Coupled heat and moisture transfer in sandy soils observed by Magnetic Resonance Imaging: effect of clay content on frost heave and thaw settlement

• The effect of clay content on frost heave and thaw settlement in sandy soils is investigated. • The Magnetic Resonance Imaging (MRI) technique is used to monitor water content profiles throughout the freeze–thaw process. • The kinetics of frost heave closely follow the kinetics of freezing front propagation. • Frost heave increases nearly linearly with clay content (from 0 to 15 %), with a rate of 0.6 mm of heave per 1 % of clay content. • Frozen fringe becomes progressively thicker as the fine content increases. Freeze–thaw processes in seasonally frozen soils involve complex interactions between heat transfer, moisture migration, phase changes and mechanical deformation. While the role of clay content in influencing frost heave susceptibility is well recognised, its effect on coupled heat and moisture transfer and thaw settlement in sandy soils remains insufficiently understood. This study investigates the impact of varying clay content on frost heave and thaw settlement in sandy soils, utilising Magnetic Resonance Imaging (MRI) to monitor water content profiles throughout the freeze–thaw process. Sandy soil specimens with 0%, 5%, 10%, 15%, and 20% of clay content were prepared, fully saturated, and subjected to unidirectional freezing and thawing under controlled thermal conditions. The results show that frost heave increased from 0 mm for clean sand to a maximum of 9.1 mm at 15% clay content. Continuous MRI measurements captured the evolution of liquid water content during both freezing and thawing phases, indicating an increase in water content after a freeze-thaw cycle from 0% for clean sand to approximately 30% at 15% of clay. These findings demonstrate that clay content significantly alters moisture redistribution and coupled heat–moisture transfer mechanisms in sandy soils, providing new insights for predicting frost-related ground deformation.