Mechanisms of freeze-thaw-induced structural degradation in black soil driven by ice lens growth using a phase-field model

Abstract

Seasonal freeze–thaw processes in black soil critically regulate soil structural stability, hydrothermal migration, and aggregate integrity, thereby influencing water retention, root-zone stability, and agricultural productivity and ecological stability. In cold-region agricultural ecosystems, freeze–thaw-induced changes in pore structure and permeability also strongly influence contaminant migration, retention, and the effectiveness of soil remediation measures by altering water flow pathways and mass transport processes. However, the multi-scale coupled mechanisms linking ice lens growth, hydrothermal transport, and structural degradation under varying initial water content and freezing intensity remain poorly understood. To address this, a thermo–hydro–mechanical phase-field coupled model (THM-PF), specifically adapted for black soil, was developed to simulate interactions among temperature variation, moisture migration, stress redistribution, and pore structural evolution. Gradient cooling experiments (−15 °C to −3 °C), combined with microscopic observations, were conducted to validate the model and elucidate coupled freeze–thaw mechanisms and structural degradation processes under controlled conditions. The results indicate that freeze–thaw fragmentation in black soil is primarily controlled by ice lens-induced segregation stress, exhibiting threshold behavior defined by the critical separation void ratio (e sep ). When this threshold is exceeded under strong freezing, the system rapidly destabilizes, with pore pressure reaching 107 kPa and porosity increasing by up to 546.47% relative to the initial state. Initial water content further regulates degradation pathways: at high water content (>32.3%), ice lens growth dominates and causes severe aggregate breakage (up to 99% after 30 cycles), whereas at low water content (<3.4%), degradation is mainly driven by mechanical friction, resulting in limited structural change (porosity increase of 21.62%). In addition, permeability heterogeneity alters heat transfer pathways by 15°–23°, leading to discontinuous ice lens distribution and enhanced spatial heterogeneity. Structural modification (e.g., gravel–sand incorporation) can reduce temperature gradients by up to 27%, thereby suppressing ice lens development. These results collectively reveal a cascade mechanism of “ice lens segregation → hydrothermal heterogeneity → structural degradation” and highlight the potential risks to soil stability, crop productivity, and contaminant transport in high-intensity freeze–thaw regions. Based on this mechanism, targeted mitigation strategies are proposed, including optimal water content control (<28%), permeability regulation through graded fillers, and layered structural design, providing theoretical guidance and technical support for soil conservation, agricultural management, ecological restoration, and contaminant transport management in black soil regions.