Abstract: To investigate the dynamic expansion characteristics and mechanism of three-dimensional cracks in expansive soil, a constant temperature and humidity environment box was constructed. The real-time images of rectangular and circular expansive soil cracks with thicknesses of 4 mm, 8 mm, and 16 mm were captured using binocular cameras. The three-dimensional shapes of the cracks were reconstructed using three-dimensional digital image correlation (3D-DIC) technology, and the expansion mechanism and development characteristics of the cracks were quantified. The results show that shrinkage centers vary in time and space when expansive soil cracks, and the number of shrinkage centers decreases with the increase of sample thickness. More shrinkage centers result in more complex crack network shape. The movement of the shrinkage center shows vertical settlement, and the settlement is related to the moisture content of the cracked soil block. The motion vectors of surrounding soil particles move towards the shrinkage center in a bowl shape. The uneven deformation of the soil surface varies with the spatial distribution of the shrinkage centers, causing shear stress and deflection in the development direction of some cracks, resulting in a diverse fracture network connection. The cracking process of expansive soil is accompanied by the curling of the cracked soil block. Regarding the formation and movement mechanism of the curling of cracked soil block, after the development and formation, the overall humidity difference is “dry outside and wet inside”. The resulting suction difference in the profile matrix causes a vertical contraction deformation gradient. The gradient generates a bending moment that drives the cracked soil block to sag and gathers towards the contraction center, promoting the curling formation of the soil block. The test and analysis results can provide references for the cognition, analysis and prediction methods of expansive soil crack development law.

Key words: expansive soil, three-dimensional dynamic expansion of cracks, three-dimensional digital image correlation technology, drying induced curling