Thermal mechanical behavior of energy piles with cap under embedded depth

Using energy piles with cap foundations under 3.0 m of backfill, four tests with different heat exchanger tube types (U&W) and operation modes (continuous and intermittent heating) were carried out. The temperature and stress changes of the energy pile and its adjacent structures were measured, and the experimental results were compared with those without buried depth conditions. The heat exchange rate per pile length with W-type heat exchanger tubes was approximately 1.39 times that with U-type tubes. The heat exchange rate per pile length during intermittent heating operation was approximately 1.30 times that during continuous heating operation. The overlying backfill showed a certain insulation effect, and the heat exchange rate per pile length was approximately 1.13 times that without burial. The soil above the pile was less susceptible to seasonal variations, and after 192 h of heating, the temperature rise at the pile top under the 3.0 m of backfill was 5.10 °C less than that without burial. The overlying 3 m of backfill also exhibited some restraining effects: this inhibited the thermal relative displacement of the pile top, with thermal expansion coefficients of 0.027 mm/°C and 0.037 mm/°C with and without burial, respectively. The corresponding thermal dissipation coefficients were 2.55 °C/m and 3.43 °C/m. The null point appeared at 0.78 times the pile length, and there were slight differential deformations in the cap with/without burial.