# Thermomechanical Behavior of Parallel Multi-U-Shaped Energy Piles Under the Summer Condition
## 1 Introduction
[17] thought that the pile body expands or contracts around the \"zero point\" in the pile when the temperature difference in the pile body is assumed to be uniformly distributed, [PERSON] et al. [32] thought that the temperature stresses caused by the heat transfer process are not uniformly distributed and create additional compressive stresses of more than 2 MPa in the middle and lower parts of the pile body. According to [PERSON] et al. [33], the binding force becomes pressure when the pile body is heated, and the side frictional resistance brought on by the pile body's expansion and deformation is oriented in opposite directions in the upper and lower portions of the pile body. This results in the greatest additional stresses in the center of the pile body. [PERSON] et al. [34] unfolded the thermal response test of energy piles through modelling tests and found that different forms of embedded tubes lead to an uneven distribution of the internal temperatures of the piles, which affects the reduction in the pile's load carrying capacity. [PERSON] et al. [35] concluded that the uneven thermal conductivity of layered soils significantly affects the temperature growth of piles, with a neutral point near the lower 2/3 of the pile. [PERSON] et al. [36] investigated the thermomechanical properties of a PCC energy pile model under alternating hot and cold cycles and found that under the cooling cycle, a large tensile stress is generated at the pile end. [PERSON] et al. [37] simulated the thermomechanical properties of a single and group of helical energy piles in the summer and winter seasons by means of a model test and found that the pile body axial force increases under summer conditions, and the opposite is true in winter conditions. By combining field experiments and numerical simulations, [PERSON] et al. [38] examined the thermodynamic characteristics of energy heaps under various soil conditions and discovered that the pile's stress-strain relationship would be impacted by the soil's elastic modulus and thermal expansion. However, most of the above studies only investigated the thermomechanical behavior of energy piles under a single soil condition and did not carry out a large number of studies on the influence of the thermomechanical behavior of surrounding soils on the thermomechanical behavior of energy piles under various soil conditions. Therefore, the understanding of multiple soil conditions is limited, and further studies are needed, which will help in the practical engineering of energy piles. Existing energy pile tests are mostly in the form of single- or double-U series and parallel connections, and there are relatively few in situ tests and three-dimensional numerical simulations regarding the thermomechanical behavior of multi-U energy piles. As large-diameter pile foundation projects become more and more common, the thermomechanical behavior of multi-U energy piles needs to be studied in depth. Therefore, this paper explores the variation rules of the pile-soil temperature, stress-strain, side friction resistance, and pile-top displacement of full-scale parallel multi-U energy piles under temperature loading by combining in-field test and numerical simulation, aiming to provide a basis and help for the design of parallel multi-U buried energy piles.