Abstract
This paper introduces a thermo-hydro-mechanical finite element model for energy piles subjected to cyclic thermal loading. We address four particular features pertaining to the physics of energy piles: three-dimensionality, embedded heat exchangers, soil constitutive modeling and pile–soil interface. The model is designed to capture the strong coupling between all important physical and thermomechanical processes occurring in a concrete pile embedding U-tubes heat exchangers and surrounded by a saturated soil mass. It encompasses solid and fluid compressibility, fluid and heat flow, thermoplastic deformation of soil, buoyancy, phase change, volume change, pore expansion, melting point depression, cryogenic suction and permeability reduction due to ice formation. The model is distinct from existing energy pile models in at least two features: (1) it can simulate the detailed convection-conduction heat flow in the heat exchanger and the associated unsymmetrical thermal interactions with concrete and soil mass; and (2) it can simulate cyclic freezing and thawing in the system and the associated changes in physical and mechanical properties of the soil mass that likely lead to thermoplasticity and deterioration of pile shaft resistance. The performance of the model is demonstrated through a numerical experiment addressing all its features.
Highlights
An energy pile is a dual-purpose structural element, functioning as a structural foundation and a ground source heat exchanger
This paper introduces a thermo-hydro-mechanical finite element model for energy piles subjected to cyclic thermal loading
We address four particular features pertaining to the physics of energy piles: three-di mensionality, embedded heat exchangers, soil constitutive modeling and pile–soil interface
Summary
An energy pile ( known as thermal pile) is a dual-purpose structural element, functioning as a structural foundation and a ground source heat exchanger. Several notable THM studies have been introduced, including those by Yavari et al [35], Di Donna and Laloui [12], Gawecka et al [16] and An ongphouth et al [3] They model energy piles with different levels of physical complexity, material constitutive relationships and pile–soil interaction. The heating (cooling) system works by circulating a fluid through the U-tube that collects (rejects) heat arising from a series of thermal interactions between the fluid, pipe wall, concrete and surrounding soil mass These distinct geometrical and physical features are ignored in the line heat source approach, eliciting three main shortcomings: (1) it ignores the conductive-convective heat flow in the U-tube that varies following the daily and seasonal thermal load demands; (2) it ignores the three-dimensionality of the problem which results from the U-tubes configuration and their associated un symmetrical heat flow and thermal stresses; and (3) it does not allow assessing the energy efficiency of the energy pile, which constitutes the.
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