Thermo-mechanical aspects of energy piles

Energy piles are deep foundations which belong to the category of shallow geothermal systems and have the double role of structural support and geothermal heat exchangers. A system of pipes embedded into the concrete with a heat carrier fluid circulating through them allows exchanging heat with the ground to heat the building during the winter and cool it during the summer. During winter, the system works thanks to a heat pump which increases the temperature coming from the soil to the one required in the heating plant of the building. During summer, heat coming from the air conditioning systems can be injected into the soil. The heat injection and extraction activities result in an additional thermal solicitation for the foundation which is seasonally and daily cyclic. Since the primary role of piles is to provide the necessary structural stability of the overbuilding, their geotechnical performance has to be guaranteed even in the presence of the additional thermal cyclic loading. The objective of this thesis is to identify and analyse the principal aspects that are involved in the geotechnical design of piled foundation which could be affected by the temperature changes. In the first part, a state of the art about the currently employed design methods for deep foundations is provided. On the basis of this analysis and knowing the design recommendations already available for energy piles, the thesis is divided into three sections. The first and second ones are devoted to the investigation of the effects of thermal (cyclic) loading on the soil and pile-soil interface behaviour, respectively, while the third one aims to study the response of single and groups of piles in clay when subjected to a thermal cyclic solicitation. The response of both a natural soil and a reconstituted clay when subjected to thermal cyclic loading in the range between 5 and 60 °C is investigated experimentally. For this purpose a new experimental setup is developed and calibrated. It includes four oedometric cells equipped for temperature control. The results show that over consolidated clays behave thermo elastically when subjected to thermal cycles: they dilate during heating, contract during cooling and do not accumulate any irreversible deformation cycle after cycle. Conversely, normally consolidated clays contract when subjected to a temperature increase (thermal consolidation) and such deformation is irreversible during cooling (thermo plasticity). Most of the irreversible deformation is developed during the first thermal cycle, even if small amounts are also accumulated during the few subsequent ones, showing an accommodation phenomenon. With the purpose of numerically reproducing the response experimentally observed, an existing thermoelastic-thermoplastic model is enhanced to take into account the thermal cyclic accommodation aspect and validated against the experimental results. The response of both clay-concrete and sand-concrete interface response at different temperatures is studied experimentally. A direct shear test device is developed to introduce the possibility to run tests under Constant Normal Stiffness (CNS) conditions and controlled temperature. In the experimental campaign, different concrete roughnesses and stress paths are considered, as well as both monotonic and cyclic solicitations. The results show that the sand-concrete interface behaviour is definitely not affected by temperature. Differently, the clay-concrete interface strength increases at high temperature and this is interpreted as a consequence of the clay thermal consolidation. The knowledge acquired from the two experimental campaigns realised on the soil and soil-concrete interface subjected to thermal loading is included into a series of numerical analyses performed with a finite element code in which a coupled thermo-hydro-mechanical formulation is implemented. Single and groups of energy piles are both studied. The enhanced thermoelastic-thermoplastic constitutive model for soils accounting for the thermal cyclic accommodation phenomenon is considered in the simulations. The pile-soil interface is modelled through a thin layer of elements behaving according to the same model used for the soil. Under normal working conditions in terms of mechanical and thermal loading, the thermoplastic behaviour of the soil induces a slight additional irreversible settlement of the single pile during the first increase in temperature, which is subtle in the case of the more rigid group of piles. The subsequent soil thermal accommodation phenomenon does not play a crucial role on the performance of the foundation, both for the single and the group of piles, which react thermo elastically starting from the second cycle. In the cases studied, the additional heating-induced compressive stress is significant, although admissible with respect to the concrete strength, while the cooling-induced reduction in compression stress leads to a zone of tensile stress in the lower part of the pile. The 3D model of the energy pile group allows a deeper understanding of the thermo-mechanical interactions between the piles. In the considered case, the thermal-induced displacement is fairly uniform over the whole foundation and the thermal-induced stresses are redistributed towards the central piles. Finally, a real case of a fully monitored group of energy piles is simulated and the numerical results are validated through the available experimental measurements. The satisfactory matching between the numerical and experimental data validates the thermo-hydro-mechanical finite element formulation approach. Taking into account the knowledge acquired throughout the previous experimental and numerical investigations, the validated model is used to study further configurations and extreme thermal loading conditions. In conclusion, in the normal working conditions of current applications, both the thermal-induced foundation displacements and piles compression stresses are acceptable with respect to the norms in use, even in soils showing a thermoplastic response. The two most critical issues are the eventual tensile stresses developed during cooling and the mechanical cyclic degradation is sandy soils. The first aspect is specific for energy piles and can be faced by accounting for that in the design practice. The second one has been largely studied in the literature for other applications and the available results could be also applied in the case of energy piles. As soon as the thermal solicitation becomes more significant, all these aspects are enhanced and taking them into consideration in the design practice becomes fundamental.