Sample thickness had an effect on not only self-sealing but also hydration cracking of compacted bentonite.
Increasing thickness slowed down the self-sealing rate, thus extending the time required for filling technological voids.
Thickening sample reduced the development degree of hydration cracks and simplified the crack networks notably.
A critical value of sample thickness was predicted beyond which no hydration cracks could be observed on the sample surface.
In the hydration process of compacted Gaomiaozi (GMZ) bentonite with technological voids, the cracking phenomenon happens simultaneously with self-sealing. In order to investigate the effect of sample thickness, a series of hydration tests were carried out in this paper and the evolutionary process of bentonite was visualized using a custom-designed device. Corresponding digital images were acquired by time-lapse photography and analyzed by image processing techniques. Experimental results showed that the sample thickness exerted an apparent effect on both the self-sealing and the hydration cracking of compacted bentonite. Increasing thickness slowed down the self-sealing process and thus prolonged the time needed to seal the technological voids. Moreover, it was found that thickening the sample inhibited the development of hydration cracks and simplified the crack network notably, indicated by a decreasing trend of crack amount, length and area. In addition, the fractal dimension, characterizing the complexity degree of crack network, decreased with the increase of thickness. By fitting and predicting, a critical value was obtained that if the sample thickness exceeded it, no hydration cracks could be observed on the surface and no penetrating cracks could be formed in the sample. These conclusions are of great importance in the design of bentonite barriers by reducing the incidence of hydration cracks.