## 3 DBP-EP Pile Soil Thermal Response Test  
_DBP-EP Thermal Response Analysis_  
Figure 4a illustrates the temperature variation of the heat exchanger tube along the depth direction over time. During the entire heating process, the water inlet temperature remains constant after the circulating water is introduced for one hour. As the circulating water moves through the heat exchanger tube, there is a noticeable change in temperature along the depth of the tube. Specifically, the temperature at the inlet side experiences a gradual decrease. This decline in temperature is consistent as the water travels deeper into the tube. Conversely, on the outlet side, the temperature shows a steady increase as it moves along the same depth direction. This contrasting thermal behavior highlights the effective heat transfer process occurring within the heat exchanger, where the circulating water relinquishes heat on its way in, while the outgoing water emerges at a lower temperature. This indicates that the circulating water continuously exchanges heat with the pile body as it flows through the heat exchanger tube. As the inlet temperature increases, the difference in temperature between the inlet and outlet also expands. When the inlet temperature increased from 30 \({}^{\circ}\)C to 35 \({}^{\circ}\)C, the temperature difference between the inlet and outlet increased by 1.27 \({}^{\circ}\)C. When the inlet temperature increased from 35 \({}^{\circ}\)C to 40 \({}^{\circ}\)C, the temperature difference between the inlet and outlet increased by 1.47 \({}^{\circ}\)C. It is worth noting that for every 1 \({}^{\circ}\)C increase in temperature, the temperature difference between the inlet and outlet increased by approximately 0.27 \({}^{\circ}\)C. Illustrated in Figure 4b is the variation in pile temperature with depth as time progresses. The pile temperature gradually increases over time, with the rate of change diminishing after 8 h. The temperature inside the pile is higher than the temperature at the pile side, and notably, the difference between these temperatures increases as the inlet temperature rises. The average temperature at the pile end was greater compared to the overall average temperature of the pile body as time increased, as shown in Table 2. This is different from the findings reported in the IBP-EP thermal response tests [30,31,50], and this discrepancy may be attributed to the fact that DBP-EP requires the heat exchanger tubes to extend downward at the end of the piles, and thus, the temperatures are greater at the end of the piles. This suggests that more heat buildup occurs inside the pile when DBP-EP is introduced at a higher inlet water temperature, which could be more pronounced if the number of heat exchanger tubes increases. Figure 5 illustrates the temperature variation of the pile-soil interface along the horizontal direction during the operation of the DBP-EP system. It can be observed that the energy pile generates a thermal exchange, leading to changes in the temperature of the surrounding soil. The data presented in the figure indicate that the operation of the energy pile has a significant impact on the temperature of the adjacent soil. Furthermore, it is evident that the extent of the temperature variations in the soil is correlated with the horizontal distance from the energy pile, suggesting that the thermal influence diminishes as the temperature point moves further away from the pile. Combined with the average temperature change curve in the horizontal direction in the figure, it can be observed that