MSCA-COFUND-CLEAR-Doc - PhD Position #CD21-13 "Numerical and Physical modelling of interactions between nearby energy geostructures"

The growing energy needs of urban areas have led to the development of innovative solutions for energy production. In Europe, 80% of energy consumption in the residential construction sector is due to space heating and cooling. In the context of climate change, it is crucial to develop renewable, local energy production solutions with low environmental impact and low risk for humans. Among these solutions, shallow geothermal energy has the additional advantage of being non-intermittent. And among the shallow geothermal solutions, one of them, developed since the 1980s, consists in using geostructures (foundations, retaining walls, tunnels, etc.) as heat exchangers.

Heat exchanger tubes are fixed to the reinforcement cages of the geostructures and connected to a heat pump thus allowing the heating and cooling of the premises supported by these geostructures or present nearby. In addition to the advantages of conventional shallow geothermal energy, energy geostructures have almost zero carbon cost, their energy role being added to a mechanical role for which they should have been realised anyway.

However, cyclic thermal loading generates stresses and strains in the geostructure as well as in the surrounding soil, due to thermal expansion. The impact of thermal effects on the mechanical behavior of structures raises some concerns. The lack of design methods has largely hampered the deployment of these solutions for years. While the thermomechanical behavior of energy piles under vertical loading has been extensively studied, many questions remain regarding more complex structures (groups of piles, diaphragm walls, tunnels, metro stations, etc.). In order to envisage a large-scale deployment of the technology, it is essential to remove these scientific barriers.

The complexity of the problem is further increased in the presence of a natural groundwater flow. On the one hand, the latter has the advantage of dissipating thermal anomalies and cities with significant underground flows such as Nantes or Sao Paulo are therefore suitable for energy geostructures. On the other hand, it also generates heat transfers between neighboring geostructures. Understanding the positive or negative aspect of these interactions is an essential aspect for the deployment of technologies at the scale of eco-districts.

In order to answer the questions raised by the thermo-hydro-mechanical behavior of energy geostructures, it is proposed here to carry out a research program combining in situ observation, physical modeling and numerical modeling:

• The CG laboratory has developed a strong expertise in the centrifuge study of scale models of geotechnical structures. This type of modeling has the advantages of reduced models (low cost, repeatability, easy instrumentation) while ensuring respect for the similarity of the behaviors observed with the behaviors of full-scale structures. In the case of thermomechanical phenomena, it also has the advantage of considerably reducing the characteristic times of the phenomena observed. Thus, the time scale (for diffusion) of a 1/N small scalled model subjected to a macro gravity flied N time higher the one of the Eart (N×g) is reduced by N square. As the Uni Eiffel centrifuge can go up to 100×g, 1 hour of centrifuge test corresponds to 1 year and 2 month. In this way, centrifuge tests can model several years of cyclic thermal loading.
• In addition to the tests carried out in a centrifuge, the PhD student will carry out an 8-month mobility at the University of Sao Paulo during which he will carry out thermal and mechanical measurements on real geothermal piles, designed to be the foundations of the future building of the CICS (Sustanaible Construction Innovation Center) named « CICS Living Lab » (http://cics.prp.usp.br/cics-living-lab/).
• The comparison of experimental data will make it possible to feed and calibrate a finite element model produced using CESAR-LCPC, a software developed at the University Gustave Eiffel. Once this model has been calibrated, a complex “metro station” type structure will be modeled.

This comprehensive study will be the subject of scientific publications but it will also aim to produce design elements for energy geostructures for the profession.

International Mobility:

A 8-month secondment at University of São Paulo at São Carlos, Department of Geotechnical Engineering, Brazil. For more information, contact the PhD thesis supervisor.