Cracking in compacted expansive soils under unidirectional wet-dry cycles: insights from X-ray computed tomography

Cracking of compacted clays during cyclic wetting–drying poses significant challenges to the stability of channel slopes. This study performed a series of unidirectional wet-dry tests to evaluate the cracking behavior of expansive soils collected from a channel slope in northern Xinjiang, China. Using computed tomography scanning and three-dimensional (3D) reconstruction, the internal crack characteristics of expansive soils were quantitatively described. The results indicate that the penetration depth of the cracks was stabilized after five cycles, reaching 31.4% of the initial specimen height. The morphologies of internal cracks revealed a transition in the cracking mode, form shallow and scattered cracks in the initial stage to deeper and more clustered cracks in the final stage. Centralized cracks were prominent in the first three wet-dry cycles, followed by a shift to crack deflection from the vertical plane in subsequent cycles. Four indices (i.e., slice crack ratio, crack length, branching number, and dead-end point) provided a satisfactory quantitative depiction of the evolution of the spatial distribution and connectivity of the cracks over the number of cycles. Additionally, the crack volume fraction and fractal dimension effectively evaluated the 3D cracking behavior of soil crack networks.