Morphological quantity analysis of soil surface shrinkage crack and its numerical simulation

Abstract: Soil cracking is a common phenomenon in nature, which has great influence on soil properties, plant growth and soil moisture transfer. This study aimed to reveal the development law and formation mechanism of dry shrinkage cracks on the soil surface. Soil crack evolution test was carried out in 1-m2 soil in Yellow River irrigation area (105°40′E, 37°28′N) of Ningxia Zhongning County. Soil was irrigated to saturate condition, and dried under sunlight to form cracks. A Canon digital camera was used to take crack pictures every 2 h for 3 days. The picture was processed through binarization, noise removal and edge extraction of fracture for extraction of morphological properties. Minkowski area, length and Euler number were to quantify crackers. The cracker evolution was simulated through morphological dilation and erosion operation. The model assumed that the soil critical strain field followed normal distribution. Friction was taken as soil static adhesion force, and grid structure contraction frequency represented soil moisture evaporation intensity in hexagonal network structure composed of nodes. In the simulation, vertical interaction between layers of the soil was ignored, and the clay was considered as elastic material. The distance between each node in hexagonal structural network was reduced gradually on behalf of the soil evaporation. The reducing of distance between each node could cause the changing of force between network nodes, which resulted in soil cracking. The model was solved using program written by C++ language under the CodeLite platform and generated Tiff image as output. By comparison of crack area, crack length, euler number density that calculated by analyzing experiment pictures and images the program produced, it could be drawn that crack morphology could be described by Minkowski area, length, euler number density effectively; each parameter had its own variation as morphological analysis results did, among which area density function was an increasing function between 0 and 1, the length density function increased and decreased, and the euler number density function increased and then decreased, and finally increased again. Furthermore, a two-dimensional soil cracking model was established based on Hooke's law. To analyze the calculation results of different soil cracks, statistically equivalent soil cracks had been investigated, 100 seeds were used to generate 100 groups of random structured soil cracks. The average level and standard deviation of each parameter of 100 groups of random structured soil cracks were also calculated and the average density value in this experiment was basically distributed between the mean value and standard deviation of the model. Coefficient of determination of area, length and euler number density was 0.893-0.928. Root mean square error was 0.002-0.039. Bias was 0.064- 0.144. Index of agreement was greater than 0.888. It suggested that the model established had high precision. Therefore, the model based on Hooke's law can effectively simulate the cracking process of soil. Simulation of surface soil cracking with numerical method is helpful to study the formation of farmland soil cracks as well as its dynamic evolution mechanism.