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GEOthermics vs GEOtechnics: from the fundamentals of thermo-hydro-mechanical behaviour of clays to the design of safe energy geostructures – GEO2

GEOthermics vs GEOtechnics: from the fundamentals of thermo-mechanical behaviour of clays to the design of safe energy geostructures

The aim of this project is to investigate experimentally the mechanical response of clays subjected to thermal loading and to provide recommendations for the design of energy geostructures in clays. Particular focus is put on the volumetric response, as it is the responsible of possible additional thermal-induced settlements of energy geostructures. The project propose an innovative approach, fondamental and multidisciplinary.

From clay microstructure to their macro behaviour at the structure scale

The general aim has to be achieved through the following partial objectives, which correspond to the three work packages of this research project:<br /><br />1. Characterise experimentally and systematically the macroscopic volumetric response to thermal loading of water saturated clays with different microstructures, at different confining pressures and over consolidation ratios and use microstructural characterisation as a mean to interpret and understand macroscopic response.<br /><br />2. Repeat selected tests by changing the factors affecting the mechanical and electro-chemical clay interparticle forces and use microstructural characterisation as a mean to interpret and understand macroscopic response, to correlate those factors to the thermal response.<br /><br />3. Provide physically based interpretation of constitutive parameters of selected constitutive models to support design of energy geostructures in clays, using new advanced discrete numerical approaches as a virtual laboratory to explore different loading paths.

EXPERIMENTAL ACTIVITY
A mechanically isotropic experimental cell with temperature control has been developed, allowing monitoring the volumetric response of a clay sample during thermal cycles. The cell was carefully calibrated and a huge effort was made to optimise the heating process in order to make it as fast as possible while ensuring drained conditions. It is now possible to heat continuously at a rate as slow as 0.03 °C/h, which was checked to be slowly enough to limit the thermal induced pore pressure to less than 10 kPa. This resulted in a significant reduction of the time needed for each thermal cycle. The cell with this heating system is operational since June 2021.
A thermal oedometric cell was built in order to perform part of the experimental campaign in parallel to the series of core isotropic tests. This includes tests with different pore fluids which have never been performed before. The oedometric apparatus is a very convenient way to observe rapidly the response of the material and gain a first insight into its thermal behaviour.
MIP measurements are done before and after thermal loading to investigate the change in the clay microstructure due to temperature and pore fluid chemical properties.

THEORETICAL DEVELOPMENT AND CONSTITUTIVE MODELLING
The goal of this part of the project is to understand the interaction mechanisms between clay particles in order to get information to feed continuum constitutive models commonly used in practical problems. The idea is to: (i) develop a physically-based particle interaction law accounting for temperature; (ii) implement it in a Coarse-Grained Molecular Dynamics (CGMD) model to be used as a virtual laboratory to interpret the experimental results, (iii) transfer the achieved knowledge into commonly used ‘continuum’ models to guide the selection of constitutive parameters.

EXPERIMENTAL ACTIVITY
The preliminary and calibration tests were completed and the first thermal cyclic tests under NC conditions on kaolin clay were performed, tegether with the corresponding MIP measurement before and after thermal solicitation. The thermal edometric device is also operational and the tests on dry kaolin already have already started. The results will be published soon.

THEORETICAL DEVELOPMENT AND CONSTITUTIVE MODELLING
An electrostatically consistent potential energy interaction function for 2D infinite (in the 3rd direction) particles has been derived and implemented in a Coarse-Grained Molecular Dynamics (CGMD) model. The code is now under validation. The results will be published soon.

EXPERIMENTAL ACTIVITY
The experimental tests are ongoing. The main tested material remains the kaolin clay and all the related experiments are planned until March-April 2022. Some of the experiments will be repeat on other soils. The tests involving other fluids will be performed in oedometric conditions.

THEORETICAL DEVELOPMENT AND CONSTITUTIVE MODELLING
The implementation of the designed function in the GCMD model is now completed. This will be followed by a validation and testing procedure. The model will be used to interpret the experimental results and then as virtual laboratory to investigate other conditions.

1. Casarella A., Pedrotti M., Tarantino A. and Di Donna A., A critical review of the effect of temperature on clay inter-particle forces and its effect on macroscopic thermal behaviour of clay. 16th IACMAG Torino, Italy, 1-4 July 2020.
2. Tarantino, A., Casarella, A. Pedrotti, M, Di Donna, A., Pagano, A. de Carvalho Faria Lima Lopes, B, Magnanimo, V. Clay Micromechanics: an analysis of elementary mechanisms of clay particle interactions to gain insight into compression behaviour of clay, 16th IACMAG Torino, Italy, 1-4 July 2020.
3. Casarella A., Tarantino A. and Di Donna A., Micromechanical interpretation of thermo-plastic behaviour of clays, 2nd International conference on energy geotechnics, La Jolla, California, USA, September 20-23, 2020.

Low enthalpy geothermal energy is a green and local source of energy. Traditional geothermal systems have high cost of installation and energy geostructures, i.e. geostructures equipped with the facility to exchange heat with the ground, represent a promising alternative. However, they generate a thermal loading to the ground, which might affect its hydro-mechanical response and eventually the geotechnical performance of the structure. The aim of this project is to investigate experimentally the mechanical response of clays subjected to thermal loading and to provide recommendations for the design of energy geostructures in clayey soil. The project proposes an innovative fundamental and multidisciplinary approach involving soil mechanics, clay science and physical-chemistry. The first Work Package aims to address the interplay between microstructure and the macroscopic volumetric response of clayey soils subjected to drained thermal cycles under ‘standard’ fully saturated conditions. The microscopic analyses will be obtained through Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscopy (SEM), carried out at different key stages of the stress-strain path to monitor the evolution of the pore-size distribution and, hence, microstructure. Both kaolinite and illite will be tested, to investigate a relatively broad range of clay types. The second Work Package will be devoted to elucidate key aspects of the micro-mechanisms behind the thermal response at the macroscale via ‘non-standard’ tests. First, selected experiments will be repeated under dry and partially saturated conditions. Then, to explore the role of electro-chemical forces generated between the negatively charges particles faces (mainly repulsive Coulomb forces), samples will be prepared using pore-fluid with different dielectric permittivity. Comparison with previous results where double-layer interactions were modified via the temperature, will allow assessing the role of electro-chemical forces on thermal behaviour at the macroscale. Finally, the role of the mechanical forces (mainly attraction Coulomb forces) developing at the edge-to-face contacts will be explored by preparing samples with alkaline pore-water, which ‘deactivates’ the edge-to-face contact. Again, the macroscopic tests will be combined with MIP and SEM data. In the last Work Package, a constitutive model selected from the literature to simulate the response of soils under thermo-hydro-mechanical (THM) loading will be reconsidered in the attempt to give the constitutive parameters a physical meaning based on the findings from micro-scale investigations. In turn, this would allow the constitutive parameters to be estimated by practitioners via ‘accessible’ routine experimental tests rather than complex and excessively time-consuming THM tests. To support the development of relationships to estimate constitutive parameters, advanced discrete numerical models calibrated against the experimental results will be used as a virtual laboratory to explore more complex loading paths. The final goal is to provide a concrete support to engineers in the design of energy geostructures in clays by providing guidance on parameter selection.

Project coordination

Alice Di Donna (Sols, Solides, Structures, Risques)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

3SR - UGA Sols, Solides, Structures, Risques

Help of the ANR 223,766 euros
Beginning and duration of the scientific project: - 36 Months

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