Evolution of unfrozen water in coarse-grained frozen soils during thermal thawing: Integrative observations and mechanistic modeling

• Dual- T 2 cutoffs (frz:1.05/2.25 ms, thw:1.05/2.77 ms) show reversed water transition. • Ultrasonic vel. drops >800% (-3→3°C) indicates ice-soil pts. thawing collapse. • Resist.- θ u model valid for clay/silt/sand, resist. sharply increases at θ u<2.89%. • NMR measures 2355% bound water increase during -5 to 0°C thaw. • Multi-param diagnose builds thermal-thaw alert frmk. for coarse-grained permafrost. Predicting thermal thawing disasters in coarse-grained frozen soil remains an urgent challenge. To address the limitations of traditional frozen soil models and the unclear multi-physics links, this study utilizes nuclear magnetic resonance (NMR), ultrasound, and resistivity tests to investigate the one-sided phase change in coarse frozen soil. Key innovations include: defining a distinct T ₂ cutoff value for coarse-grained soils and quantifying the asynchronous phase transition behavior of bound water, capillary water, and free water in coarse-grained soils. Revealing a sequence diametrically opposed to the progressive phase transition in fine-grained soils (free water freezes before thawing, while bound water does the opposite). Ultrasonic waves reveal a distinctive brittle failure threshold (-3°C) during the thawing process of coarse-grained soils, characterized by a sharp decline in wave velocity (828.85%). This abrupt change contrasts markedly with the gradual variation observed in fine-grained soils, providing a unique precursor to thermal instability caused by ice-cemented collapse. A universally applicable resistivity ρ -unfrozen water model θ u (R² > 0.90) has been established, effectively overcoming the limitations of traditional models in coarse-grained soils. This model accurately captures critical states such as conductive network fractures ( θ u < 2.89%). The multi-parameter diagnostic system provides a framework for thermal thawing hazard warnings in coarse-grained frozen soil engineering. This is significant for monitoring and reducing instability in railways, roads, and pipelines in frozen soil.