Performance-based Design of Energy Pile Foundations

Over the past twenty years, an increasing amount of research has been performed to understand the multiphysical behaviour and to address the geotechnical and structural design of so-called energy piles, i.e. deep foundations that can serve any superstructure as both structural supports and geothermal heat exchangers. The coupled application of thermal and mechanical loads to energy piles, due to their multifunctional operation, represents a challenge. Currently, knowledge about the response of energy piles subjected to thermal and mechanical loads is accessible, along with some design guidance. However, this knowledge is fragmented and no recognised performance-based design framework is available. Looking at such challenge, this paper presents a theoretical and experimental analysis of the multiphysical behaviour of energy piles, as well as a performance-based design framework for such foundations. The work highlights that thermal loads involve effects that can be neglected in the design of energy piles at ultimate limit states and can be considered relevant only at serviceability limit states. Based on this result, the performance-based design of energy piles at ultimate limit states reduces to a conventional pile design process while the design at serviceability limit states must account for a number of proposed provisions and verifications.