Thermo-mechanical radial expansion of heat exchanger piles and possible effects on contact pressures at pile-soil interface

This letter shows that the increase of heat exchanger pile capacity in response to heating, observed in several small-scale laboratory studies, cannot be directly attributed to the increase of contact pressure at the soil-pile interface. The main thermo-hydro-mechanical processes that influence the capacity and behaviour of heat exchanger piles include thermal hardening of the soil, thermally induced water flow, excess pore pressure development and volume changes upon thermal consolidation. Due to the lack of understanding of the behaviour around the soil-pile interface, thermo-mechanical interactions between the heat exchanger pile and the ground are not taken into account appropriately in energy foundation design. However, in situ and reduced-scale experiments provide evidence about temperature-induced changes in pile capacity, presumably as a result of the altered stress state around the test pile. A finite-element analysis was conducted to quantitatively assess the radial stresses and strains undergone by a heated pile embedded in deformable soil. The study indicates that radial contact pressures typically increase less than 15 kPa, which cannot fully explain the increase in shaft resistance observed in heating tests. Further analyses are underway to characterise the mechanisms that govern pile load-displacement behaviour and the limit state.