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Energy piles are a relatively new technology that have dual function as heat transferring and load bearing. Due to the influence of temperature cycles, additional thermal stress and relative displacement of the pile will be generated; this is different from the load transferring mechanism of the conventional pile. In order to study the thermodynamic characteristics of the energy pipe pile under dual working conditions and temperature cycles, field tests were carried out on the PHC (prestressed high-strength concrete) energy pipe pile without constraining on the top of the piles. Displacement gauges were arranged on the top of the pile, and concrete strain gauges (temperature, strain) were embedded in the pile.
2. Experimental Study of Thermal Response of Vertically Loaded Energy Pipe Pile
- (1) During the operation of the PHC energy pile in the summer and winter, the temperature of the pile gradually changed. The temperature in the initial stage changed rapidly; as the test progressed, the temperature increment gradually decreased. In short, the temperature was low at the two ends and high in the middle;
- (2) Under both the summer and winter conditions, the thermal stress gradually increased with the pile’s temperature, due to the joint constraints of the soil around the pile and the soil at the pile top; the thermal stress was small at both ends and large in the middle, and this is probably because the pile top is located at the dense sand layer and the restraint rigidity is relatively large. At the end of the summer test, the maximum compressive stress at 8 m of the pile was 3.446 MPa; in the winter, the maximum tensile stress at 8 m of the pile was 2.69 MPa. The thermal stress has a linear relationship with the temperature increment. The relationship between the maximum compressive stress and the temperature difference is ; the relationship between the maximum temperature tensile stress and the temperature difference is . The results showed that the temperature stress generated under the dual temperature cycle conditions would not affect the safety of the pile foundation and the superstructure;
- (3) The pile is constrained by the soil around during the temperature cycle and cannot be deformed freely, leading to side friction resistance. In the summer, the soil around the upper part of the pile produced negative side friction resistance, and the soil around the lower part of the pile produced positive side friction resistance; the situation is the opposite in the winter. The neutral point is about 10.75 m away from the pile top, and it did not move during the whole test, indicating that the neutral point of the pile is close to the end of the pile with a higher degree of restraint. The side friction resistance of the pile increased with the increase in the temperature difference and increased along with the neutral point to the two ends of the pile, and the absolute value of the soil friction resistance of the upper part of the pile was larger than the that of the lower part. Under the winter condition, the side friction resistance degraded from 4.5 to 11.5 m during the test. In addition, the results show that the position of the neutral point of the pile is affected by the pile end restraint condition, and it is always close to the end of the pile with stronger restraint;
- (4) During the test, the displacement of the pile top and other measuring points had been increasing, and the displacement of the point above the neutral point was greater than the displacement of the point below the neutral point. The test results show that at the end of the summer test, the displacement of the pile top was 0.7 mm, about 0.175%D. In the winter, as the temperature of the pile gradually decreased, the displacement of the pile top gradually increased. At the end of the test, the displacement of pile top gradually increased, and the maximum displacement was 0.4 mm, about 0.1%D.
The entry is from 10.3390/su13137411
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