## 1 Introduction
This modification improved heat exchange efficiency, increased heat storage capacity, and enhanced temperature stability in both the piles and the surrounding soil. However, it is worth noting that thermal interference among the heat exchange tubes may influence heat transfer calculations due to the small size of the model pile. [PERSON] et al. [30] conducted a thermal response test on a PCM pile in unsaturated clay. The results clearly show that using PCM energy piles is an efficient engineering approach for improving heat exchange capacity and saving underground space resources compared to traditional concrete piles. Their investigation revealed that PCM piles exhibited a reduced temperature fluctuation range compared to traditional piles, effectively reducing heat accumulation in the surrounding soil. However, the pile accumulated plastic strain during the temperature cycling process. While PCM can enhance the energy density of piles and improve heat exchange efficiency compared to traditional concrete piles [31], it also reduces the thermal conductivity of piles. Furthermore, there is still a lack of clarity in characterizing the phase change process for PCM in concrete. The ambiguity primarily arises due to the dispersion of PCM when incorporated into the concrete, making it challenging to assess its thermal behavior through experiments. The size limitations of model tests and the high cost of field experiments have limited research on energy piles. Consequently, many scholars have employed numerical methods to further investigate energy piles' thermodynamic characteristics [31; 32; 33]. [PERSON] et al. [34] and [PERSON] et al. [35] conducted multidimensional analyses using the finite element method and comprehensively validated their findings through experiments. Their research confirmed that the heat within the soil medium resulted in only a slight vertical temperature gradient on the pile surface and a relatively constant temperature was observed across each horizontal cross-section. [PERSON] et al. [36] conducted a comparative simulation analysis of the thermal properties of various backfill materials, including soil, paraffin, ordinary acid, and reinforced acid, in a ground source heat pump (GSHP) system. Their findings revealed that, compared to soil, the use of PCMs as backfill materials reduced the thermal effect radius by approximately 15% to 35%. [PERSON] and [PERSON] [37] and [PERSON] et al. [38] indicated that the spacing between piles should be greater than 6.8 times the diameter of each pile to reduce the impact of interference. There are few existing studies on the simulation of the phase change process in phase change piles, so it is urgent to establish a simulation method for phase change piles. To address the issues of inadequate thermal conductivity and observe the phase transition process within PCM inside a pile, two types of energy piles were designed in this study: a fiber-reinforced concrete pile (FR pile) and a fiber-reinforced phase change pile (FRPC pile). The equivalent substitution method established a thermo-hydro-mechanical multi-physics coupling three-dimensional numerical model to simulate the phase transformation process. The thermal performance of the FR and FRPC piles was investigated through a combination of model tests and numerical simulations. By analyzing the phase change process and temperature distribution in both piles and soil, this study provides valuable insights into designing and implementing phase change energy piles.