Structural change and volumetric shrinkage of clay due to freeze-thaw by 3D X-ray computed tomography

Artificial ground freezing (AGF) has found extensive applications in construction of civil structures, and has been frequently used in the construction of subways in soft ground within urban settings. Ground heave and subsidence due to AGF of clay soils are of great concern. Understanding of both short-term and long-term settlement mechanisms is still very limited and estimation of settlement amount needs further improvement. This paper describes an apparatus for conducting unidirectional freeze-thaw experiments in a closed system, presents testing results on structural change in terms of moisture redistribution, void ratio and dry density variation within high-quality unsaturated clay specimens, and discusses the mechanism of structural change and limitations of the experiment. In particular, soil specimens before and after freeze-thaw were scanned by three-dimensional X-ray Computed Tomography (X-ray CT) to reveal detailed structural changes. Non-uniform volumetric shrinkage, referred to as freeze-necking, was observed on an unsaturated clay specimen. A short-term volumetric shrinkage ratio was defined and assessed using the three-dimensional X-ray CT image. It was found to be closely related to freezing temperature and have a linear relationship with the freeze equilibrium time. The volumetric shrinkage was likely induced by moisture migration, and could be useful for estimating short-term settlement caused by AGF in unsaturated clay.