The sandy clayey purple soil is susceptible to alterations in particle size distribution due to hydraulic erosion, such as rainfall and fluctuations in reservoir water levels. These changes can affect the soil’s strength. To elucidate the strength characteristics of sandy clayey purple soil in the influence of the absence of different particle groups, consolidated drained triaxial shear tests of saturated sandy clayey purple soil with five distinct particle groups were conducted. The fractal dimension was employed to quantify the physical properties of varying particle sizes. The relationship between the strength characteristics of purple soil with different particle groups and the fractal dimension was obtained. The results show that: (1) There is no significant change in the internal friction angle of the soil shear strength index under the missing effect of different grain groups, but the cohesion and fractal dimension show a hyperbolic relationship; the initial tangent modulus of different grain groups is affected by confining pressure and fractal dimension, and there is a quadratic function relationship between parameter n and fractal dimension. Under low confining pressure, the stress-strain relationship of different grain groups is obviously different. With the increase of confining pressure, the stress-strain characteristics tend to be consistent. (2) Based on the Duncan-Chang E-ν model, the nonlinear elastic hyperbolic constitutive model of saturated sandy clayey purple soil under the effect of different particle groups is established by fitting the cohesion-fractal dimension hyperbolic equation, and the validity of the model is verified.

Purple soil is often characterized by a loose structure and high erodibility, particularly prone to desiccation cracks under the alternating action of the rainfall and evaporation. These cracks can significantly damage the integrity of the soil structure and further degrade its water-holding capacity. This study aims to systematically clarify the influence of the initial cracks on the water-holding capacity of the purple soil. The undisturbed ring knife samples were prepared with the initial crack parameters, including the crack ratio, crack length, average crack width, and crack number. The soil-water characteristic curve (SWCC) of each sample was obtained using the axis translation. The Fredlund-Xing model was adopted to conduct fitting analysis on the measured SWCC data. Meanwhile, the grey correlation analysis was employed to explore the influence of the various crack parameters on the characteristic parameters of the SWCC and the fitting parameters of the Fredlund-Xing model. Additionally, the low-field nuclear magnetic resonance (NMR) scanning tests, including the nuclear magnetic resonance T2 spectra analysis and pore throat distribution analysis, were carried out to verify the correlation between soil pore distribution and crack development. The results indicated that the influence of the initial cracks on the water-holding characteristics of the purple soil was concentrated mainly in the low suction section of the SWCC, indicating the rapid dehydration that was accompanied by the "plateau period". In contrast, the initial cracks had a minimal impact on the transition and residual section of the SWCC, where the water-holding characteristics remained relatively stable. Further analysis showed that the air-entry value of the soil was inversely proportional to the crack ratio and dehumidification rate. The Fredlund-Xing model sand showed an excellent fitting performance for the SWCC of the purple soil with the initial cracks, particularly with the determination coefficient generally higher than 0.85. The parameter, α, which was closely related to the air-entry value, was inversely proportional to the crack ratio and dehumidification rate. While the parameters n and c showed no significant linear correlation with any crack parameters. The grey correlation analysis revealed that the crack ratio was the primary dominant influencing factor on the water-holding capacity of the purple soil (correlation degree > 0.67), followed by the average crack width, whereas the crack number shared the weakest correlation with the water-holding capacity. The initial cracks also destroyed the internal structure of the soil. Thus, a dual-pore system was formed with the interconnected crack space and intact non-cracked soil matrix. The proportion of the large pores was dominated by the macro-cracks, while there was a relative decrease in the proportion of the medium and small pores that were controlled by the soil matrix. The preferential water flow effect of the crack system was reduced in the soil air-entry value at the low suction stage. The "plateau period" also occurred in the SWCC. The NMR scanning tests confirmed that there was a close correlation between pore distribution and crack development: The nuclear magnetic resonance T₂ spectra and pore throat distribution verified the negative effect of the cracks on the soil pore distribution. A multi-scale verification on the SWCC was integrated with the macro water-holding, micro pore structure, and model fitting parameters under crack disturbance. The dominant role of the crack network was clarified as a preferential water flow path to regulate the soil hydraulic behavior. The influence mechanism of the initial cracks on the water-holding of the purple soil can provide a solid theoretical basis and key parameter support for soil and water conservation, soil improvement, and accurate thresholds of the landslide disaster early warning in purple soil distribution areas.

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.

