Effect of freezing-thawing cycles on the microstructure of soils: A two-dimensional NMR relaxation analysis

Structural deterioration of a soil subjected to freezing-thawing cycles can significantly influence its engineering performances. The nuclear magnetic resonance (NMR) technique provides a powerful tool for studying the structural evolution of water-saturated porous media such as soils. In the present work, three soils were tested under repeated freezing-thawing cycles. The process was monitored by measuring T-2 distribution curves, T-1-T-2 correlation spectra and distributions of log (T-1/T-2) with NMR technique, and then adsorptive water contents were calculated. The variation of the adsorptive water contents increased as the fraction of sand in the soil increased, implying that the higher the sand content, the more significantly the soil microstructure varied. The T-2 distribution curves for soil with sand fraction of 30% did not significantly change, whereas those for soils with higher sand fractions (50% and 70%) were quite different. In addition, Peak 2 changed from a flattened peak to a sharp peak, implying a more uniform microstructure after cyclic freezing-thawing. The peak intensity in T-1-T-2 correlation spectra changed quite significantly during the freezing-thawing process, implying that the two porosity systems in the soil exchanged water during the process. Significant change of soils usually occurred only in the first 5 freezing-thawing cycles. The microstructure variation for soil with sand fraction of 70% was larger, and then soils with sand fractions of 30% and 50% were less in order, suggesting a ranking of variation degree in agreement with sand fraction.