Elena Ravera

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

Lyesse Laloui, Elena Ravera

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.

2022

Cyclic thermomechanical behaviour of pile-soil interface and design methods for energy pile foundations

Elena Ravera

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.

EPFL2021

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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.

2020