Thermo-plastic response of energy piles during long-term monotonic cooling

Realistic numerical modeling of energy piles in soft soil requires advanced constitutive relationships capable of capturing the inherent thermo-plastic behavior of the surrounding ground. In this study, a newly developed rate-dependent, thermo-plastic constitutive model, called AVISA-T, is employed within the Plaxis Finite Element (FE) code to simulate the response of a well-instrumented energy pile embedded in multilayered soft soils subjected to thermo-mechanical loading. Following material parameter calibration and model prediction validation using non-isothermal laboratory tests on the soils surrounding the pile, the model was employed to simulate full-scale in-situ tests. In these simulations, the pile was initially subjected to either 0 % or 60 % of its bearing capacity and then exposed to continuous cooling over a period of up to three months. The AVISA-T model effectively reproduces the development of contractive and expansive strains, as well as compressive and tensile stresses that coexist along the pile shaft, including the accumulation of residual strains and stresses. In the absence of axial mechanical load, both residual contractive and expansive strains were observed, accompanied by irreversible uplift of the pile head, primarily attributed to non-uniform, unrecovered temperature changes. Moreover, under higher mechanical loading, the model captures dragdown effects resulting from thermal shrinkage of the surrounding soil, which contributes to the accumulation of permanent strains, stresses, and settlements. A comparison between simulations using the common Modified Cam Clay (MCC) model, the AVISA model without thermal effects and the AVISA-T model highlights the importance of using models including thermal plasticity for engineering practice.