Fenglei Du

Accurately quantifying the thermal-hydraulic efficiency of large-diameter energy piles is essential for the design of high-capacity geothermal systems, yet the significant thermal inertia of these structures challenges traditional thermal response analysis. This study presents a rigorous experimental investigation into the thermal performance of a 1.5 m diameter energy pile equipped with a single helical pipe configuration. A series of thermal response tests were conducted using a PID-controlled equipment, maintaining injection temperatures at sequential stages of 32.0°C, 35.0°C, and 38.0°C over 150 hours. The heat exchange capacity, pipe flow resistance, and energy conversion efficiency were analyzed. To address the difficulty of reaching thermal equilibrium in large-diameter piles, a non-linear exponential decay model was implemented to predict steady-state temperatures for both return water and pile cross-sections. Results indicate steady-state heat exchange capacities were reliably predicted as 94.5 W/m, 98.0 W/m, and 112.0 W/m for the three temperature stages, respectively. The energy conversion efficiency ranged from 0.68 to 0.73; while hydraulic analysis confirmed a stable unit head loss of 0.037 m/m. This work establishes a methodological framework for determining the lower-bound energy supply capacity of energy piles, providing critical parameters for system optimization and distinguishing the thermodynamic behavior from conventional geotechnical assessments.

Energy piles, combining structural support with ground heat exchange, are increasingly used for sustainable building energy solutions. However, the thermo-mechanical behaviour of large-diameter, extra-long energy piles under real operational conditions remains inadequately understood. This study investigates the bearing characteristics of a 1.5 m diameter, 58.3 m long cast-in-place concrete energy pile installed beneath a liquefied natural gas tank, subjected to staged mechanical and thermal loading. A full-scale in situ experiment was conducted, simulating realistic load conditions, including static mechanical loading, combined thermal–mechanical loading during heating and cooling cycles, and cyclic loading effects. The results indicate that the pile exhibited high heat exchange efficiency, with rates of 115.6 and 130.3 W/m during heating and cooling, respectively. Axial expansion due to thermal loads led to the formation of a neutral plane at approximately −39 m depth, redistributing shaft resistances along the pile. Under heating, negative shaft resistance developed in the upper (−1.5 to −14 m) and lower (−39 to −51 m) parts, while positive resistance was concentrated in the mid-depth. Upon reloading, the negative resistance diminished, demonstrating mechanical dominance in pile behaviour. The pile head settlement reached −11.6 mm under maximum static loading (24 000 kN), with an additional −7.3 mm settlement observed during combined heating–loading. These findings confirm the feasibility of large-diameter energy piles for deep foundations, highlighting their stable thermo-mechanical response and informing future design optimisation for cyclic thermal and mechanical actions.

In this work, field-scale experiments were carried out on two energy piles to investigate the mechanical behavior of the piles when subjected to thermomechanical loads under a liquefied natural gas tank. The results showed that the super-long and large-diameter energy pile exhibited a better heat transfer performance. After pile heating or cooling, the temperature in the mid-depth of the pile increased or decreased rapidly, and at the two ends, the temperature varied relatively slowly. Regarding the observed axial strain, energy pile 1 exhibited predominately compressive deformation during the coupled heating–loading process, while a certain tensile deformation was found near the pile toe of energy pile 2 during the coupled cooling–loading process. Moreover, for both energy piles, positive shaft resistances appeared predominately under both the pure mechanical and coupled thermomechanical conditions, and occasional and local occurrences of negative resistances could be related to the ground conditions on site. The settlement and bearing capacity values of the two energy piles were not significantly affected by the coupled thermomechanical loads, and thus, the serviceability of the gas tank would not diminish.

High-filled red soil typically lacks sufficient bearing capacity, which can pose significant challenges when constructing building foundations. One economical and effective method for the reinforcement of high-filled red soil is the dynamic compaction (DC) method. However, the design parameters for reinforcing high-filled red soil using the DC method are largely based on experience, which indicates the significant value of field results of related engineering practice. In this paper, we report a field study that was carried out to investigate the effect of impact energy on the treatment of super-high-filled ground with red soil in southwestern Yunnan, China, where three pilot DC tests were designed and conducted with three different impact energies (4000 kN·m, 8000 kN·m and 15,000 kN·m). To evaluate the reinforcement effect and optimize the DC operational parameters, a series of in situ tests, including settlement monitoring, standard penetration tests, dynamic penetration tests, surface wave velocity tests and plate-load tests, were carried out. Furthermore, the improvement depth of DC was discussed. The results of the field study show that the characteristic value of the ground bearing capacity of the three test zones could reach 250 kPa, which coincides with the design requirement, although the improvement depth of testing zone III fails to reach the required depth. This study helps to improve the in situ recycling of high-filled soil, thereby promoting the sustainable development of engineering construction.