## 3 Background and Methodology  
In general, the TRT (Thermal Response Test) method is employed to estimate the thermal properties of energy piles [11]. However, the accuracy of TRT for energy piles can be lower than that for BHEs because of the complicated components in energy piles. [PERSON] et al. [13] developed a numerical approach for calculating the thermal resistance of an energy pile. The parameter discussed in [PERSON]'s model contains the pipe wall thermal resistance, the concrete properties and the position of the heat exchanger loops. However, there are still components of the thermal resistance that were not considered, such as the in-pipe layer resistance and the thermal resistance at the surface of the pile. If all these components are fully simulated, the TRT data could be used to determine the ground thermal properties more accurately by numerical back analysis. This study aims to illustrate the heat transfer at the concrete-soil surface of EGSs. In the simplest case, heat transfer across an interface follows Figure 1.  
\[Q=h(T_{2}-T_{1}) \tag{1}\]  
where \(Q\) is the heat flux (W/m\({}^{2}\)), \(h\) is the heat transfer coefficient (Wm\({}^{-2}\)K\({}^{-1}\)), and \(T_{2}\) and \(T_{1}\) are the temperature values at the two sides of the interface. One of the concerns in energy pile design relates to the expansion and contraction of the pile caused by temperature changes. As shown in Figure 1, it can be assumed that the tightness of contact between the soil and the concrete decreases when the pile contracts. As a result, the heat transfer across this interface could become inefficient. Conversely, in the pile heating period, the concrete pile body is hotter than the soil, and thus the pile expands and causes the heat transfer coefficient to increase. Normal operation of the energy piles includes both heating and cooling modes. Accordingly, the heat transfer coefficient from the pile body to the surrounding soil may vary, and thus the heat extraction/injection rate of the energy pile may not be constant. Ideally the design operation plan should take this into account. [PERSON] et al. [19] has noticed this difference by studying a radial expansion of an energy pile in US, and potential effects on contact pressures were discussed. However, to the best of author's knowledge there has been no research about the contact thermal resistance. Ignorance of this issue could lead to differences between the design and the real operation in the long run. The thermo-mechanical issue of an energy pile was discussed in work by [PERSON] et al. [20] in their project in Lambeth College. Lambeth College is situated in South London and uses all 143 of its bored pile foundations as energy piles. Prior to its construction a trial was carried out by Cementation Skanska, Geothermal International Ltd., and Cambridge University, which involved the installation of a test pile that was subjected to extreme heating and cooling cycles. The intention of the trial was to improve the understanding of what impacts heating and cooling cycles have on the structural and/or geotechnical performance of a pile, and to analyze the thermodynamic response of the system [17]. The mechanical behavior of an energy pile was recorded with a heating-cooling trial. In the pile, the strain was detected by means of both VWSG (Vibrating Wire Strain Gauges) and OFS (Optical Fiber Sensors) sensors. The details of the thermal-mechanical loading test of Lambeth College pile can be found from [PERSON] work [20]. The test shows clearly that in the pile cooling mode, the concrete pile body contracts at both axial and radial directions for the strain curves are under zero. Meanwhile, in the pile heating period the pile expands, and the strain is positive. Another TRT test was conducted in 2012 at the Shell Centre, London, to determine the overall thermal conductivity of an energy pile. The TRT operators faced similar problem in the data fitting from the hot/cold trial.