Microstructural insight into the effect of fine content on frost heave in clayey sand mixtures

This study investigates the complex thermo-hydro-mechanical response of frost-susceptible soils subjected to freezing and thawing, using in situ X-ray computed tomography. Five saturated clayey sand mixtures with varying kaolin contents have been subjected to two freeze-thaw cycles by means of a cooling plate. X-ray computed tomography enabled the reconstruction of three-dimensional images of the samples at different deformation stages during the thermal process, revealing the influence of fine content on the microstructure evolution. The soil response to freeze-thaw cycles is shown to be highly heterogeneous, with local axial deformation exceeding 100%, especially in the central regions of samples with higher fine content. During freezing, frost heave ratios reached 25%, 47%, 66%, for mixtures with 5%, 10% and 20% of kaolin, respectively, and up to 78% for the mixture with 50% kaolin, the main cause of deformation being the water migration driven by fine pores. Upon thawing, the soil rarely returns to its initial undeformed state, often resulting in a significantly altered microstructure. Image analysis provided both qualitative and quantitative insights into the evolving non-homogeneous density patterns and the localized phenomena occurring throughout the transient process.