Temperature effects on the design parameters of a geothermal pile

In geotechnical engineering, geostructures with thermo-active functions establish direct thermal exchange between the ground and buildings. They can transfer energy from or into the ground to heat or cool a building. However, adapting foundation piles, completely or in part, to produce energy piles results in heat exchange with the soil, which changes the temperature of the soil and could thereby and affects the geotechnical properties and load bearing capacity of the geostructure. Most calculations of the bearing capacities of deep foundations conducted in France are currently based on in-situ testing results using a pressuremeter. Using finite element method to model the pressuremetric behaviour of a compacted soil subjected to thermo-mechanical variations is the main motivation for this work. In this study, several pressuremeter tests were conducted on a compacted illitic soil in a laboratory tank at temperatures between 1° and 40°C. The impact of temperature variation on the limit pressure (Pl), the creep pressure (Pf) and the Ménard pressuremeter modulus (EM) were determined. The results showed a significant decrease for both limit pressure (Pl) and creep pressure (Pf) with the increase of temperature. Numerical simulations of these tests were used to calibrate a bilinear constitutive model, taking into account temperature effects on soil compressibility within a coupled thermo-mechanical framework. Thereafter, a case study of a heat exchanger pile was simulated using the proposed approach

Characterisation of ground thermal and thermo-mechanical behaviour for shallow geothermal energy applications

Increasing use of the ground as a thermal reservoir is expected in the near future. Shallow geothermal energy (SGE) systems have proved to be sustainable alternative solutions for buildings and infrastructure conditioning in many areas across the globe in the past decades. Recently novel solutions, including energy geostructures, where SGE systems are coupled with foundation heat exchangers, have also been developed. The performance of these systems is dependent on a series of factors, among which the thermal properties of the soil play one of major roles. The purpose of this paper is to present, in an integrated manner, the main methods and procedures to assess ground thermal properties for SGE systems and to carry out a critical review of the methods. In particular, laboratory testing through either steady-state or transient methods are discussed and a new synthesis comparing results for different techniques is presented. In-situ testing including all variations of the thermal response test is presented in detail, including a first comparison between new and traditional approaches. The issue of different scales between laboratory and in-situ measurements is then analysed in detail. Finally, thermo-hydro-mechanical behaviour of soil is introduced and discussed. These coupled processes are important for confirming the structural integrity of energy geostructures, but routine methods for parameter determination are still lacking

Comportement thermo-hydromécanique des sols fins : applications aux géostructures énergétiques

The increasing use of geostructures equipped with geothermal exchangers is in line with the energy transition and environmental concerns. However, the cyclic thermal solicitations of soils raise many questions, as they may substantially modify the mechanical and energy sustainability of these systems. A multicoupled study of the thermo-hydromechanical behavior is thus necessary. In this work, the effect of cyclic temperature variations on the hydromechanical parameters was studied. A multi-scale and multiphysics experimental approach is employed to improve the predictive models of the THM behavior of energetic geostructures.Le développement de l’utilisation des géostructures équipées d’échangeurs géothermiques est en phase avec la transition énergétique et les préoccupations environnementales. Cependant, les sollicitations thermiques cycliques des sols engendrent de nombreuses questions, car elles pourraient modifier sensiblement la durabilité mécanique et énergétique de ces systèmes. Une étude multicouplée du comportement thermo-hydromécanique s’avère donc nécessaire. Dans ce travail, l’effet des variations cycliques de température sur les paramètres hydromécaniques a été étudié, à l’aide d’une approche expérimentale multiéchelle et multiphysique, afin d’enrichir les modèles prédictifs du comportement THM des géostructures énergétiques

Soil-atmosphere interaction: cracking of a compacted soil under the effect of a thermo-hydric stress

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

Temperature effect on the mechanical parameters of a clayey compacted soil

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Évolution des paramètres pressiométriques d’un massif argileux non saturé soumis à des variations monotones et cycliques de la température

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/m3. 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 1er 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 1er cycle

The performance of a Horizontal Ground Heat Exchanger (HGHE) under the seasonal ground energy storage behaviour

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

Thermal energy storage in embankments: Investigation of the thermal properties of an unsaturated compacted soil

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