Energy pile technology is a popular means of heating and cooling buildings and has high potential applicability. However, energy pile design does not currently consider the thermo-mechanical interactions between the energy pile and surrounding soil appropriately due to a lack of understanding of behavior around the soil pile interface, which has seen little in-situ testing. In this study, a 74 day full scale in situ test of a large diameter over length energy pile (1000-mm-diameter, 44-m-long) in silty clay was carried in Nanjing, China, followed by a one year follow up monitoring period. The thermal load was applied by the constant temperature method, and contact pressures at the soil pile interface and pore water pressures in the surrounding soil were monitored to analyze the mechanism affecting shaft resistance. The results indicate that, during heating, there was a slight change in the radial effective contact pressure at a clay–pile interface but a 55.3 kPa maximum effective pressure change at a sand–pile interface. This suggests that the contribution of changes in the radial effective pressure at sand–pile interfaces to shaft friction should not be ignored.