Freeze-necking and volumetric change of clay during freezing by 3D x-ray computed tomography

In artificial freezing engineering, the freezing temperature is an important factor affecting soil frost heave deformation, and studying its impact is of great significance. The frost heave ratio of soil is a crucial factor for designing and predicting soil frost heave. However, it only considers vertical deformation while neglecting radial deformation. This paper introduces a simple unidirectional freezing apparatus specifically designed for three-dimensional x-ray computed tomography (CT) scanning, which allows for the investigation of internal structural changes in clay during freezing at four different freezing temperatures (i.e., -3 degrees C, -5 degrees C, -7 degrees C, and -9 degrees C). Freeze-necking of the soil was observed during freezing. An image processing method was proposed to segment the soil samples, and parameters such as length, equivalent diameter, and volume were measured to assess changes during freezing. The observed variations in necking depth and equivalent diameter indicate that freeze-necking is uniform. As the freezing temperature decreased, the necking depth reduced from 72.4 mm to 38.1 mm, and within this necking depth, the equivalent diameter decreased progressively from the bottom to the top. Moisture content increased near the cold end of the soil and decreased near the warm end, suggesting that freeze-necking is due to moisture migration within the soil. Considering freeze-necking, the volumetric frost heave ratio was defined to characterize soil frost heave deformation. This ratio also decreases as the freezing temperature decreases, and the values are smaller than those of the traditional frost heave ratio. The discrepancies become more pronounced at higher freezing temperatures, reaching up to 1.8% at -3 degrees C. The results indicate that lower freezing temperatures can reduce frost heave deformation, and freeze-necking requires greater attention in engineering at higher freezing temperature.