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.