Understanding the thermo-mechanical response of heat exchanger piles

Although the adaptation of deep foundations into heat exchangers is receiving growing attention, geothermal research is focusing mostly on energy optimization aspects. Meanwhile, the primary bearing function of the foundation must remain unaltered. Also, provided that soils are naturally prone to dilation or contraction upon heating-cooling cycles, ensuring all the time the lateral contact between pile and soil is a critical issue. The paper is a contribution to the verification and optimization of mechanical dimensioning of heat exchanger piles. Besides, the soil behavior in the vicinity of the geothermal structure is assessed with respect to temperature diffusion throughout the medium. An experimental programme is carried out on a real scale instrumented heat exchanger pile, featuring cyclic variations in temperature in the embedded pipes. A thermo-hydro-mechanical (THM) elastoplastic model for unsaturated porous media is introduced to simulate the behavior of the heat pile and soil. A finite element analysis of the in-situ test pile is then carried out to assess the validity and capabilities of the innovative modeling framework. The numerical results show good comparability with field measurements. Further numerical investigation on a pile group predicts the pile and soil response to thermal loads beyond the conventional temperature range seen previously. Simulations predict thermo-plastic behavior which is harmful for the soil-foundation contact.