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Purple soil is one kind of agricultural soil that is widely distributed in the Three Gorges Reservoir area. The drying desiccation cracks in the purple soil can be easy to form under the rainfall-evaporation cycle. Hydraulic and mechanical properties of the soil can also be significantly exacerbated by natural disasters, such as soil erosion. Three Gorges Reservoir area also shares the complex topographical characteristics. Geometric size has been the most common environmental factor in the development of desiccation cracks in purple soil. Additionally, the recurrent persistent heavy rainfall often leads to severe hydraulic erosion in the purple soil, leasing to the dispersion of soil aggregates, the migration of fine particle runoff, and significant soil structural damage. The soil saturation can be localized, due to waterlogging. This study aims to investigate the geometric size effect of desiccation cracking under hydraulic erosion using the response surface method (RSM). A series of indoor drying experiments were conducted on 13 groups of purple soil mud samples with different thicknesses (10, 20, and 30 mm) and diameters (100, 200, and 300 mm). The crack morphologies were quantitatively characterized using digital image processing. The results show that: 1) Geometry posed an outstanding effect on the crack ratio, length density, average width density, and fractal dimension of desiccation cracks. Thickness was the main influencing factor on crack ratio and fractal dimension, while there was a more pronounced effect of the diameter on the crack length and width development. All crack ratio, average width density, and fractal dimension showed linear growth with an increase in thickness, whereas, ρl decreased linearly. length density decreased linearly with an increase in diameter, while average width density, and fractal dimension decreased nonlinearly, and crack ratio increased linearly. There was an interaction effect of diameter and thickness on crack ratio, average width density, and fractal dimension; 2) The cracks mainly presented in three forms of “I”, “T”, and “Y”, where the “T” type cracks were the highest proportion. The smaller the thickness was, the denser the crack distribution and the more irregular the crack network was. The smaller the diameter was, the more single the crack network and the more the cracks distributed toward the edge of the soil mass. The proportion of single crack area was mainly less than 0.2%, with a frequency exceeding 0.5. The probability density of the proportion of single crack area decreased exponentially with the increase of crack area. The frequency distribution followed the Expdec function curve. A random orientation was found in the initiation and propagation of cracks. However the size of the soil specimens significantly affected the uniformity of the crack direction distribution; 3) The geometric dimensions had a significant impact on the ease of cracking in purple soil, indicating a smaller average cracking stress with a larger diameter. The soil was more prone to desiccation cracking. However, the large thickness was selected to diminish the geometric size effect of diameter on the average cracking stress. The mechanical mechanism behind crack development was determined, according to the inter-particle unsaturated soil mechanics. The soil shrinkage and eventual cracking were attributed to the imbalance in the tensile stress field caused by an increase in the matric suction, due to the moisture evaporation. Geometric dimensions significantly impacted the rate of moisture evaporation. The finding can provide a strong reference for the morphology and development of cracks in purple soil in the Three Gorges Reservoir area. [ABSTRACT FROM AUTHOR]

The sandy clayey purple soil is widely distributed in the middle and upper reaches of the Yangtze River, serving as one of the main soil resources in the mountainous areas of southwest China. Intense physical weathering, loose structure, and low erosion resistance are the main characteristics of sandy clayey purple soil. Therefore, environmental disturbance, such as the water fluctuation near reservoirs, heavy rainfall, and groundwater, often induces soil erosion, landslide, settlement, and soil-water disasters of purple soil. A serious threat has been posed on the village buildings and roads, even the agricultural production. The properties of water immersion disintegration with the sandy clayey purple soil can also be an important reason for water and soil disasters in the southwest mountainous areas. It is necessary to clarify the disintegration characteristics and reinforcement for the water-soil disasters prevention and control. In this study, a disintegration test was performed on the purple soil samples with different initial dry densities, water content, and grain gradation using a self-developed instrument. Meanwhile, the disintegration and evolution of sandy clayey purple soil were also analyzed from the perspective of unsaturated effective stress. Bacillus megaterium was selected to reinforce the soil samples with the Microbial Induced Calcite Precipitation (MICP), as the more suitable as Sporosarcina pasteurii in the sandy clayey purple soil areas. A Scanning Electron Microscope (SEM) was then utilized to characterize the morphologies of the soil sample, thereby determining the MICP improvement on the disintegration characteristics of sandy clayey purple soil. The results show that: 1) Four stages were divided in the whole process of immersion and disintegration of sandy clayey purple soil, including the air-water conversion, equilibrium, disintegration development, and disintegration residual stage. 2) The initial dry density, water content, and grain gradation obviously affected the disintegration characteristics of sandy clayey purple soil. Specifically, the disintegration rate and the average disintegration velocity decreased, with the increase of initial dry density and water content. In addition, the average disintegration velocity of soil increased by the content of fine particles. 3) The evolution of water and air was ranging from the pore water closed, double connected, and pore air closed morphology, with the increasing of the initial saturation. Water was rapidly absorbed into the pores under the matric suction, where the pore pressure was changed significantly. Subsequently, the effective stress of unsaturated soil rapidly reduced to the negative, leading to an interparticle compressive stress (the negative tensile stress). Once the tensile stress reached the value of effective cohesion, the unsaturated strength of purple soil was lost completely. Finally, the soil sample was then destroyed under disintegration. The more severe disintegration was also obtained with the decrease in the initial saturation of a soil sample. The decay process of the unsaturated effective stress depended greatly on the initial saturation after the purple soil was immersed in water. Specifically, the average disintegration velocity attenuated exponentially with the increase of the initial saturation. 4) The disintegration rate and the average disintegration velocity of the MICP treated soil samples decreased by 73 to 97 percentage points and 84%-99%, respectively, compared with the untreated soil. Calcium carbonate crystals formed by solidification and deposition greatly reduced the micro-cracks and large pores in the soil structure. As such, a denser pore structure was achieved to enhance the strength of intergranular cementation for the higher resistance to the disintegration of the soil. Consequently, the MICP technology can serve as an effective measure to prevent the water and soil disasters of the sandy clayey purple soil in southwest mountainous areas. [ABSTRACT FROM AUTHOR]