Failure mechanism of fine-grained soil-structure interface for energy piles

The response of the soil-structure interface can significantly affect the performance of any geotechnical structure. Thermal cycles are a new factor that influence the response to all structures that have an energy function in addition to the structural one, such as energy piles foundations. In this study, a theoretical interpretation of the failure mechanism at the pile-soil interface subjected to cyclic thermal loads and a dedicated constitutive model is presented. The phenomenon is characterised by large localised strains concentrated in a thin layer around the pile surrounded by soil behaving under oedometric conditions. The theoretical framework refers to direct shear tests at a constant normal stiffness, as well as oedometric tests on clayey soil. Observations indicate a negligible effect of the temperature cycles on the soil-structure interface response for typical temperature ranges of energy piles (delta T = −10, +20 °C). Therefore, an isothermal mathematical formulation in the framework of elastoplasticity is proposed for the analysis of both conventional and energy piles. The effectiveness of the interface model in reproducing the observed behaviour is confirmed via experimental tests. The proposed constitutive model is applicable in engineering practice via the finite-element or the load-transfer method. Additionally, its calibration requires exclusively conventional geotechnical testing.