Effects of thermal-mechanical coupling on the thermal conductivity of sandy silt: insights from NMR and SEM analysis

Existing studies mostly focus on the analysis of the thermal conductivity of frozen soil, and few reveal the mechanism of thermal conductivity change from a microscopic perspective. In this paper, based on the thermal conductivity test of sandy silt under freeze-thaw conditions, the microscopic pore structure characteristics were revealed by the nuclear magnetic resonance (NMR) and scanning electron microscope (SEM) tests. The results demonstrated that the thermal conductivity of sandy silt was closely related to dynamic changes in unfrozen water content and ice content, in which −5 °C was the key turning point of the sudden drop of unfrozen water content and the ice content close to saturation, and the thermal conductivity of freeze-thawed sandy silt could be divided into the rapid growth trend (20 °C ∼ -5 °C) and the slow growth trend (−10 °C ∼ -30 °C). From the microscopic point of view, this trend was closely related to three factors: pore size distribution, particle surface morphology, and particle contact state. During the rapid growth trend stage, the micropores inside the sandy silt accounted for a larger proportion, and the filling effect of micropores improved the overall connectivity of the soil body. The surface morphology of particles gradually changed from sharp to rounded, which increased the contact area between particles. Meanwhile, the contact state of the particles in this stage changed from loose to dense, further optimizing the heat transfer path. All three factors played a facilitating role. During the slow growth trend stage, the increase in macropores of sandy silt was larger (up to 0.5 % compared to unfrozen), and the surface morphology and contact state of the particles tended to be relatively good. However, due to the high freezing point of unfrozen water in the sandy silt, it was not possible to provide water recharge through water migration in real-time, which led to the decrease of ice content and the development of macroporous structure, and this defective pore network was difficult to construct continuous and efficient thermal conductivity channels, although it was accompanied by the improvement of particle contact or particle surface morphology. This study reveals the intrinsic relationship between the variation of macroscopic thermal conductivity and the micro-structure characteristics and provides a basic guideline for explaining the variation of thermal conductivity from the microscopic level.