Experimental study on the thermo-mechanical behavior of steel pipe energy pile groups with and without phase change material

Energy piles have attracted global interest as a low-carbon, environmentally friendly and cost-effective underground energy structure. However, there were fewer studies on the thermo-mechanical behavior of steel pipe energy pile groups. The thermo-mechanical behavior of steel pipe concrete energy pile groups (SPEPG) and steel pipe phase change concrete energy pile groups (SCEPG) have been analyzed and compared by means of an indoor model test under thermal imbalance cycles (heating with different amplitude than cooling). The results indicated that by the third heating phase, the heat exchange power of the SCEPG stabilized at 73.3 W, which was 16% higher than that of the SPEPG. During the cooling phase, the heat exchange power of the SCEPG was stable at 96.7 W, which was 18% higher than that of the SPEPG. Furthermore, the average heat exchange efficiency of each pile in the pile group was lower than that of a single pile, with the maximum heat exchange power occurring when the pile spacing was four times the pile diameter. In terms of mechanical performance, the maximum thermally induced stress applied to the SPEPG and SCEPG were 485.0 kPa and 441.3 kPa, respectively, accounting for 2.3% and 2.1% of the design tensile strength of the steel pipe pile. The pile head displacement of the steel pipe energy pile groups was primarily determined by the displacement after the first cycle, and the accumulation rate of pile head displacement decreases as the number of cycles increases. After three cycles, the pile head displacement of the SCEPG decreased by 9.1% compared to that of the SPEPG. This study provides a reference for the application of steel pipe energy pile groups with and without phase change material in hot climate regions.