A full-scale experimental site with four energy test piles was built on the campus of the Swiss Federal Institute of Technology in Lausanne, Switzerland. This site was used to investigate interaction effects within a group of energy piles. First, the ground constraints were evaluated by testing the piles without any structure on top. Next, each pile was individually tested once the overlying structure was built which provided information on the structural constraints and allowed the quantification of the pile-structure-pile interactions. Finally, the four test piles were simultaneously heated to quantify the group effects. A thermal response test was performed on one pile and the thermohydraulic response of the soil between the piles was monitored with piezometers and thermistors. Load redistributions may occur in mixed foundations, i.e. with conventional and energy piles, because of differential displacements. Conversely, heating an entire foundation increases the individual pile displacements but reduces the differential displacements and consequently the pile thermal efforts. Radial strains may also have a significant impact on the axial thermomechanical response of the piles in stiff soil layers. Heat barely propagated farther than a couple of meters with no significant pore water pressure variation observed.