# A Study on the Softening Shear Model of the Energy Pile-Soil Contact Surface
## 2 Materials and Methods
### Model Verification  
Hua Xincheng AB Lot Project, located at the intersection of Jiangdong Middle Road and Hexi Street in Hexi New Town, Nanjing, covers an area of about 163,000 m\({}^{2}\), with a total construction area of about 1,000,000 m\({}^{2}\), foundation pit excavation area of 98,000 m\({}^{2}\), with a perimeter of about 1271.3 m and an excavation depth of about 16.5 m, and with a total of 4378 drilled piles. The energy pile is 44 m long and 1000 mm in diameter with a full-depth reinforcement cage. Three U-shaped high-density polythene pipes with a diameter of 25 mm and a thickness of 2.3 mm were embedded in the energy pile as the ground heat exchanger. Due to the reinforcement cage being installed in sections, the heat exchanger was not attached to the reinforcement cage. The procedure of heat exchanger installment is as follows. First, install the reinforcement cage. Second, push 3 U-shaped pipes to design depth by the modified tremie. Third, pour the concrete. The geological substrate of the project is typical of the Yangtze River floodplain; the geological condition and the groundwater level was close to the ground surface during testing. The physical parameters in the test are shown in Table 1. As shown in Table 2, the material of energy pile is concrete; the actual test results show that the elastic modulus is 32,800 MPa; Poisson's ratio is 0.2; the coefficient of expansion is 1.0 \(\times\) 10\({}^{-5}\); tensile strength is 2.39 MPa; compressive strength is 26.80 MPa; and biaxial compressive strength is 29.48 MPa. Due to the limitation of monitoring conditions, this in situ test selects 2630\(\#\) energy pile as the monitoring pile. The pile diameter is \(\Phi\)1000 mm, the length of the pile is 58 m, and the concrete strength is C40. The heat exchanger pipe of the 2630\(\#\) energy pile is laid in the form of 3U parallel connection, the heat exchanger pipe adopts the HDPE (high-density polyethylene) pipe with the size of dn25 mm \(\times\) en2.3 mm, the depth of the heat exchanger \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Soil Layer** & **Name** & **Thickness (m)** & **Density (kg/m\({}^{3}\))** & **Condition** \\ \hline \hline  & Miscellaneous fill & 3.8 & 1750 & Loose \\  & Silty soft clay & 18.7 & 1770 & Flow plasticity \\  & Silty clay & 13.8 & 1790 & Local flow plasticity \\  & Silty fine sand & 5.5 & 1920 & Sand \\  & Silty fine sand & 10.0 & 1930 & Sand \\  & Coarse sand mixed gravel & 17.2 & 1917 & Gravel \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Basic characteristics of the test site. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Materials** & **Density (kg/m\({}^{3}\))** & **Thermal Conductivity (W/(m\(\cdot\)K))** & **Specific Heat J/(kg\(\cdot\)K)** \\ \hline HDPE & 1100 & 0.42 & 1465 \\ Concrete & 2750 & 1.80 & 800 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Energy pile model thermophysical parameters. pipe is 32 m under the top of the pile, the thermal response test is carried out by using the thermostatic method for the heat exchanger pipe in the pile, and the initial temperature of soil at the test site is about 20 \({}^{\circ}\)C. Thermal response testing of the heat exchanger pipe in the energy pile was carried out using the thermostatic method, and the initial temperature of the soil at the test site was measured to be about 20 \({}^{\circ}\)C.  
PT100 platinum resistance thermometers, vibrating wire steel stress gauges, vibrating wire concrete strain gauges, and vibrating wire earth pressure gauges are installed in 2630# energy piles. Seven platinum resistance thermometers are installed along the pile body, with burial depths of 15.5 m, 22.5 m, 29.5 m, 36.5 m, 42.5 m, 51.5 m and 57.0 m. At the same depth of the thermometers, vibrating wire type reinforcement stress gauges and concrete strain gauges are symmetrically installed to monitor the axial stress and radial strain, with a total of 14 reinforcing bar stress gauges and 14 concrete strain gauges.