Numerical Simulation of Pore Water Distribution Under Electric Field in Unsaturated Soils

A numerical model predicting the moisture distribution under external electric field in unsaturated soils is established. Key parameters including matric potential, hydraulic and electrical permeability coefficient and electric conductivity are discussed. The simulation results are in good agreement with the measured data from literaturea, which verify exactness and suitability of the model. In general, the moisture contents decrease with time under the action of external electric force and matrix suction. A slight increase in moisture contert was observed in the anodic area at the beginning of the treatment. The electro-osmosis treatment effect can be enhanced by improving the voltage gradient or weaken the voltage loss at electrode. The limit value of electroosmosis treatment in moisture content is observed and the method can only be applied on the soil with moisture content higher than the limit value.

The Effects of Rainfall, Soil Type and Slope on the Processes and Mechanisms of Rainfall-Induced Shallow Landslides

Landslides are a serious geohazard worldwide, causing many casualties and considerable economic losses every year. Rainfall-induced shallow landslides commonly occur in mountainous regions. Many factors affect an area’s susceptibility, such as rainfall, the soil, and the slope. In this paper, the effects of rainfall intensity, rainfall pattern, slope gradient, and soil type on landslide susceptibility are studied. Variables including soil volumetric water content, matrix suction, pore water pressure, and the total stress throughout the rainfall were measured. The results show that, under the experimental conditions of this paper, no landslides occurred on a 5° slope. On a 15° slope, when the rainfall intensity was equal to or less than 80 mm/h with a 1 h duration, landslides also did not happen. With a rainfall intensity of 120 mm/h, the rainfall pattern in which the intensity gradually diminishes could not induce landslides. Compared with fine soils, coarser soils with gravels were found to be prone to landslides. As the volumetric water content rose, the matrix suction declined from the time that the level of infiltration reached the position of the matrix. The pore water pressure and the total stress both changed drastically either immediately before or after the landslide. In addition, the sediment yield depended on the above factors. Steeper slopes, stronger rainfall, and coarser soils were all found to increase the amount of sediment yield.

Effect of Salt Content on the Evaporation and Cracking of Soil from Heritage Structures

The degree of damage to heritage structures in arid areas can be particularly serious due to long-term exposure to a harsh arid climate. In this paper, the characteristics of evaporation and cracking of soil taken from heritage structures with different NaCl concentrations are experimentally investigated by subjecting saturated soil samples taken from the Wang-Jing tower in Weihui City, Henan Province. Making soil sample of water content of 110% is used to test the rate of evaporation at a constant temperature and humidity, through which the changes in the evaporation rate, water content, fracture development, and fracture fractal dimension are obtained. The results show that, (1) with higher water content, NaCl can affect the evaporation process by increasing the matrix suction of the soil of the Wang-Jing tower and then affect the cracking process of soil. When 0%, 1%, 2%, and 4% NaCl are added, the residual water content of the samples is 3.15%, 4.23%, 4.82%, and 5.89%, respectively, which show an obvious trend of increasing water content; (2) the period with a stable fracture fractal dimension of the samples is shortened with an increase in NaCl concentration, and its maximum fractal dimension is reduced in turn; and (3) at a lower water content, NaCl crystallizes and precipitates in the pores of the soil, which provide a cementitious effect among the soil particles, thus inhibiting crack development.

Stability analysis of unsaturated clay slope based on the coupled effect of temperature and saturation

In view of the clay slope instability caused by environmental temperature and rainfall infiltration in summer and autumn, the mechanism of the effect of temperature and saturation on unsaturated clay slope stability was discussed. To achieve this objective, this study uses the theoretical and numerical methods. Based on the theory of unsaturated clay and the law of heat conduction, the relationship between cohesion and matrix suction of unsaturated clay was derived considering the influences of temperature and saturation. Considering the effect of temperature and saturation, the coupling equation of shear strength of unsaturated clay was constructed under conditions of rainfall infiltration. The coupling equation of temperature and saturation was used in the strength reduction method, and the slope stability was analysed by FLAC software. The results show that the cohesion of unsaturated clay slope increases with the increase of depth, and the safety factor considering the coupled effect of temperature and saturation is smaller than that without considering the temperature effect; the clay slope is unstable considering the coupled effect of temperature and saturation, but stable without considering the coupled effect when the saturation of soil reaching 80%.Thematic collection: This article is part of the Role of water in destabilizing slopes collection available at: https://www.lyellcollection.org/cc/Role-of-water-in-destabilizing-slopes

Experimental Study on the Effect of Key Factors on the Soil–Water Characteristic Curves of Fine-Grained Tailings

The soil–water characteristic curve (SWCC) is an essential parameter for studying the mechanical properties of unsaturated tailings, and it plays an important role in stability assessment and prediction of unsaturated tailings dams. In this paper, the matrix suction was measured indirectly by a filter paper-based method to investigate the effects of key factors (gradation, temperature, and initial dry density) on the soil–water characteristic curves of fine-grained tailings, and the Van Genuchten model was adopted to obtain the empirical equation of SWCC and to verify the accuracy of experimental results. The results showed the following: 1) the Van Genuchten model fits well the relationship between matric suction and volumetric moisture content of fine-grained tailings, indicating that experimental data determined by filter paper-based method is accurate and appropriate; 2) at the same volumetric moisture content, the matrix suction increased with decreasing average particle size, and the decrease in temperature will increase the matrix suction and water-holding capacity of fine-grained tailings, and the matrix suction of the tailings in summer is 38.3% lower than that in winter under the natural volumetric moisture content (14.0%); 3) when the saturation degree of fine-grained tailings is less than 60%, the greater the initial dry density, the better the water-holding capacity and matrix suction.

