Abstract: Crack is a crucial factor that affects the mechanical properties of expansive soil. Expansive soils taken from Nanyang Expressway (Henan Province, China) were compacted to big-size samples with various original water content and compactness, and then the samples were desiccated gradually in a closed greenhouse where a camera was used to record the propagation of cracks on the surfaces of samples. The captured images were analyzed quantitatively using self-compiled program of MatLab to obtain the characteristics of surface cracks which were further used to explore the laws of crack propagation. The results suggest that the course of crack propagation in compacted expansive soil samples can be divided into three stages: in the first stage minor cracks aroused and developed quickly; then main cracks emerged and cracks reached peak in the second stage; at last the cracks withdrew and remained stable in the third stage. The higher the original water content was, the faster the cracks on the surfaces of expansive soil samples developed, the higher the extent of crack development was, and the longer the crack development lasted; the higher the original compactness, the smaller the surface crack ratio and total length of expansive soil samples were, but the larger the crack width was. The above results can be explained by the mechanism of superficial evaporation and water transformation in the soil.

Key words: expansive soil, crack propagation, digital image, binary image, crack ratio, compactness, initial water content