Low field NMR based relative permeability and drying model for unsaturated granular materials

• Published in: Engineering geology Vol. 352; p. 108071
• Main Authors: Cha, Wonjun, Park, Junghee, Woo, Sang Inn
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05.06.2025
• Subjects: air flow; climate change; Drying; Evaporation; evolution; geometry; glass; mercury; Nuclear magnetic resonance; nuclear magnetic resonance spectroscopy; permeability; Pore characterization; porosimetry; porosity; Relative permeability; sand; Unsaturated soils; vapors; Pore characterization; Relative permeability; Nuclear magnetic resonance; Evaporation; Unsaturated soils; Drying
• ISSN: 0013-7952
• DOI: 10.1016/j.enggeo.2025.108071

Climate change and airflow variations critically influence soil-atmosphere interactions and subsurface evaporation processes. This study investigates the role of particle and pore sizes in the drying dynamics of unsaturated coarse-grained granular media under low-humidity airflow, employing coupled nuclear magnetic resonance NMR and matric suction measurements. Comprehensive experiments analyze grain size impacts on (1) drying rates, (2) matric suction evolution, and (3) T2 relaxation times. Results reveal that finer-grained specimens retain higher asymptotic saturation due to stronger capillary forces, while smaller mean grain sizes d50 correlate with elevated matric suction in constant-suction zones. A geometric constant (a = 4), derived from mercury intrusion porosimetry, BET specific surface analysis and NMR spectroscopy with various coarse-grained materials, enables direct conversion of T2 relaxation times to pore diameter dp and assuming simple cubic packing allow to estimate d50. This constant underpins a log-normal pore size distribution model that aligns with suction measurements. We propose a three-stage drying model integrating surface evaporation, capillary flow, and soil-internal vapor diffusion, validated against experimental data for glass beads and sand. NMR-derived hydraulic properties enable accurate predictions of drying curves, advancing non-destructive characterization of unsaturated soils for geotechnical applications. •A novel drying model is developed that integrates three transport mechanisms: surface evaporation, capillary flow, and evaporation within sediments, enhancing predictions of drying behavior in unsaturated coarse-grained soils.•NMR measurements effectively characterize pore structures, allowing for accurate estimation of mean pore diameters and their relationship with matric suction during the drying process.•A new NMR-based relative permeability model is proposed, leveraging the relationship between peak relaxation time and degree of saturation, providing a robust method for estimating relative permeability in unsaturated coarse-grained soils.