Numerical Study on the Thermo‐Hydro‐Mechanical Behavior of Energy Micropiles in Hypoplastic Soft Clay Under Cyclic Thermal Loading

Coupled nonlinear thermo-hydro-mechanical finite element simulations were carried out to investigate the behavior of energy micropiles subjected to thermal loading cycles. Two kinds of problems were analyzed: The case of an isolated micropile, for which comparison with previous research on medium-size isolated energy pile is provided, and the case of large groups of micropiles, with the aim of investigating the interaction effects. In both problems, micropiles were considered installed in a thick layer of very soft, saturated clay, characterized by isotropic or anisotropic hydraulic conductivity. Two advanced existing hypoplastic models, one incorporating the thermal softening feature, were used to describe the clay behavior in both problems. The settlements of the micropile head were found to increase during thermal cycles under constant mechanical load, showing a sort of ratcheting. For micropile groups, the settlement increase rate was faster as the spacing between micropiles was reduced. The excess pore water pressures developed at the micropile-soil interface played a significant role on the deformation and displacement fields of the soil-micropile systems, especially in the case of micropile groups, affecting the shear strength developed at the micropile-soil interface. The consolidation process was faster when the hydraulic conductivity was anisotropic, meaning that the development of excess pore water pressure was reduced in this case. As the spacing between the micropiles increased, i.e., as thermal interaction decreased, the heat flux exchanged by amicropile of the group during one cycle approached the heat flux exchanged by an isolated micropile in the same period.