Analysis of unfrozen water in cation-treated, fine-grained soils using the pulse nuclear magnetic resonance (P-NMR) method

Abstract

Unfrozen water within frozen soils is a key component that determines a soil's thermophysical response to changing physical and environmental conditions. This research focuses on the use of pulse nuclear magnetic resonance (P-NMR) for measuring unfrozen water content within frozen soils. The research is divided into two components: 1) improvements made to the P-NMR testing method, including refinements in the laboratory set up and testing procedure, and experimental validation of the normalization method; and 2) determination of unfrozen water content of fine-grained, cation-treated samples at various sub-freezing temperatures using the improved P-NMR methodology. Previous P-NMR testing used the first return data from the free induction decay signal intensity to calculate unfrozen water content; however, this approach may overestimate unfrozen water due to inclusion of ice content. This research used the normalization method for calculating unfrozen water, which proved to be repeatable with excellent agreement between P-NMR-derived unfrozen water and physical gravimetric water content data. Cation treatments of five standard clays and one heterogeneous soil were prepared to determine how the physicochemical structure of clays, including the adsorbed cations, controls the amount of unfrozen water. Results indicated that cation treatments have negligible effect on the unfrozen water content of kaolinite, and minimal effect on illite, chlorite, and the heterogeneous soil. Conversely, soils that are partially or completely composed of smectite demonstrated the largest unfrozen water content when treated with Na⁺ cations, and a marked reduction with the K⁺ treatment. Using the results of the standard clay testing, the unfrozen water content for the natural, heterogeneous soil was estimated, which matched measured values within 4%. This suggests that the unfrozen water content of a heterogeneous soil with a known mineralogy may be approximated from a database of measured standard clay unfrozen water contents of standard clays.