Experimental and numerical investigations on thermo-mechanical behaviors of energy pile group under different operational strategies

Energy piles are often operated in the form of pile group, and their thermo-mechanical (TM) behaviors are strongly affected by the operational strategies. A model experimental bench of energy pile group with the layout of 3 × 3 was established to investigate the TM behaviors of energy pile group under different start-stop time ratios and pipe connection forms. The test results show that under the current test conditions, raising the start-stop time ratio leads to the increase of heat storage and extraction amount by the pile group, but the temperature of soil around the pile is not well recovered, which in turn results in the rise of deformation degree of pile. For the connection forms of pile buried pipe, the heat storage amount under three kinds of series connection forms is less than that under the conventional parallel connection form. However, the mechanical properties of pile group under the series connection forms are better than those of parallel connection form, which helps to alleviate the pile deformation degree. A 4 × 4 group pile model was developed to further find the effects of two operation strategies, namely, opening partial piles at different locations in winter mode and non-uniform intensity operation of piles in the inner and outer zones, on the TM characteristics of energy pile group. The results showed that opening partial piles increased the axial force of pile in the running piles relative to opening all the piles. Meanwhile, opening eight side piles extracted more heat per month during the heat extraction period relative to opening four internal piles and four corner piles. The four kinds of internal and external zoned non-uniform strength operation modes have different energy storage and thermodynamic properties. The most suitable mode should be selected considering the actual situation.