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International audience; This paper explores the use of geotextiles as insulation material for energy geostructures made of compacted soil. This work is based on measurements of the thermal conductivity of four nonwoven needle-punched geotextiles of varying thicknesses made from virgin or recycled fibres. These values were measured using the hot-plate method. Then, to assess the use of these geotextiles under a covering soil layer, compression tests were performed. For each sample, the relation between thickness and vertical stress was thus established. The thermal conductivities varied from 0.04 to 0.06 W/m/K depending on the geotextile type and the compression stress applied. Subsequent measurements focused on the thermal conductivity of a bilayer compacted soil plus geotextile. Results revealed a water migration towards the geotextile and therefore a larger thermal conductivity. These results highlighted the main importance of compression load and moisture environment in the use of geotextiles as insulation products.

Reusing excavated material in geotechnical engineering reduces the carbon impact of a project. Such materials are usually placed in a compacted state in order to achieve the mechanical and hydric characteristics required to guarantee the safety of the structures. A good geotechnical knowledge of the materials is therefore necessary as well as a good anticipation of their behaviour over time. Indeed, in some situations, as in the case of waste storage, a low hydraulic conductivity is required. The use of crushed rocks rich in clays (argillite), possibly improved by adding bentonite, could be interesting. However, this addition, beneficial in terms of hydraulic conductivity, could be damaging from a mechanical point of view by the development of cracks at the interface atmosphere-compacted soil. For this purpose, samples compacted at the normal Proctor optimum are exposed to a relative humidity of 46% and a temperature of 22.5°C. The thickness, mass and surface condition (cracking) were monitored during the drying process, and measurements were taken in the thickness of the specimen after 29 hours of exposure. The results make it possible to compare the two materials at the same compaction energy. The argillite sample shows a significant shrinkage but no cracks at this scale. On the different hand, with the addition of bentonite, a significant cracking was observed and analysed. These results provide information on the hydromechanical behaviour of unsaturated fine soils at the atmosphere-compacted soil interface.

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.

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; 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.

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

International audience

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, the 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 at optimal water content (31.3 %) and 90% of maximal 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.

In geotechnical engineering, the proper design of thermo-active geostructures requires a better understanding of the thermo-hydro-mechanical behaviour of natural and compacted soils. For design purposes, mechanical parameters are usually obtained by pressuremeter in-situ tests. In the present study, mini-pressuremeter tests were conducted in laboratory to characterise compacted soils. The objective was to examine and quantify the influence of temperature cyclic changes on pressuremeter parameters of two different compacted soils. A heating-cooling cycle (20-40-20 °C) was applied to a clayey compacted soil, and three heating-cooling cycles (20-50 °C) were applied to a compacted loam. The obtained results showed a decrease in limit pressure (Pl) and creep pressure (Pf) with increasing temperature for both tested soils, while the variation of the pressuremeter modulus (EM) was less significant. Through the temperature range tested, a reversibility of the effect of a heating cycle is obtained. These results were quite similar for both studied materials and the application of several thermal cycles did not modify the reversibility of the evolution., L’incorporation d’échangeurs de chaleur aux géostructures pose la question de l'impact des variations de température sur les paramètres géotechniques des sols. Le dimensionnement des capacités portantes des fondations profondes peut être basé sur les résultats d’essais pressiométriques ou pénétrométriques. Dans cette étude, des essais mini-pressiométriques ont été réalisés sur des massifs de sol compactés dans un modèle réduit thermo régulé. Un matériau argileux est soumis à un cycle de réchauffement (20-40-20°C) tandis qu’un matériau limoneux est soumis à trois cycles thermiques (20-50-20°C). Une diminution nette de la pression limite (Pl) et la pression de fluage (Pf) sur chaque sol est mesurée tandis que l’évolution du module pressiométrique (EM) est beaucoup plus faible. Dans ces intervalles de température, l’effet des cycles de chauffage est réversible. La normalisation des résultats montre une similarité des évolutions pour les deux matériaux.

International audience

International audience

International audience