Centrifuge modelling of heating effects on energy pile performance in saturated sand

The operation of energy pile in summer can expel excessive heat of the buildings into the ground by the use of a heat pump. Despite of having been implemented for decades, the design of energy pile still relies heavily on empiricism as there is still limited understanding about heating effects on pile capacity. A series of centrifuge model tests in saturated sand is reported in this study to investigate heating effects on the settlement patterns as well as capacities of single piles. In total, four in-flight pile load tests under different temperatures (i.e. 22℃, 37℃ and 52℃) were carried out. Variations of pile capacity were interpreted with the help of a non-linear elastic analysis. The test results show that after heating at zero applied axial load, toe resistance of pile was mobilized as a result of constrained downward thermal expansion of pile. Heating to a higher temperature caused thermally induced the neutral plane (NP) to shift towards the pile toe. It is also found that for pile under a maintained working load, a temperature increase of 30℃ made the pile head initially heave by 12.5 mm and gradually settle by 5.1 mm after 4 months of continuous heating. The post pile settlement is believed to be caused by thermal contraction of sand. Subsequent pile load tests showed that pile capacity increased by 13% and 30% with an incremental temperature of 15℃ and 30℃, respectively. With an increasing temperature, shaft resistance increased but at with a reducing rate. At a higher temperature increment, In contrast, toe resistance increased more rapidly than shaft resistance due to a larger downward expansion of the pile at higher temperature increment. For simplicity, earth pressure coefficient with a value of 1.1K0 and 1.3K0 was found to be suitable for estimating the capacity of model pile with a temperature increment of 15℃ and 30℃, respectively.