Thermally induced tensile hoop stresses in energy piles: Implications for design and operation

Energy piles, also known as thermoactive piles, serve a dual purpose: providing structural stability and harvesting shallow geothermal energy. As such, their design must account for both structural and thermal loads. However, current practice typically considers only the axial components of thermal loads. This study aims to investigate thermal loads in three dimensions, with a particular focus on tensile hoop stresses in the clear concrete cover. Through numerical modelling, this study examines the positional and temporal variations of hoop stresses and elucidates the underlying mechanisms. The findings demonstrate that significant tensile hoop stresses can develop within the clear concrete cover during the operation of energy piles, and that these stresses shift positions with seasonal operational changes. Therefore, it is crucial to consider hoop stresses in the design of energy piles to prevent the exceedance of concrete’s structural tensile capacity, a major design concern. A sensitivity analysis was conducted, varying material properties, geometric aspects and operational preferences to identify the relationships between influential parameters and hoop stresses. The study concludes with design and operational recommendations based on these findings.