Thermal Conductivity Evolution of Sandy Silt Under Thermal–Mechanical Coupling: Insights from the Phase Transition Temperature Zone

Abstract

Understanding the evolution of frozen soil thermal conductivity under thermo-mechanical coupling is critical for predicting heat transfer in porous media. In this paper, the thermal conductivity evolution of sandy silt under thermal–mechanical coupling was systematically investigated using a thermal constant analyzer, focusing on the phase transition zone (PTZ, − 0.5 °C, − 1 °C, and − 5 °C). We quantified the effects of temperature (20 °C, − 0.5 °C, − 1 °C, and − 5 °C) and stress ranges (0 kPa, 25 kPa, 50 kPa, and 100 kPa) on thermal conductivity of sandy silt using thermal constant analysis and in-situ magnetic resonance imaging (MRI), while revealing microstructural drivers via pore-scale moisture distribution. The findings demonstrated an asymmetric thermal conductivity evolution between the unfrozen zone and the phase transition zone, with pronounced nonlinear behavior at subzero temperatures. One of the most significant enhancements (up to 55.7 %) occurred in the phase transition zone, where ice formation and stress-optimized particle contact cooperatively promoted heat transfer. The discrete ice crystal (− 0.5 °C) triggered a gradual increase in the thermal conductivity. At − 1 °C, the ice lens body gradually formed a continuous network, causing a sudden jump. Eventually, it was gradually frozen at − 5 °C, and the ice skeletal restructuring enabled steady enhancement through heat path optimization. The phase transition zone led to a significant increase in the microporosity of the sandy silt, forming an interconnected pore network that enhanced the heat transfer pathway. The results provide essential information for evaluating the thermal conductivity of fine-grained soils under freeze–thaw conditions and offer fundamental insights into heat transfer within porous media undergoing phase transition change.