Revisiting the effect of coefficient of thermal expansion in energy pile–soil interactions

When pile foundations and the ground around them are thermally-activated as part of a shallow geothermal energy system, new and somewhat complex changes in pile-soil interaction occur. In this paper, the influence of the relative expansion and contraction between the pile and surrounding soil, as expressed via the ratio of the coefficient of thermal expansion (CTE) of the soil to that of the pile, is revisited. Previous steady-state thermo-mechanical analysis by the authors suggested that this may be a very significant parameter especially when the CTE ratio was somewhat greater than 1 but more recent work has shown there are a number of other factors which may work to either mitigate or exacerbate this effect. This paper brings together recent work relating to this effect, to provide guidance on the impact of the form of the thermal loading in time and the initial mobilisation of the pile shaft resistance on when the CTE ratio may be significant in the functioning of thermally-activated pile foundations.