Contributions of constraints to mechanical fields of energy pile foundation

In most practical cases, the free deformation tendency of energy piles is constrained by the cap and surrounding soil. As a result of these constrained thermal deformations, additional forces are introduced into the system and are balanced by transfer to the ground via soil-structure interfaces. Consequently, interface conditions and structural element constraints play crucial roles in the behavior of energy pile foundations, and their evaluation is essential. This study evaluates the effects of constraints on the response of a large-scale piled raft foundation in homogeneous stiff saturated clay to cyclic thermal loads using coupled Thermo-Hydro-Mechanical Finite Element modeling. Particularly, the effects of interface stiffness and mechanical load variations in the evolution of mechanical fields, including stresses, displacements, and load-sharing ratios, were investigated. Despite the magnitude independence of the initial stresses, the greater constraining effect of the stiffer soil-structure interface and stronger pile-raft connection led to substantial excess loads, especially at shallow depths. Nevertheless, due to the performance of floating piles, the thermal axial stress variations in deep regions were virtually identical. Moreover, the larger tendency of the soil than the piles to undergo thermal deformations was found to be the primary determinant of the resultant load redistribution, which led to the soil-raft interface being a significant factor in determining vertical raft displacements. In the most severe case, the thermal axial stress variation range and stabilized excess settlement of the foundation were approximately three times the mechanical stress and one-fifth of the mechanical settlement, respectively.