèle est capable de décrire correctement le comportement hydromécanique des sols non saturés mais ne tient pas compte de quelques aspects particuliers observés sur des sols gonflants. Un deuxième modèle (BExM) a été alors proposé pour prendre en considération ces aspects. L'objectif principal du travail présenté ici est : (i) d'implanter le modèle élastoplastique BExM pour les sols gonflants non saturés dans le code de calcul aux éléments finis Code_Bright ; (ii) de valider le modèle numérique à partir d'essais réalisés au laboratoire sur des sols gonflants ; et (iii) d'appliquer ce modèle aux simulations des problèmes pratiques. Pour atteindre cet objectif, une nouvelle famille de procédures numériques adaptées au modèle BExM a été introduite dans le code. L'équation de la fonction de charge de ce modèle pour un état donné de contrainte déviatorique a été déterminée de manière à faciliter les calculs de ses dérivées. La validation numérique du modèle a été vérifiée par des simulations numériques d'essais œdométriques à succion contrôlée, réalisées sur trois sols gonflants différents. Les résultats des simulations numériques de ces essais ont montré les aptitudes du modèle numérique implanté à reproduire correctement les résultats expérimentaux. Enfin, le modèle a été appliqué à deux problèmes pratiques : le stockage de déchets radioactifs dans les couches géologiques profondes et l'effet du retrait-gonflement du sol sur une fondation superficielle. Les résultats obtenus ont montré la capacité du modèle implanté à modéliser des problèmes couplés hydromécaniques.