Energy piles are shallow geothermal systems integrated into structures already required for structural functions of civil works that provide both geothermal energy supply and structural support to the built environments. This research work is driven by the need to convey scientifically based answers that meet the needs of practice for the thermomechanical design of energy piles. Two main challenges are part of this thesis: (i) the understanding of the mechanism that govern the behaviour of energy pile-soil interface and (ii) the development of theoretical methods for design purposes. The multifunctional roles of this technology require to investigate new thermomechanical effects related to daily and seasonal temperature variations during their lifetime, thermally induced effects on pile and soil behaviour, as well as structure-pile-soil interaction. The failure mechanism at the pile-soil interface subjected to cyclic thermal loads is investigated from experimental and constitutive perspectives. The effects of thermal cycling and negative temperatures on pile-soil interface behaviour are presented. Experimental procedures and constitutive models for more advanced design analyses are proposed to help develop reliable long-term predictions of the behaviour and performance of energy pile groups. The results suggest that (i) there is no reduction of the shear strength of the fine-grained soil-concrete interface when subjected to cyclic thermal loading in saturated condition, and (ii) no analysis can be considered complete for a robust design of the system without addressing the risk of freezing. The utility of developing theoretical methods enables the examination of complex practical problems in a systematic, albeit approximate, way for routine design. The majority of theoretical procedures has been originally presented for conventional pile groups with some of them modified to address the different type of loads characterising energy pile groups. However, some gaps are still present in the literature especially with regard to the analysis of pile-soil-slab interactions under thermal loads and the analysis of stress-strain behaviour of energy piles in a group. The development of theoretical methods aims to provide analysis tools for the design of energy piles with respect to the previously mentioned shortcomings. Two methods, the interaction factor method and the load transfer method are mathematically formulated to expand and enrich the framework of available analytical and semi-analytical methods. (i) The interaction factor method integrates the available formulation of the method and allows the analysis of the displacement interaction under a broad range of design conditions addressing the pile-soil-slab interactions. (ii) The load transfer method provides a semi-analytical solution to study the thermomechanical response of an energy pile in a group.