Soil-water retention behaviour of fine/coarse soil mixture with varying coarse grain contents and fine soil dry densities

An interlayer soil identified in the French conventional rail track corresponded to a mixture of fine soil and coarse grains. To investigate the role of fines in the soil-water retention property of such mixture, different coarse grain contents fv and dry densities of fine soil ρd-f were considered. The filter paper method was applied to measure the matrix suction. Mercury intrusion porosimetry tests were performed for the microstructure observation of fine soil. In terms of gravimetric water content of fine soil wf with matrix suction Ψ, the soil-water retention curve (SWRC) was significantly affected by ρd-f for Ψ < 715 kPa, while independent of ρd-f for Ψ > 715 kPa. Interestingly, this threshold Ψ corresponded to a delimiting pore diameter of bi-modal microstructure of fine soil, which separated micro-pores from macro-pores. In terms of degree of saturation Sr with Ψ, the SWRC was significantly affected by ρd-f in the full suction range, while independent of fv. These findings help better understand the results on samples with the dry density of mixture ρd kept constant: an increase of fv resulted in a decrease of ρd-f and the suction changed accordingly. In that case, both fv and Ψ affected the mechanical behavior.

The Influence of Drying-Wetting Cycles on the Suction Stress of Compacted Loess and the associated Microscopic Mechanism

In order to better understand and analyze the unsaturated stability of loess fillings, it is necessary to study the changes in suction stress before and after the drying-wetting cycles. In this study, the soil-water characteristic curve (SWCC) of compacted loess before and after drying-wetting cycles was tested using the filter paper method. Then, the suction stress was calculated and the microstructure of the loess sample was determined by the scanning electron microscope（SEM）and nuclear magnetic resonance (NMR). The results showed that the drying-wetting cycles had an important influence on the suction stress characteristic curve (SSCC) and microstructure of compacted loess. The change in suction stress before and after the drying-wetting cycles can be explained by the loess microstructure. The drying-wetting cycles did not significantly change the basic trend of the compacted loess's suction stress, but it increased the porosity and the diameter of the dominant pore (i.e., the inter-aggregate pore) of the sample, and reduced the suction stress when the same matrix suction was applied. The main significant change in suction stress with matrix suction occurred within the range of the dominant soil pores. The larger the diameter of the dominant pore, the smaller the suction stress under the same matrix suction. In addition, this study also proposes a new method for calculating suction stress based on the pore size distribution（PSD） parameters, which is more convenient than traditional calculation methods based on SWCC parameters.

Experimental Study on Response Law and Failure Process of Slopes in Fully Weathered Granites Under Precipitation Infiltration

To investigate the response law and failure process of slopes in fully weathered granites under precipitation infiltration, a typical fully weathered granite slope is selected for sampling in Fengkai, Guangdong. The physical simulation experimental study of rainfall-induced landslide is conducted, in which Weber criterion is used as the similarity criterion for precipitation. The research results reveal that under precipitation infiltration, the fully weathered granite slope responds quickly. Further, the water content increases sharply, and the matrix suction quickly dissipates. After dissipation, the matrix suction transforms into pore water pressure, which accelerates the deformation of the slope. The wet peak has a large infiltration depth in the slope, and the acceleration of deep part is lower than that of the shallow part. Under the action of precipitation, the fully weathered granite model undergoes four stages of failure. Firstly, gullies and cracks appear. Secondly, cracks propagate and link up. Then, the soil on the slope surface swells and ruptures. Finally, the slope slides locally until the entire slope creeps, collapses, and transforms into a "soil flow." Based on the analysis of precipitation similarity, the landslide will be triggered in fully weathered granite slope by precipitation when the precipitation intensity comes up to 155 mm/d, and the landslide occurs at an accumulated precipitation of 304 mm. Overall, the results can provide a reliable theoretical basis and abundant experimental data for the prevention, monitoring, and forecasting of geological disasters in granitic areas.

Plot test in a 50-year abandoned mine tailings dam

Knowing the evolution of water content in a tailings dam is essential when analysing its stability. The case study is a small tailings dam abandoned more than 50 years ago, located in the Andes region of San Juan province, Argentina, where the climate is arid. This paper presents the results obtained in a plot test to study the interaction of tailings with the atmosphere, and to explain the reasons that generate internal zones in the dams with surprisingly high degrees of saturation, despite the strong water deficit of the site. The plot was hydraulically isolated on the four lateral faces where ten capacitive sensors were installed at different depths (volumetric moisture, matrix suction and temperature). At the beginning of the test, a major rainfall event was simulated with a sprinkler irrigation system. The plot was then allowed to interact freely with the atmosphere for an additional period of one and a half years. The records obtained suggest that the stratigraphy of the hydraulically deposited tailings has a preponderant influence on the interaction with the atmosphere. Heterogeneous flow phenomena and multiple capillary barriers during the infiltration and evaporation processes, respectively, appear to control the long-term moisture state of the tailings dam.