Abstract: Expansive soil is a widely distributed soil with obvious dilatancy, shrinkage and crack-rich properties. Its crack initiation and propagation are affected by many factors. In this study, the image processing and crack quantitative analysis program developed by MATLAB were used to explore the size and temperature effects of the crack evolution of expansive soil samples. Firstly, twenty-five expansive soil mud samples with different initial states were prepared. Then, the evolution of cracks on the surface and the corresponding water content of the samples during the shrinkage due to water loss were recorded using a designed camera system. Finally, using crack quantitative analysis program, some indexes including fracture degree, aspect ratio, average crack width and fractal dimension, were quantitatively analyzed under the size effect or temperature effect, and the size-temperature combined action, respectively. It is shown that the fractal dimension value of the crack remains stable, and is only slightly affected by the thickness of the samples. But there exists an approximate logarithmic relationship between the fractal dimension and the moisture content. In the process of water loss and shrinkage, the final fracture index of the expansive soil was mainly affected by the thickness of the sample, and the surface size also had a certain impact on the final value of the fracture length to diameter ratio and width. Temperature accelerates fracture development and stability. However, the impact of average width value and temperature on the final index was not obvious. The thickness was the most significant factor, followed by the surface size, and finally temperature. Additionally, the microcosmic mechanism of shrinkage and cracking process of expansive soil were also analyzed according to this experiment.

Key words: expansive soil, fracture, size effect, temperature effect, quantitative characterization