In order to study the strength characteristics of alluvial silt in the lower Yellow River Channel at Luokou, Jinan, by varying the moisture content five times, direct shear tests were conducted and soil–water characteristics curves were obtained to explore the relationships between moisture content and matric suction as well as the bishop coefficient and shear strength. The soil–water characteristics curve test shows that the water retention curve of silt samples in the lower Yellow River can be fitted by the VG (Van Genuchten) model with the appropriate fitting coefficients. The direct shear test reveals that the relationship between shear stress and shear displacement alters from the shear softening type to the hardening type with the increase in moisture content and normal stress. The cohesion has a nonlinear inverse relationship with moisture content while a small variation is reported in the internal friction angle. Finally, a simple shear strength equation for silt in the lower Yellow River is proposed in relation to moisture content, to define the relationship between the effective stress parameter and the matric suction for future engineering purposes.
The mechanical behaviours of unsaturated soils are highly related to the water content and pore water and air distributions. Under the context of climate change, geo-disasters related to soil moisture change attract more and more research attentions. Due to the heterogeneity of soil textures and the complicated morphology of liquid phases, it is crucial to understand the microstructural features of unsaturated soils. This work presents a study based on a miniaturized suction controlled triaxial device which is suitable for micro-CT image analysis. A fine sand is sheared in this device under different suction levels while CT scans are taken at different strain stages. After image 3D reconstruction, image trinarization, label analysis, contact detection and other customized image analysis and calculations, the micro-mechanisms of unsaturated granular soils upon triaxial shearing are investigated. It is observed that the inter-particle contact coordination number is reduced after shearing due to the dilation behaviour and the sample with the highest capillary strength has the highest coordination number. Although there is an initial fabric anisotropy due to gravity and sample compaction, triaxial loading will further enlarge the fabric anisotropy of the solid phase and the solid fabric anisotropy is also associated with shear strength. With the development of shear band, water drains out and the quantity of small-volume liquid clusters in the liquid bridge increases. This shifts the distributions of inter-facial areas. The effective stress tensor is interpreted microscopically based on small RVEs. Based on the CT image analysis, the suction-induced stress component is not an isotropic term and the anisotropy of the water phase is increased with triaxial deformation as well as decrease in degree of saturation when there are more isolated water bridges formed around solid contacts.
<p>Rainfall infiltration is the main inducing factor for the instability of unsaturated soil slopes, and root water uptake and reinforcement play an important role in preventing shallow landslide. In order to explore the influence of vegetation root on the soil hydraulic and mechanical properties under rainfall, a self-designed soil permeability coefficient measuring device considering the effects of vegetation was used to study the soil water characteristic curve (SWCC) and permeability coefficient of <em>Festuca Arundinacea</em>, <em>Ophiopogon Japonicus</em>, <em>Ligustrum Vicaryi</em> and bare soil under two different rainfall conditions (3.0mm/h and 5.0mm/h) were studied. Then, the direct shear tests of root-soil composite with different water contents and root contents were carried out. Finally, the slope stability under different rainfall and vegetation was simulated by GeoStudio. Results show that: root water uptake can effectively reduce soil water content and increase soil suction, and its influence range is about 2-3 times the length of the root system. Root water uptake can also significantly improve the soil water retention capacity. The air entry value of vegetation soil is larger than that of bare soil, and the permeability coefficient of vegetation soil is about one order of magnitude lower than that of bare soil. Among the three different types of vegetation, the effect of <em>Festuca Arundinacea</em> and <em>Ophiopogon Japonicus</em> on soil water content and suction is more significant than <em>Ligustrum Vicaryi</em>. Root reinforcement mainly increases the soil shear strength by improving the cohesion of the root-soil composite, but has little effect on the internal friction angle. The cohesion of the root-soil composite is affected by soil water content, root content and root distribution, which increases with the increase of root content, and decreases with the increase of water content. When the roots are vertically distributed, the cohesion of the root-soil composite is greater than when the roots are placed horizontally and inclined. Vegetation can effectively improve the stability of the shallow slope under various rainfall conditions, but has little effect on the stability of a deep slope. The safety factor of all three types of vegetated slopes is higher than that of bare soil slopes.</p>
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