The intensification of human activities an‎d the growing dependence on fossil resources in recent decades have led to environmental crises such as global warming, greenhouse gas emissions, an‎d climate instability. Under these circumstances, the use of renewable energies, including geothermal energy, has received increasing attention as a sustainable approach. One of the modern methods of utilizing this energy is the application of energy piles in foundations, which, in addition to bearing structural loads, function as heat exchangers between the ground an‎d the circulating fluid. Their thermo-mechanical behavior under the simultaneous influence of thermal an‎d mechanical loads is complex an‎d requires detailed investigation. In this regard, the present study aims to examine the thermo-mechanical behavior of an energy pile an‎d the amount of extractable energy through the design an‎d implementation of a physical laboratory model. For this purpose, a closed-ended aluminum tube with an embedment depth of 350 mm was used as the model pile, equipped with an internal helical copper tube an‎d a water circulation system. The soil used in the tests was an inorganic clay with low plasticity (CL), whose characteristics were determined based on index tests. The specimens were prepared under unsaturated conditions such that the applied water content was 13%, lower than the optimum moisture content of 21.5%, to better simulate field conditions. For model preparation, the soil was compacted in ten 5-cm layers to achieve a dry unit weight of 1.43 g/cm³. To record temperature changes an‎d settlement, multiple LM35 IC temperature sensors with an accuracy of 0.1 °C were installed inside the pile an‎d in the surrounding soil, while an analog dial gauge with an accuracy of 0.01 mm was mounted on the pile head. The experiment was conducted in two stages. In the first stage, five heating–recovery an‎d five cooling–recovery cycles were performed without mechanical loading to investigate the effect of temperature variations on the pile’s thermal behavior. In the second stage, the same cycles were carried out under a constant mechanical load equal to approximately 50% of the reference bearing capacity (1.095 kN, equivalent to 111.66 kg) to analyze the thermo-mechanical behavior an‎d assess the influence of thermal cycles on pile settlement. The main outputs included the temperature transfer pattern along the pile an‎d surrounding soil, the amount of harvested thermal energy, an‎d the pile head displacement under coupled thermal–mechanical loading.The results indicated that the model boundaries were effectively insulated. During heating, the internal pile temperature was approximately 1.8 °C lower than the inlet water temperature, while during cooling it was about 0.8 °C higher. The outlet water temperature during heating was 1 °C lower than the inlet water temperature an‎d higher than the internal pile temperature, whereas during cooling, the outlet water was approximately 1 °C warmer than the inlet water an‎d slightly higher than the internal pile temperature. This behavior appears to result from heat conduction, the high thermal capacity of the pile an‎d soil, an‎d the limited residence time of the circulating fluid. Applying mechanical load reduced the inlet–outlet temperature difference an‎d consequently decreased the heat transfer rate. The harvested thermal energy during heating an‎d cooling without mechanical loading was 3.06 MJ an‎d 3.27 MJ, respectively, which decreased to 2.77 MJ an‎d 1.90 MJ under load—demonstrating the system’s considerable efficiency even in the presence of mechanical stress. Additionally, the settlement under the stan‎dard load after five heating–recovery an‎d five cooling–recovery cycles decreased by about 50%, indicating enhanced soil cohesion, improved effective strength of the clay, an‎d partial improvement of pile–soil contact due to thermal cycling.

هاجر شرع اصفهاني , محمدرضا خان محمدي

محمدعلي روشن ضمير , امير اكبري گركاني