# Thermomechanical Behavior of Parallel Multi-U-Shaped Energy Piles Under the Summer Condition
## 2 Overview of the Field Testing of the Parallel Multi-U Energy Pile  
### Project Overview  
The pile material used for this test is C30 concrete, and the pile type chosen is a bored pile, which has a wider diameter at 0.8 m and a length of 25 m. Five heat exchanger tubes are connected in parallel via the water collector and distributor, and the tubes are tied in a symmetrical distribution in the middle of the steel cage. The starting temperature of the pile soil must be determined before the thermal response test. The average temperature of the inlet is taken as the beginning temperature of 19.5 \({}^{\circ}\)C after room-temperature water has been fed into the heat transfer tubes prior to the heat transfer test and the water temperature has stabilized. According to the geological conditions gathered by [PERSON] et al. [39], the average thermal conductivity of the soil layer at the test site was determined to be 2.01 W/(m\(\cdot\)K). The test site's soil layers were divided into seven groups, and Table 1 lists the fundamental physico-mechanical characteristics of each soil layer and energy pile.  
In this test, the circulating water was heated by an electric heater in the HGNY-03 geotechnical thermal response tester to simulate the summer working conditions. Polyethylene (PE) pipes with an inner diameter of 23 mm and thickness of 2 mm were used in the piles. Inside the pile body, the temperature was measured by DST distributed temperature measurement fibers, which were placed on both sides of the heat transfer pipe wall and at the center of the pile shaft. There are five layers of strain sensors in the depth direction, starting from 4 m; one layer of strain gauges is laid every 3 m, the last layer is 7 m, and each layer of strain gauges consists of two vertically arranged vibrating string concrete strain gauges, which are symmetrically distributed on the reinforcement cage. The temperature data selection position is the same as the strain sensor position. The heat exchanger pipe and vibrating string concrete strain gauges are laid out as shown in Figure 1. The temperature measurement equipment used in this test is the UK DST distributed temperature measurement fiber optic system, the measurement accuracy is \(\pm\)0.5 \({}^{\circ}\)C@100 m, the mass flow meter is set in the HGNY-03 geotechnical thermal response  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{**Depth Z (m)**} & \multicolumn{2}{c}{**Thermal Conductivity (kg/m\({}^{3}\))**} & \multicolumn{2}{c}{**Specific Heat Capacity (\(C_{p}\)/K \(\cdot\)kg)**} & \multicolumn{2}{c}{**Coefficient of Thermal Expansion (MPa)**} & \multicolumn{2}{c}{**Young’s**} & \multirow{2}{*}{**Poisson’s Ratio \(\mu\)**} & \multirow{2}{*}{**Porosity**} \\  & & \multicolumn{2}{c}{**(W/(m \(\cdot\) K))**} & & & & & \\  & & & & & & & \\ \hline Fill & 0–4 & 2.01 & 1800 & 1600 & \(1\times 10^{-6}\) & 160 & 0.3 & 0.3 \\ Fine sand & 4–7 & 2.01 & 1830 & 1625 & \(1\times 10^{-6}\) & 170 & 0.3 & 0.3 \\ Clay & 7–10 & 2.01 & 1730 & 1580 & \(1\times 10^{-6}\) & 190 & 0.3 & 0.3 \\ Gravel & 10–13 & 2.01 & 1850 & 1540 & \(1\times 10^{-6}\) & 180 & 0.32 & 0.3 \\ Mudstone 1 & 13–20 & 2.01 & 1680 & 1605 & \(1\times 10^{-6}\) & 250 & 0.3 & 0.3 \\ Mudstone 2 & 20–25 & 2.01 & 1880 & 1555 & \(1\times 10^{-6}\) & 260 & 0.3 & 0.3 \\ Mudstone 3 & \(>\)25 & 2.01 & 2030 & 1595 & \(1\times 10^{-6}\) & 280 & 0.25 & 0.3 \\ File (concrete) & 0–25 & 2.01 & 2500 & 1250 & \(1\times 10^{-5}\) & 30,000 & 0.2 & \\ Thermal pipe & & 0.45 & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Material parameters of piles and soils in field tests.  
tester, and the measurement accuracy is \(\pm\)1%. The strain sensor model is a JY-4058-type vibrating string strain gauge with a resolution \(\leq\) 0.2%FS.