## 5 Conclusions  
In this paper, the thermomechanical behavior of DBP-EP under different temperature loads and mechanical loads are investigated, and the changing rules of the thermal variations in the soil surrounding the DBP-EP, the mechanical response of the pile itself, the earth pressure at the pile toe, and the displacement occurring at the pile's top during the heating phase across various operational conditions are analyzed by means of indoor model tests. The following conclusions were obtained:  
1. By examining the mechanical response of DBP-EP and monitoring the variations in side friction resistance at the base of the pile over time under various top load conditions, it becomes evident that when the pile top remains unrestrained, the side friction resistance at the pile's bottom exhibits a gradual upward trend. In contrast, when the top of the pile is restrained, the alterations in side friction resistance at the bottom exhibit minimal changes. This phenomenon can be attributed to the application of a consistent load on the pile's top prior to the commencement of the test, leading to a densified state of the sandy soil surrounding the base. This indicates that the approach of applying the top load during the heating of energy piles may influence thermomechanical behavior, to a certain degree. 2. When the DBP-EP is subjected to temperature load, it is observed that the influence of the inlet temperature on the vertical load at the pile toe diminishes progressively as the constant load applied at the top of the pile increases. Specifically, the end bearing capacity--representing the resistance provided by the soil at the pile tip--shows a gradual decline in percentages in relation to the growing load at the top of the pile. When the pile top load is increased from 0.26 kN to 0.78 kN, the percentage decrease in the vertical load at the pile toe is 2.2%, 4.76%, and 8.51%, respectively, and it can be found that the percentage decrease increases multiplicatively. Such findings suggest  
Figure 9: Diagram of pile top displacement with time under different working conditions. (a) \(F_{top}=0\) kN. (b) \(F_{top}=0.26\) kN. (c) \(F_{top}=0.78\) kN. that as the load atop the pile intensifies, the downdrag effect exerted by the sandy soil encircling the pile toe on the vertical load at that location gradually diminishes. 3. As the operational duration of DBP-EP increases, the rate of pile top displacement rises more rapidly during the initial phase of operation. Subsequently, after approximately 8 h of running, this rate of displacement growth gradually declines. Once the pile stabilizes after 24 h of operation, it is observed that for every 1 \({}^{\circ}\)C rise in inlet temperature, the final displacement of the pile top reduces by roughly 0.03%.D. This indicates that altering the inlet temperature and the load on the pile top, along with choosing suitable operational conditions, can effectively prevent significant displacement of the pile top. The above conclusions summarize the thermodynamic behavior of DBP-EP under different temperature loads and mechanical loads. It can be found that the thermodynamic behavior of DBP-EP is also affected by various factors, such as pile material, soil type, and operation mode. Therefore, it is necessary to carry out further related research in the future to provide guidance and suggestions for the practical engineering applications for DBP-EP. Methodology, Z.C. ; conceptualization, J.Y., Z.C. and Y.Z. ; software, J.Y. ; validation, J.Y., Z.C. and Y.Z. ; data curation, J.Y. ; formal analysis, J.Y. ; writing--review and editing, J.Y. and Z.C. ; investigation, Z.C. ; resources, Z.C. ; writing--original draft preparation, J.Y. ; supervision, Z.C., Y.Z. and Y.L. ; visualization, J.Y. ; project administration, Z.C., Y.Z. and Y.L. ; funding acquisition, Z.C. All authors have read and agreed to the published version of the manuscript. This research was funded by the Key Research and Development Program of Hubei Province (No.