Energy pile foundation technology is developed based on the fact that the ground below a certain depth (i.e. 10 m) has a relatively constant temperature throughout a year. Energy pile serves as heat transfer medium, which can discharge excess heat into the ground in hot summer and extract heat from the ground in cold winter. It is reported that the use of energy piles can reduce energy consumption by 66% as compared with conventional heating and cooling systems. However, during seasonal operations of energy piles, the cyclic temperature changes in piles and surrounding soils may induce excessive settlements of piles. Currently, the understanding of thermo-mechanical interaction between energy pile and soil is still quite limited.

The objectives of this research are to investigate displ...[ Read more ]

Energy pile foundation technology is developed based on the fact that the ground below a certain depth (i.e. 10 m) has a relatively constant temperature throughout a year. Energy pile serves as heat transfer medium, which can discharge excess heat into the ground in hot summer and extract heat from the ground in cold winter. It is reported that the use of energy piles can reduce energy consumption by 66% as compared with conventional heating and cooling systems. However, during seasonal operations of energy piles, the cyclic temperature changes in piles and surrounding soils may induce excessive settlements of piles. Currently, the understanding of thermo-mechanical interaction between energy pile and soil is still quite limited.

The objectives of this research are to investigate displacement and capacity of single energy piles under heating and cooling operations in centrifuge. The first series of centrifuge model tests was conducted in saturated medium dense Toyoura sand. In total, four in-flight pile load tests under different temperatures (i.e. 22°C, 37°C and 52°C) and different loading sequences were carried out. Each aluminum pile had a diameter (D) and an embedded length (L) of 0.88 m and 18.8 m, respectively. Ten pairs of foil gauges and thermocouples were installed uniformly along each pile shaft to measure axial load distributions and temperature profiles. The second series of centrifuge tests was to investigate the effects of heating and cooling cycles on the long-term displacement pattern of single energy piles in lightly (overconsolidation ratio (OCR)=1.7) and heavily (OCR=4.7) overconsolidated clays. Five thermal cycles with the temperature in each energy pile varying between 9°C and 38°C were applied by a newly developed heating and cooling system. Each pile had an embedded depth of 16.8 m. Four pore pressure transducers (PPTs) were installed at 0.9D away from each pile surface to monitor the variations of pore water pressure (PWP) in clay during consolidation and thermal operation of energy piles.

It is found that for a model pile in saturated sand, after heating at zero applied axial load, toe resistance was mobilized as a result of constrained downward thermal expansion of pile. Moreover, heating to a higher temperature caused the neutral plane (NP) to shift towards pile toe due to a larger degree of mobilization of end-bearing resistance. When a pile was subjected to a maintained working load, the measured peak heave was 1.4%D after the temperature of pile increased by 30°C. The heave gradually reduced to 0.8%D after 4 months of continuous heating at the constant temperature. Subsequent pile load tests show that pile capacity increased by 13% and 30% with an incremental temperature of 15°C and 30°C, respectively. With elevated temperatures, shaft resistance increased but at a reduced rate. While toe resistance increased more rapidly than shaft resistance due to a larger downward expansion of the pile. It is observed that an earth pressure coefficient with a value of 1.1K₀ and 1.3K₀ was found to be suitable for estimating the capacity of aluminum model pile with a temperature increment of 15°C and 30°C, respectively. For both energy piles in saturated clay under a constant working load, ratcheting displacement mechanisms were observed during the five cycles of heating and cooling. The pile in lightly overconsolidated clay settled continuously but at a reduced rate and the settlement eventually reached 3.8%D. This may be due to stress reduction caused by plastic contraction of clay and thermally accelerated creep at pile-soil interface. In comparison, a relatively smaller settlement of 2.1%D was observed for pile in heavily overconsolidated clay.