Thermomechanical analysis method for energy piles with skin friction softening and hardening behavior

Various approaches have been proposed to analyze the thermomechanical behavior of individual energy piles. Although these approaches can account for pile-soil interactions, there is a lack of approaches for continuously describing the full nonlinear range of the load-transfer curve for individual energy piles under thermomechanical loading, including both skin friction softening and the hardening behavior. Therefore, this study developed an analysis method for individual energy piles by considering skin friction softening and hardening behaviors. The developed approach was verified by comparing the simulation results with those of three well-documented field tests, alongside laboratory and centrifuge model tests. The simulation results show a maximum percentage error between the simulation results and the field measurement results is 8.8 %, which is much smaller than that of other methods, indicating a relatively high degree of consistency between the simulation and the actual situation. Besides，the results suggest that the proposed method can capture the full nonlinear range of the load-transfer curve and essential aspects of the pile in terms of the stress and displacement induced by the thermomechanical operation. Finally, a parametric analysis was conducted to study the effects of the model parameters on the energy pile thermomechanical performance. Results show that increasing the dimensionless parameter changes the pile axial thermal stress and displacement oppositely; increasing the residual ratio boosts axial thermal stress, reduces displacements and stress, and moves the neutral point (NP) towards the pile head.