The thermo-mechanical (TM) behaviour of the energy pile (EP) group becomes more complicated in the presence of seepage, and the mechanism by which seepage impacts the EP group remains unclear.In the current work, a 2 x 2 scale model test bench of EP group was set up to investigate the TM behaviour of EP group with seepage. The test results indicate that the heat exchange performance of EP group with seepage can be significantly enhanced, but also leads to obvious differences in the temperature distribution of pile and surrounding soil along the seepage direction, and thus causes evident differences in the mechanical properties between the front pile and the back pile in pile group. Compared with the parallel connection form, the thermal performance of EP group with the series connection form is slightly attenuated. However, the mechanical properties of various piles in the EP group differ significantly. Under the action of seepage, the mechanical balance properties of various piles in the forward series form are optimal, followed by the parallel form, and the reverse series form is the least optimal. A 3-D CFD model was established to further obtain the influence of seepage and arrangement forms on EP group. The findings indicate that seepage can not only mitigate thermal interference between distinct piles but also expedite the process of heat transfer from pile-soil to reach a state of stability. Concurrently, the thermal migration effect induced by seepage will be superimposed along the seepage direction, resulting in the elevation of thermal interference of each pile along the seepage direction, and the superposition of thermal migration effect increases with the time. Under the same seepage condition, the cross arrangement can enhance the thermal performance of EP group, optimize the temperature distribution of pile and soil, and thus the imbalance of mechanical properties among pile groups can be reduced. In addition, the concepts of thermal interference coefficient and heat exchange rate per unit soil volume are introduced to facilitate a more precise evaluation of the thermal interference degree of each pile in the pile group and the heat exchange performance under different pile arrangement forms.The standard deviation and mean value in the statistical method are used to evaluate the equilibrium of mechanical properties of pile group, which is more intuitive to compare the differences in mechanical properties of pile groups under different working conditions.

Energy pile groups transmit geothermal energy and have attracted widespread attention as one of new building energy-saving technologies. Accurately predicting the time-dependent behaviors of energy pile groups is a challenge, given the complex thermal and mechanical interactions between piles, surrounding soils and the pile cap. This study presents a semi-analytical solution for analyzing energy pile groups within heat exchangers. Utilizing the transformed differential quadrature method, a flexible coefficient matrix for the saturated surrounding soils is acquired, which accounts for both consolidation and heat transfer. The piles are segmented, and the discrete solving equations considering thermal stresses and expansion are formulated. To accurately reflect the interactions among piles-to-piles, piles-to-soils and piles-to-pile cap, the coupled matrix equations are constructed with involving both the displacement coordination and the force equilibrium at the pile-soil interface as well as the pile cap. The validity of the proposed solution is confirmed through comparisons with results from onsite tests and simulations using COMSOL. Pivotal parameters including temperature variations, pile spacing, and the relative stiffness are discussed through examples. Compared with traditional simulation and field test, the proposed solution enables fast and accurate prediction of displacement and load distribution across pile groups, facilitating the safety evaluation of heat exchangers.

The operational performance of energy pile (EP) group with seepage is strongly influenced by seepage parameters. In this paper, a model test system of 2 x 2 EP group with seepage is built to study the influences of seepage water level and seepage velocity on thermo-mechanical behaviour of EP group. Also, a numerical model of EP group considering seepage is developed to obtain the variations of thermo-mechanical behaviour of EP group under different seepage parameters. The findings demonstrate that an augmentation in seepage water level can enhance the heat exchange performance of EP group, but it also exacerbate the imbalance of mechanical properties between piles in the short term, in which the seepage only have a significant effect on the temperature of piles and soil below the seepage water level. Increasing seepage velocity and circulating flow rate can strengthen thermal performance of EP group and improve the equilibrium of pile axial force and displacement between the pile groups, but increasing seepage velocity also increases the imbalance of mechanical properties between the front and back rows of pile group. At the same time, compared to the circulating flow rate, the change in seepage velocity has a dominant impact on the thermo-mechanical characteristics of EP group. Moreover, when the seepage angle is within 0-45 degrees, increasing the seepage angle can effectively improve the heat transfer performance of EP group, and the temperature distribution of pile and soil is obviously different for different seepage angles, in which the mechanical properties of EP group have the best equilibrium when the seepage angle is 30 degrees for current simulation conditions.

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 x 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 startstop 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 x 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.

The mechanical response of energy pile groups in layered cross-anisotropic soils under vertical loadings is studied with the aid of the coupled finite element method- boundary element method (FEM-BEM). The single energy pile is simulated based on the finite element theory, which then is extended to energy pile groups. The global flexibility matrix for soils is obtained by considering the coupling effects of vertical and thermal loadings. The coupled FEM-BEM equation for the interaction between energy pile groups and soils is derived based on the displacement compatibility condition at the pile-soil interface. According to the displacement coordination condition and force balance in the rigid cap, the displacement of the cap and axial forces of pile groups can be solved. The presented theory is validated by comparing the calculated results with numerical simulations and field test results in existing literature. Finally, effects of the thermal loading, pile-soil stiffness ratio, pile spacing, cross-anisotropy of Young's modulus and the stratification are discussed.