Experimental study of localized hydro-thermal-mechanical coupling characteristics of unsaturated silt under step-freezing conditions

Abstract

The deformation of unsaturated silt under freezing conditions is governed by coupled hydro-thermal-mechanical (HTM) processes, particularly during phase transitions between unfrozen water and ice. Unlike previous studies that have largely focused on constant-temperature freezing, this study conducted unidirectional freezing tests under step-freezing and open-system conditions to replicate frost heave phenomena typical of seasonally frozen regions. The key variables, including initial and final water content, frost depth, heave displacement, and external water uptake, were continuously monitored. Particle image velocimetry captured the localized deformation patterns, which closely matched displacement measurements. The results indicated that a higher initial water content led to deeper frost penetration, more frost heave, and increased heave rates, while simultaneously reducing external water replenishment. Significant latent heat release occurred in soils with high water content during freezing, correlating with the increased frost heave responses. Frost heave dominated within the frozen zones, whereas compressive deformation was observed in the unfrozen regions. The specimens with a low water content tended to develop single-layer segregation cracks, whereas those with a higher water content exhibited multi-layer cracking. These findings, significantly different from those of constant-temperature freezing studies, revealed more varied cracking patterns than uniform patterns observed in constant-temperature tests. These findings could provide quantitative insights into localized deformation mechanisms under phase change, enhance the understanding of frost heave risk, and support the development of deformation control strategies in cold-region geotechnical applications.