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.