# Finite Element Analysis of Combined Energy Piles with Long and Short Heat Exchanger Tubes
## 1 Introduction  
In 2020, China put forward the goal of \"achieving a carbon peak by 2030 and achieving carbon neutrality by 2060\" [1]. In this context, geothermal energy has received significant attention as a sustainable clean energy that can minimize carbon emissions [2; 3]. Presently, many countries regard the use of geothermal energy ground source heat pump technology as an important means of energy conservation and emission reduction. A great deal of related research has been carried out from different aspects. For instance, [PERSON] et al. [4] and [PERSON] et al. [5] explored the simulation techniques of ground source heat pump technology under various conditions. The energy pile technology--developed based on the ground source heat pump technology--combines the bearing function and geothermal energy utilization. It can save construction costs as no drilling is involved [6] and has been increasingly applied in the recent years. The pile foundation types of the existing energy pile technology mainly include cast-in-place piles and precast piles [6; 7; 8; 9]. In the deep coastal soft soil area, the construction of cast-in-place piles produces more mud, affecting the environment. The conventional precast piles will crush and disturb the surrounding soil, and the construction depth is usually within 40 m. The pre-bored grouted planted pile first drills holes in the ground, sprays cement soil, and then sinks the precast pipe pile. The construction depth has reached 80 m, and its small environmental impact is being promoted and applied on a large scalein southeastern coastal areas of China. The pre-bored grouted planted energy pile is a new type of energy pile based on the development of pre-bored grouted planted pile. This technology is developed using spiral drilling and spray slurry mixing to form cement soil, and the precast pipe pile and heat exchanger tubes are tied and put into the drilling hole. The drill hole is filled with muddy cement soil, and the heat exchanger tubes are fixed on the outside of the precast pipe pile and planted into the cement soil together. Due to the presence of cement soil surrounding the precast pile, the construction of the heat exchanger tubes is convenient and safe. The pre-bored grouted planted energy pile technology has the characteristics of green environmental protection, convenient construction, and high bearing capacity [10; 11]. However, there are also problems, such as heat collection and thermal interference of soil, which are caused by excessive heat discharge from the pile group, which will affect the use efficiency of the energy pile [12; 13]. At the same time, the temperature change will change the lateral friction resistance and axial stress of the pile body and affect the bearing performance of the energy pile [14]. Some soil layers in the coastal area have flowing groundwater, and the flow of groundwater can alleviate the imbalance of formation temperature to a considerable extent [2]. However, the groundwater seepage in the coastal area is usually buried deep, and the conventional energy pile cannot take advantage of this favorable geological condition. To be specific, the implantation depth of the pre-bored grouted planted energy pile is generally 40-60 m in China, and the confined water in the coastal soft soil area is generally located below 60 m. [PERSON] et al. [15] and [PERSON] et al. [16; 17] proposed the deep-buried pipe energy pile technology, which is different from the traditional method. The basic idea behind the new technology is to bend the heat exchanger tubes in the main circuit into a U-shape, the bottom of which is not buried in the pile; instead, it take the form of a buried pipe in the heat exchange well. The heat exchange well is set in the center of the pile body and extends to 100 m below the surface. The upper part of the heat exchanger tubes is wrapped in concrete, and the lower part is wrapped by rock and soil.