Long-term soil thermal imbalance analysis of the energy pile considering ambient thermal boundary

Long-term operation is crucial for energy piles, significantly affecting the pile thermal performance and soil thermal field. However, research on long-term behavior remains limited. This study conducted model tests with a scaled energy pile, involving 1 week of continuous thermal injection, followed by a validated 3D thermo-hydro-mechanical numerical model. The model simulated a 10-year operation under annual ambient temperature in Shanghai and typical building thermal loads. Test results showed ambient temperature significantly influenced the thermal field distribution, even in deeper soil layers within a short period. Long-term simulations revealed that constant or insulated soil surface conditions overestimated pile temperatures and caused excessive soil temperature rises. Soil thermal imbalance expanded from the pile vicinity to surrounding regions, with thermal accumulation shifting downward and forming a transition zone in the upper soil. Regions with soil temperatures exceeding the initial level were defined as the thermal impact zone, simplified as a circular area at the same depth. Over time, the thermal impact range stabilized annually and varied linearly with depth. However, the prolonged operation caused the range to progressively contract toward the pile, indicating depth-wise “shrinkage”. These observations suggest energy pile design can be optimized to mitigate thermal interference on adjacent piles.