Thermal Interference and Thermo-Mechanical Performance of Energy Pile Groups: Effects of Pile Spacing, Layout, and Soil Thermophysical Properties

Thermal interference among neighboring piles can substantially alter the performance of energy pile groups, and its influence on heat transfer and deformation depends on pile position, spacing, layout, and ground properties. This study developed a three-dimensional thermo-mechanical numerical model to investigate these effects for energy pile groups equipped with U-type, W-type, and 3U-type heat exchangers. The thermal response was validated against a field thermal performance test, with a relative deviation of about 10%, and the mechanical component was additionally assessed against measured pile stress data from an independent single-pile test. Simulations over 700 h under summer heat rejection conditions showed that thermal interference intensified as the temperature fields of adjacent piles expanded and overlapped. Central piles experienced stronger interference than edge piles, and the resulting increase in outlet temperature was most pronounced for the 3U-type group. Increasing pile spacing reduced thermal field overlap and pile head displacement, although the additional benefit became limited beyond about 4–5 m within the 700 h simulation; this range is not a general design threshold. Differences between the investigated nine-pile square system and eight-pile staggered system were smaller than those associated with pile spacing. Because the two systems contained different numbers of piles, these differences are interpreted as system-level responses rather than isolated layout effects. For the central pile of the 3U-type group, increasing soil thermal conductivity, specific heat capacity, and density generally improved heat rejection and reduced the predicted pile head displacement within the investigated parameter ranges, with specific heat capacity producing the largest absolute variation in outlet water temperature under the investigated 700 h summer heat rejection conditions. These results indicate that energy pile group performance is governed by the combined effects of spatial configuration and ground thermal properties and cannot be inferred from isolated pile behavior alone.