Background  Soil bund has been widely constructed as important soil conservation practice on sloping farmlands in the purple-soil area, while the stability of the soil bund affects its soil conservation benefits. The propagation and closure evolution of cracks reduces the stability of soil bund following drying and wetting alternations, thereby affecting the associated soil conservation benefit provided. This study aims to reveal the propagation and closure evolution pattern of cracks developed on purple-soiled bund following alternative drying and wetting condition using indoor simulation test, and to determine the relationship between crack evolution and stability of soil bund.

Methods  Undisturbed soil samples were collected from in-situ sloping farmland soil bund in Xiema town, Beibei district, Chongqing. Alternative drying and wetting was simulated in the laboratory, then the cracks images were extracted dynamically, Photoshop and image software were used to calculate the characteristic parameters of cracks (area density, area-perimeter ratio, shape index and fractal dimension).

Results  1) During the drying process, the cracks intensity (area density and area-perimeter ratio) and complexity (shape index and fractal dimension) firstly increased significantly (1-9d) and then slowly increased to a stable state (>9-15d), while cracks intensity and complexity indexes showed a significant positive correlation (P < 0.01). 2) During the wetting process, the evolutionary process of cracks morphology could be divided into 3 stages, i.e., (Ⅰ) secondary cracks closure stage, (Ⅱ) cracks intersection disconnection stage, and (Ⅲ) main cracks narrowing stage. The overall average declining rate of cracks intensity and complexity were 87% and 61%, respectively, following a sequence of stage Ⅰ (62%)>stage Ⅱ (47%)>stage Ⅲ (33%), and stage Ⅰ (52%)>stage Ⅱ (18%)>stage Ⅲ (17%) for cracks complexity. 3) The average values of cracks intensity and complexity was proportional to the number of wetting and drying alternations. During the initial two wetting and drying alternations, the cracking degree and network complexity changed sharply, and then gradually decreased, reached the maximum after the 4th of alternation, and the bunds peel appeared at the 5th wetting process.

Conclusions  This study shows that propagation and closure evolution of cracks subjected to alternative drying and wetting is an important factor reducing the stability of the soil bunds. With the increase of the number of the alternations, the evolution intensity and network complexity of cracks gradually increase, leading to the instability of the soil bunds. This findings can provide implications for the construction and maintenance of purple soil bunds for soil conservation purpose.