Freezing and thawing characteristics of frozen soils: Bound water content and hysteresis phenomenon

The freezing/thawing characteristics of three soils with different compositions are investigated based on the nuclear magnetic resonance (NMR) proton spin spin relaxation time (T2) distribution and free-induction decay (FID) measurements. A threshold value of T2 is identified for defining the bound water, and the bound water contents in the soils are calculated. It is demonstrated that the bound water content varies insignificantly with temperature in all the tested soils. The hysteretic characteristics of soils are analyzed during a freezing/thawing cycle, and possible mechanisms for the hysteresis are discussed. It is found that the hysteresis loop can be divided into three distinct zones, in which the hysteresis can be accounted for by metastable nucleation, capillary and adsorption effects, and adsorption effect, respectively. A NMR-based procedure is proposed for determining the unfrozen pore water content. In particular, it is shown that the thickness of the water films decreases with the temperature in the fully-saturated frozen soils.