Outstanding Issues in Understanding the Behaviour of Thermally-Activated Pile Foundations-Paper

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Thermally activated (TA) piles use building foundations for shallow geothermal heating and cooling. Their response to a single, monotonic temperature change is generally well understood: greater restraint from the surrounding soil reduces pile movement and increases thermal stress. Remaining uncertainties concern long-term cyclic heating and cooling, particularly whether cycles cause excessive settlement or structural stress.<br /><br />The paper reviews tests and presents numerical analyses of isolated piles and pile groups. Analyses indicate that cyclic, irreversible settlement and rising thermal compression are strongly linked to how much shaft resistance is already mobilised by the building’s mechanical load. When shaft resistance is highly mobilised before thermal operation, cooling can reduce shaft restraint, while repeated heating and cooling can progressively transfer load to the pile base. This produces settlement and increasing stress, resembling ratcheting observed in some small-scale tests. Keeping initial shaft-resistance mobilisation below roughly 80–90% appears to limit this response, although further work is needed to confirm the threshold.<br /><br />Pile spacing and group effects also matter: nearby piles can suppress shaft resistance and reduce thermal stress variation, though movements may increase. The authors caution that simplified design approaches may miss soil thermal behaviour, group interactions, or the importance of initial shaft mobilisation. They recommend using suitable thermo-mechanical soil–structure interaction analyses where needed, while carefully checking their assumptions.<br /><br />Key unresolved issues include the generality of the proposed mobilisation limit, thermally induced volume changes in soils, possible pore-pressure effects in fine-grained soils, and the mechanisms behind ratcheting observed in experiments. Further full-scale cyclic testing and improved modelling are needed to establish reliable design criteria for the long-term geotechnical and structural safety of TA pile foundations.

Keywords

thermally activated piles

geothermal heating and cooling

cyclic thermal loading

settlement

thermal stress

shaft resistance mobilisation

soil-structure interaction

pile groups

ratcheting

full-scale cyclic testing