Purple soil is widely distributed in southwestern China and is one of the important natural resources. Because of the loose parent rock and low degree of differentiation, purple soil is characterized by a loose structure that is easily eroded and high soil fertility. Soil erosion is very serious in the purple soil region, which is one of the main sources of sediment in the upper reaches of the Yangtze River, and has a certain impact on the agricultural production and engineering development in the purple soil region. After erosion, purple soil shows the characteristics of reduced dry density of soil and serious loss of fine particles, which affects its water-holding characteristics. In order to explore the changes of soil water-holding properties of purple soil after rainfall erosion, this study was carried out to analyze the effect of missing grain groups on the water-holding properties of sandy clayey purple soil under rainfall conditions by selecting three test methods, namely, axial translation technique, filter paper method, and saturated salt saturated saline solution vapor equilibrium technique for the three grain groups which are the most susceptible to changes in the rainfall conditions, and carrying out the soil-water characteristic curve (SWCC) test with the absence of grain groups within the full suction range to analyze the effect of the missing grain groups on the water-holding properties of sandy clayey purple soil. The results show that: 1) the missing grain group reduces the water-holding characteristics of sandy clayey purple soil, which is most obvious in the boundary effect zone and transition zone, and has less effect on the residual zone. The mass of the missing grain group is inversely proportional to the air intake value and the residual water content. 2) Using the grain size parameters (constrained diameter、average diameter、median diameter、effective diameter) to characterize the soil grading condition, the relationship between the soil grading curves and the characteristic values of SWCC was established. It was found that except for effective diameter, the other parameters were linearly inversely related to the air intake value and residual water content; 3) The data obtained from different methods were fitted using the Fredlund-Xing model, and it was found that the results of the data obtained from a single test method fitted better, but the SWCC significantly deviated from the test data points in the boundary effect zone and the transition zone. Root mean square error (RMSE)、percent bias (PB) and mean absolute percent relative error (MAPRE) were introduced as evaluation indexes, and it was found that the suction range was the main factor affecting the fitting effect of SWCC, and within a certain suction range, the larger the range of data points involved, the better the model fitting effect was, and after a certain suction value was reached, the suction range had very little effect on the model fitting effect. It is proposed that to obtain the accurate SWCC of purple soil quickly, only the maximum suction value needs to be determined, and the number of test points with high suction is reduced, which can effectively shorten the SWCC test period. The results lay a foundation for the subsequent study of the role of soil erosion and water-holding characteristics of purple soil, and also provide reliable ideas for the prevention of soil erosion and the application of soil-water characteristic curves in the Three Gorges Reservoir area.

Purple soil, one very representative type of agricultural land, has been widely distributed in the Sichuan Basin and the Three Gorges Reservoir area, such as Chongqing. Among them, purple soil often exhibits a loose structure, low strength, poor corrosion resistance, and water stability. Therefore, it is prone to soil and water loss under the frequent alternating action of dry and wet conditions caused by heavy rainfall and hydraulic erosion. Enzyme Induced Carbonate Precipitation (EICP) technology can be expected to effectively enhance the pore structure, strength, stiffness, and corrosion resistance of the soil, thereby achieving soil solidification. In this study, self-extracted soybean urease was used to induce calcium carbonate precipitation, in order to solidify the purple soil. A series of tube tests were carried out to investigate the relationship between temperature and urease concentration. Furthermore, an orthogonal test was conducted to explore the optimal calcium yield combination (C_R group), as well as the maximum calcium production combination (C_P group) under the combined influence of soybean urease concentration (A), calcium chloride concentration (B), and urea concentration (C). Subsequently, curing tests, wetting-drying cycles tests, and unconfined compressive tests were performed on the purple soil samples to investigate the impact of different curing combinations and numbers of wetting-drying cycles on sample quality, appearance, unconfined compressive strength, and stiffness. The evolving nature of the curing process was characterized to analyze its mechanism by using scanning electron microscopy (SEM), X-ray energy dispersive analysis (EDS), and low-field nuclear magnetic resonance imaging (NMR). The results showed that: 1) The yield of calcium and the productivity of calcium carbonate are dependent on the concentrations of urease, calcium chloride, and urea. The optimal combination of calcium yield (C_R group) was a urease concentration of 100 g/L, a calcium chloride concentration of 1.0 mol/L, and a urea concentration of 1.5 mol/L. The maximum combination of calcium production (C_P group) was a urease concentration of 150 g/L, a calcium chloride concentration of 2.0 mol/L, and a urea concentration of 2.0 mol/L. 2) EICP treatment significantly enhanced the unconfined compressive strength and stiffness of purple soil (elastic secant modulus E₅₀). Compared with the Control group (C_W group), the unconfined compressive strength increased by 104.47% in the C_R group and by 60.03% in the C_P group; while E50 increased by 86.36% in the C_R group and by 36.56% in the C_P group. The cured sample shared excellent durability after seven wetting-drying cycles. Among them, the unconfined compressive strength remained at 440.65 kPa for the C_R group samples and at 507.92 kPa for the C_P group samples; E₅₀ reached 24.02 MPa for the C_R group and 27.57 MPa for the C_P group. 3) SEM and NMR tests were carried out to quantitatively characterize the pore structure of soil. Microscopic analysis showed that EICP was used to solidify the purple soil with generated calcium carbonate particles sized between 0.1-2.0 μm. In small pores, calcium carbonate was used to fill and cement them. While in large and medium pores, the generated calcium carbonate was provided cementation and film coating, resulting in the transformation of large pores into medium-sized ones. Thus, the proportion of small and large pores decreased greatly, while the proportion of medium pores increased. As such, a more uniform pore structure was obtained in the purple soil, in order to effectively improve its integrity and compactness. Nuclear magnetic imaging analysis also intuitively proved that the EICP technology shared a better curing effect on the purple soil. This finding can provide the scientific basis and recommendations for engineering applications, such as foundation construction, slope reinforcement, and soil erosion protection in the area of purple land.