Freezing-thawing of porous media: An extended finite element approach for soil freezing and thawing

This paper introduces a thermo-hydro-mechanical computational model for freezing and thawing in porous media domains, with focus on freezing and thawing in soil. The model is formulated based on the averaging theory and discretized using a mixed discretization scheme, where the standard and extended finite element methods are simultaneously employed. It is capable of capturing the strong coupling between all important phenomena and processes occurring during relatively high freezing-thawing rates in porous media. Solid and fluid compressibility, buoyancy, phase change, thermomechanical behavior, water volume change, pores expansion, cryogenic suction, melting point depression and water migration to the freezing zone are all considered in the model. The cryogenic suction, in particular, is central to the occurrence of many of these phenomena and processes, and thus treated as a primary state variable, and discretized using the partition of unity method to make sure that it can be captured accurately. The paper presents detailed formulation of the governing equations and the numerical discretization. Verification and numerical examples are given to demonstrate the accuracy and computational capability of the model in describing the behavior of a soil mass subjected to boundary conditions resembling those occurring in the vicinity of an energy pile. The numerical examples show that the model is effectively mesh-independent and can simulate all important phenomena using relatively coarse meshes.