A thermo-poromechanical model for simulating freeze–thaw actions in unsaturated soils

This study presents a new fully coupled thermal-hydraulic-mechanical (THM) model for variably saturated freezing soil, which examines the freeze–thaw (F-T) actions. The model is derived based on the general form of continuum mechanics for porous media. The mass balance equations cover the conservations of the total water and dry air, where liquid water, ice, and vapor are involved in the total water balance equation. The effective stress law for the unsaturated frozen soil is included in the model to quantify poromechanical behaviors. The pore pressure contains components from pore water pressure, pore air pressure, and ice pressure. A new model for characterizing the unfrozen water content based on temperature and air-water capillary pressure is proposed. The THM formulation is based on multidimensional derivation, thus is versatile to be extended to cases including warm temperature conditions or large deformation behavior. The model was implemented in a 2D finite element package and validated by a set of published laboratory experimental data. The numerical code is also applied to simulate the freeze–thaw actions in highly unsaturated loess located in the northwest of China, where the quasi-distributed fiber optic sensing data is collected for field-scale validations. Our simulated thermal-hydro-mechanical responses match well with in situ monitored results and confirm that freezing-induced heaving is still significant in such highly unsaturated soil.