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Thermally induced group effects characterise closely spaced energy piles. It has been observed experimentally that the behaviour of energy piles subjected to mechanical and thermal loads, in which the piles are located sufficiently close to each other, is different from the behaviour of single isolated piles. Therefore, civil engineers encounter new challenges in the geotechnical design of such foundations. This leads to the necessity to develop practical tools to address their analysis and design. The conventional load transfer method is one of the commonly used methods for the analysis of axially loaded conventional piles. Thus, the purpose of this study has been to propose a formulation of the load transfer method to consider the thermally induced effects among energy piles in groups. The soil response is characterized in a lumped form by ascribing the behavioural features of the soil to interface elements. The individual response, in terms of strain and stress of an energy pile in a group, can be addressed for the first time through the application of the displacement factor in the load displacement curve of the single isolated energy pile. A validation through a full-scale field test reveals the capability of the approach to provide the necessary information in the analysis and design phases of the foundation for one-way thermal loads.

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Understanding the behaviour of soil-structure interfaces is critical for addressing the analysis and design of energy geostructures. In this study, the interface failure mechanism of energy piles (where a shear band is detached from the surrounding soil that behaves under oedometric conditions) is experimentally analysed in laboratory for saturated conditions. The choice of material (clayey soil and concrete), temperature range, and stress level is based on conditions that are likely to be encountered in practice. Specifically, cyclic thermal tests under constant vertical effective stress in oedometric conditions as well as constant normal stiffness (CNS) interface direct shear tests (in which samples have been subjected to thermal cycles between 10 and 40 °C) are presented. From a practical perspective, the results show very low volumetric strain variations and negligible effects on shear strength. The volumetric aspects do not appear to have significant impact on the shear resistance of the interfaces against cyclic thermal loads. Fundamental insight on the effects of thermal cycles on the concrete-soil interface behaviour which are relevant to energy piles are presented. In addition, the proposed interpretation procedure provides a basis for the standardisation of thermomechanical testing in geotechnical engineering.