Abstract
Energy piles represent a sustainable synergy between structural foundations and geothermal energy systems, yet their application in high-load infrastructure remains limited by a lack of data on large-scale geometries. This paper presents a full-scale field study on a rock-socketed energy pile (length 44.75m, diameter 1.50m) equipped with a double-helix heat exchanger subjected to 240hours of constant heating (7.4kW). The results demonstrate a high unit-length heat exchange rate of 145.70W/m, with a comprehensive thermal conductivity of 2.93W/(m‧K), confirming the thermal efficiency of helical configurations in extra-long piles. Despite the intense boundary constraints of a rock-socketed tip, the observed thermomechanical strain maintains a remarkably robust linear correlation with temperature (R² ≥ 0.99) across all sections. Based on this relationship, a novel steady-state analytical framework is developed to extrapolate short-term field data into long-term structural design limits, specifically predicting an additional steady-state axial force of 2548.71 kN at the tip and 4.10mm expansion at the top, with the zero-displacement point located near the rock-socketed section. Comparison with a nearby conventional pile static load test confirms that the thermal-induced mechanical response remains well within safe structural limits. The double-helix configuration ensures uniform temperature distribution, enhancing both thermal efficiency and structural stability. This study provides valuable insights and steady state analytical framework for the design and application of extra-long and large-diameter energy piles in high-load infrastructure.
•A full-scale field test on a 44.75m long, 1.50m diameter rock-socketed energy pile with a double-helix heat exchanger is conducted under 240hours of constant heating.•The double-helix configuration achieves a high heat exchange rate of 145.70W/m and a comprehensive thermal conductivity of 2.93W/(m·K), demonstrating superior thermal performance.•A robust linear relationship (R² ≥ 0.99) between thermal-induced strain and temperature is observed across all pile sections, enabling a novel steady-state analytical framework to predict long-term thermomechanical behavior from short-term data.•Under steady-state conditions, the additional axial force at the pile tip is predicted to be 2548.71 kN, with a top expansion of 4.10mm and the zero-displacement point located near the rock-socketed section.•Comparative analysis with a conventional pile static load test confirms that the thermal-induced mechanical response remains within safe structural limits, supporting the feasibility of large-diameter energy piles in high-load infrastructure.