Thermo-mechanical behavior of steel pipe energy piles under thermal imbalance cycles

There is an effect on the thermo-mechanical behavior of the energy pile from the thermal imbalance conditions (heating with different amplitude than cooling). In this study, 5 cycles with a heating amplitude of 25 degrees C and a cooling amplitude of 15 degrees C were performed on ordinary concrete energy piles (OCEP), steel pipe concrete energy piles (SPEP), and steel pipe phase change energy piles (SCEP), respectively. The results show that imbalanced heating and cooling conditions alter the temperature distribution of the energy pile, causing irreversible displacement accumulation. However, including steel tubes reduces the loss of load bearing capacity caused by the heating and cooling cycles. The maximum increase in pile temperature was 6.7 degrees C for OCEP, 7.9 degrees C for SPEP, and 7.1 degrees C for SCEP. The final normalized displacement rates were -0.462 %, -0.558 %, and -0.515 %, respectively. The ultimate bearing capacity of the piles was reduced by 10 %, 9.1 %, and 9.1 %, respectively. The heat exchange efficiency of the 5th heating and cooling phases of SCEP increased by 67.4 % and 65 % compared to OCEP and by 26.3 % and 20 % compared to SPEP, finally stabilizing at 72 W/m and -66 W/m, respectively. This study provides a qualitative analysis of energy piles operating under heating and cooling imbalance conditions.