Search

Soil cracks thoroughly affect the soil shear strength, which is a critical parameter in the analysis and modeling of slope stability. However, the relationship between soil cracks and shear strength in collapsing gullies remains inadequately studied. In this study, soil collected from collapsing gully areas was used to quantitatively investigate the parameters of soil desiccation cracks and the effects of soil cracks on shear strength. The results showed that the development degree of soil cracks increased gradually with decreasing soil water content. During desiccation, surface crack evolution was dominated by the formation of more cracks when soil water content was higher than 20%, and then experienced crack width extension for at lower soil water content. Three-dimensional crack analysis showed that cracks in the undisturbed soil of the red soil layer were very densely developed, and the cracks rates were all greater than 9%. Soil water content and cracks controlled the shear failure characteristics under triaxial compression, and the characteristics of soil shear failure can be summarized into three categories: composite softened soil, ideal elastoplastic soil, and curve-softened soil. The shear strength (τc) of undisturbed red soil samples had significant negative correlations with the crack rate (CR), total crack length (CL), crack connectivity (CC), and crack fractal dimension (FD). Linear fitting analysis showed that the effect of cracks on soil shear strength could be described as follows: τc = 1548.8−22.4 × WR−2.67 × CR−4.65×CC, which is helpful for the numerical simulation analysis of the stability of collapsing gully walls. [ABSTRACT FROM AUTHOR]

Purpose: Collapsing gully erosion is a specific form of soil erosion that is widely distributed in the hilly granitic region of tropical and subtropical southern China and resulted in extremely rapid water and soil loss. The aim of this study was to investigate the correlations between soil physicochemical and shear properties and the clay mineralogical of different profiles of the non-eroded soils (without soil erosion) and collapsing gully soils in Changting County, Fujian Province, southeastern China. Materials and methods: A total of 32 sampling soils collected from four pedons of non-eroded and collapsing gully soils were subjected to conventional soil analyses for physicochemical properties. The soil shear strength of collected soils was measured using a triaxial shear apparatus according to the unconsolidated-undrained (UU) method. The clay mineralogical of different profiles soils was examined with an X-ray diffraction (XRD) analysis. Results and discussion: The results showed that non-eroded soils had superior physicochemical characteristics. The cohesive force of the non-eroded soils was generally greater than that of collapsing gully soils. The XRD patterns of the clay fraction indicated that kaolinite, illite, hydroxy-interlayered vermiculite (HIV), and gibbsite were the dominant clay minerals in the studied soils. Pearson's correlation analysis showed that the cohesive force of the studied soils had significant and positive correlations with CEC, exchangeable Al3+ + H+, Fed, Ald, and Fet; the correlation coefficients (R value) for cohesive force were greater than those of internal friction angle. The stepwise multiple linear regression analyses indicated that exchangeable Al3+ + H+ and Ald were the dominant factors affecting cohesive force. Conclusions: Compared with collapsing gully soils, non-eroded soils had superior physicochemical and shear properties, indicating that non-eroded soils were better able to resist soil erosion. The findings obtained in the present study were of fundamental significance in understanding the correlation between shear strength and the soil physicochemical properties in the non-eroded soils and collapsing gully soils of tropical and subtropical China. [ABSTRACT FROM AUTHOR]

Purpose: Collapsing gullies, which involve considerable erosion and extreme landform changes, frequently occur in the granite region of Southern China. Capturing the evolution of collapsing gullies is useful and effective for predicting erosion amount and landform changes. However, the evolution of collapsing gullies is too complex to simulate using conventional models. The aim of this study is to modify the traditional cellular automata (CA)-Markov model for simulating the evolution of collapsing gullies and then quantify their morphology using landscape pattern metrics. Materials and methods: A hillslope eroded by collapsing gullies located in Longmen Town of the granite region of Southern China is used as a case study. Three digital elevation models (DEMs) were derived on 11 March 2017, 21 July 2017, and 2 December 2017 from a remotely piloted vehicle and global positioning system (GPS) real-time kinematics. Rainfall data for the corresponding time was recorded by a tipping-bucket rain gauge. Using these data, the CA-Markov model for simulating the evolution of collapsing gullies was developed, and then the most accurate one was chosen to predict the evolution on 2 December 2018. Evolution of the case study hillslope was interpreted and assessed using landscape metrics to capture the erosional trends of collapsing gullies. Results and discussion: The area differences of the modified CA-Markov model are lower than those of the traditional model while the kappa coefficients of the modified CA-Markov model are higher than those of the traditional model; that is, the modified CA-Markov model performs better for simulating and predicting the evolution of collapsing gullies. The kappa coefficients also demonstrate that both scouring and gravity impact collapsing gullies, and scouring force is more effective than gravity. Based on the evolution prediction, the erosion amount of collapsing gullies in the subsequent year is predicted to be 904.1 m3. Spatial pattern analysis showed that the mid-lower part of a hillslope eroded by collapsing gullies will continue to be intensively eroded and the ground surface will become more fragmented. Conclusions: The use of a modified CA-Markov model and landscape pattern metrics provide an improved and effective method for understanding the spatial and temporal variations of collapsing gullies landform and ground surface, as well as better capturing the erosional trends of collapsing gullies. [ABSTRACT FROM AUTHOR]

Except for the catalyst and the temperature, the nature of the solvent also affects the aldol condensation, favouring α,β-unsaturated carbonyl compounds in alcoholic solvents or β-hydroxyl carbonyl compounds in tetrahydrofuran. [ABSTRACT FROM AUTHOR]