Investigating the performance of geothermal energy piles using coupled thermo-hydro-mechanical finite element analyses

Abstract

Harvesting shallow geothermal energy by means of energy piles coupled with ground source heat pump systems for heating and cooling buildings has increased in recent years. However, the structural integrity of such systems subjected to thermo-mechanical loads or heating-cooling cycles should be studied. Therefore, a comprehensive understanding of their structural and geotechnical performances is vital for successful applications. This thesis aims to investigate the responses of concrete energy piles subjected to thermal and thermo-mechanical loads using fully coupled thermo-hydro-mechanical (THM) finite element analyses. The axisymmetric models were carried out for two case studies of full-scale energy pile tests. Two hypothetical energy piles in Winnipeg were also analyzed to study their performances by considering local geological and climatic conditions. In general, it was found that the THM numerical models could capture considerably well the behavior of energy piles during cooling and heating cycles in comparison with the field data published in the literature. The thermo-mechanical loads did have significant effects on pile responses. From sensitivity analyses, it was found that there were considerable effects of the thermal expansion of concrete and soil stiffness on the thermo-mechanical pile responses. The pile head restrained conditions also affected the behavior of energy piles with stronger effects in the upper part of the pile near the pile head. From the simulations of the energy piles in Winnipeg, settlements of the pile head kept on increasing with increasing numbers of thermal cycles (ratcheting settlement phenomena). It was also found that the ultimate geotechnical pile capacities generally increased when the pile was heated but reduced when cooled.