# Finite Element Analysis of Combined Energy Piles with Long and Short Heat Exchanger Tubes
## 2 Technical Introduction  
Studies have found that the longer the length of the energy pile, the better the heat transfer performance [13; 24]. However, the length of the pile foundation is limited by economic constraints, construction progress, and other factors and cannot be too long. Considering the above situation, a new technology based on the combination of the pre-bored grouted planted energy pile and bored buried pipe ground source heat pump, i.e., deep and shallow combined buried pipe energy pile, is proposed: after the completion of the pre-bored grouted planted energy pile, a ground source heat pump system is laid in the hollow part of the precast pipe pile, and the U-tube heat exchanger of the ground source heat pump is buried 70-100 m below the surface or through the soil layer containing groundwater [5]. Figure 1 is a schematic diagram of the work of the deep and shallow combined buried pipe energy pile. Figure 1: Schematic of deep–shallow combined buried pipe energy pile. The deep and shallow combined buried energy pile installations all include heat exchanger tubes, hoses, and water circulation heat exchange systems. The top view of the deep and shallow combined buried energy piles is shown in Figure 2.  
The deep and shallow combined buried energy pile represents an advancement in the pre-bored grouted planted energy pile techniques. The process commences with a drilling machine creating a hole, followed by an expansion of the bottom of the hole based on the predetermined diameter and height. Then, cement slurry is injected as the drill is retracted. The piles are connected by welding, and the heat exchange tubes, positioned externally on the piles, are symmetrically affixed to both the pile sides. The precast pipe piles are inserted into the cemented soil, predominantly depending on their inherent weight. Subsequently, an additional hole is drilled within the hollow section of the precast pipe pile to the designated depth, after which an inner heat exchange pipe is introduced. Accounting for geological conditions and economic factors, the advocated burial depth of the inner heat exchange pipe ranges between 70-100 m. This heat exchange pipe is connected with the water circulation heat exchange system via a hose, forming two distinct sets of heat exchange systems inside and outside the pile. Therefore, the deep-shallow combined buried pipe energy pile technology integrates pre-bored grouted plant energy piles with borehole buried pipe ground source heat pumps. Incorporating these two heat exchange tube types produces savings in drilling and soil processing costs previously associated with the static drilling energy pile. This system capitalizes on the construction attributes of the static drilling energy pile as it optimizes the groundwater conditions in the coastal soft soil regions. For instance, in the Ningbo region, the soil stratum with optimal groundwater flow resides at depths of 60-70 m. The elongated heat exchange pipe traverses this groundwater area, as illustrated in Figure 3.  
By leveraging the unique construction features of static drilling and planting piles, this combined buried energy pile offers distinct advantages over conventional energy piles. In particular, the peripheral region of the static drilling and planting pile is composed of a cement-soil slurry circle that simplifies the placement of the external U-shaped heat exchange pipe on the outside of the precast pile but also minimizes resistance during the pile sinking. Additionally, the center of the static drilling and planting pile, which is a precast pipe filled with cement-soil slurry, is conducive for placing the inner U-shaped heat exchange pipe. This internal U-shaped pipe can achieve significant depths, optimizing the groundwater conditions in coastal soft soil regions for heat exchange with deep groundwater, thereby delivering a superior heat exchange capacity. This paper first presents the deep-shallow combined buried pipe energy pile, elucidating the rationale behind the development of this novel energy pile.