Thermo-hydraulic behaviour of energy pile in chalk: First experimental characterisation and field-scale performance assessment under unsaturated conditions

Energy piles—foundation elements that simultaneously provide structural support and exchange heat with the ground—offer a direct response to the growing heating and cooling demands of north-western Europe, where chalk is the primary bearing stratum for pile foundations. Despite the widespread occurrence of chalk and its frequently unsaturated state, its coupled thermo-hydraulic behaviour has not yet been characterised as a host medium for energy piles: existing estimates of chalk thermal conductivity correspond to discrete saturation states rather than to a continuous conductivity–saturation relationship established experimentally, representing a gap in geothermal foundation design. This study addresses that gap by delivering the first chalk-specific experimental dataset, constitutive framework, and validated numerical model for energy pile performance in unsaturated chalk. Thermal conductivity was measured on intact chalk specimens using a transient needle-probe across controlled degrees of saturation, and the data were used to calibrate a thermal conductivity function. A fully coupled thermo-hydraulic formulation was implemented in the finite-element framework COMSOL Multiphysics and verified against two independent benchmarks: a field thermal response test on an energy pile in saturated clayey sand and a laboratory model energy-pile test in unsaturated sand. The validated model was then applied to a three-dimensional field-scale energy pile in three chalk formations, and compared with representative sand, silt and clay, to examine the effect of saturation on thermal performance under a seasonal heating–cooling cycle. For the investigated configuration, increasing GWT depth from 0 to 29 m reduces the end-of-extraction heat rate by 6.1%, 8.8% and 11.9% for Estreux, Château-Landon and Seaford chalk, respectively, compared with 3.6% for clay, 8.6% for silt and 26.0% for sand. At 29 m, the chalk cases retain 159.4–169.9 W m − 1 , compared with 135.5 W m − 1 for sand. This crossover highlights the potential of water-retentive chalk under deep groundwater conditions, subject to the adopted material properties and modelling conditions.