# A Study on the Softening Shear Model of the Energy Pile-Soil Contact Surface
###### Abstract
However, when the normal stress was greater than 100 kPa, the shear strength of the contact surface increased by 20% after 10.5 thermal cycles. [PERSON] et al. [13] found that cyclic thermal loading increased the peak shear strength of the pile-soil contact surface but had no significant effect on the residual shear strength. Figure 2: Common heat exchanger tube arrangements for energy piles. By studying the load transfer characteristics of the pile-soil contact surface under the effect of temperature, it is found that the calculation idea of the load transfer method can also be applied to the settlement and axial force calculation of energy piles [14]. The load transfer method is widely used in traditional pile settlement and axial force calculations, and its basic idea is to divide the pile into several elastic units, each of which is connected to the soil body by a nonlinear spring, which is used to simulate the load transfer relationship between the pile and the soil [15]. In addition, some scholars have analyzed the influence of the discretization used on the results obtained, and found that the way in which discretization is performed can affect results [16; 17]. [PERSON] et al. [18] found that the interaction of shear displacements between groups of piles decreases under conditions of nonlinear deformation of the soil body. [PERSON] et al. [19] found that the effect of thermal loading on the displacement of end-bearing energy piles is greater than that on friction energy piles. [PERSON] et al. [20] proposed a finite element model that can represent the mechanical properties of the pile-soil interface. [PERSON] et al. [21] were the first that used the load transfer method for the calculation of internal forces and displacements of energy piles by utilizing the condition of temperature-induced conservation of external forces at the neutral point to compute internal forces and displacements induced with temperature loading. In the above research studies, the pile-soil contact surface is equivalent to a spring model to study the shear characteristics of the pile-soil contact surface. The existing studies have not considered the effect of softening on the shear characteristics, but the softening phenomenon has an important effect on the bearing performance of energy piles. The softening phenomenon during the shear process of pile-soil contact surface was observed [22], and the softening phenomenon of the soil on the pile side leads to a decrease in the rate of change of the pile axial force along the depth [23]. Therefore, [PERSON] et al. [24] proposed a load transfer model under softening and, based on this model, derived the equation for the relationship between pile top load and settlement; [PERSON] et al. [25] proposed a nonlinear method to represent the relationship between pile lateral friction resistance and displacement by means of a nonlinear model in the form of a segmented function; [PERSON] et al. [26] derived a load transfer model considering both hardening and softening effects; [PERSON] et al. [12] proposed a load transfer model considering cyclic hardening and softening. Further research found that the shear properties of pile-sand contact surface were affected by cyclic temperature loading but not by constant temperature loading and that the shear strength increased with increasing temperature [27; 28]. In this paper, a finite element numerical model of thermal-hydro-mechanical of energy piles under multi-layer geological conditions was established, and field tests of ultra-long energy pile (1000-mm-diameter, 44-m-long) were carried out to reveal the temperature distribution and mechanical properties of the energy pile under typical working conditions. On this basis, the axial deformation mechanism of energy pile was proved. Furthermore, based on load transfer method, a softening shear model of the energy-soil interface under large shear displacement was proposed.