Thawing mechanism of frozen loess soil based on a nuclear magnetic resonance study

Permafrost thawing is a common natural phenomenon in cold regions, where it has significant impacts on ecosystem stability and the sustainability of human society. This study elucidates the melting process of frozen soil and the importance of water content during the thawing process at the pore scale based on nuclear magnetic resonance (NMR) investigations. Additionally, thermodynamic theory is applied to interpret the link between the pore ice melting process and the NMR T₂ relaxation signals. The NMR signal intensity has been used to estimate the thawing degree of frozen soil, however, the mechanism underlying the shift in the T₂ signal peak has not been revealed. In this study, a pre-freezing thawing experimental platform was established to capture pore-scale characteristic thawing (temp gradient -30^oC, -20^oC, -15^oC, -10^oC, -5^oC, -3^oC, -2^oC, -1^oC, 0^oC, 1^oC, 5^oC, 15^oC) of four different loess soil samples with various saturation levels ranging from 25% to 100%. The results show that the T₂ distribution clearly demonstrates three distinct thawing mechanisms in frozen soil thawing: (1) surface water melting corresponds to an increase in the T₂ peak amplitude; (2) bulk water melting corresponds to a broadening of the T₂ peak; (3) pore water migration from large pores to small pores corresponds to a shift in the T₂ peak. Furthermore, measurements from unsaturated samples (25%, 50%, 85% saturation) provide insights into the importance of water content in the thawing process. Collectively, our method for interpreting thawing behaviors of soil provides a non-invasive and high-resolution method to understanding the dynamic soil-water behaviors in cold regions and can further help establish advanced freeze-thaw induced landslides monitoring framework.
Keywords frozen soil; pore ice; melting mechanism; nuclear magnetic resonance; loess