Search

The presence of water and salt transport in complex hydrochemical environments allows for continuous changes in the suction state of clay (matric suction, osmotic suction, and their interactions). The interaction between salt solution and negatively charged clay matrix alters the interparticle physicochemical forces, which makes the conventional suction stress theory for unsaturated soils not able to well characterize the local interparticle forces in unsaturated saline clays. For this reason, the relationship between osmotic pressure and osmotic suction of clay when the pores contain a general salt solution is first revealed from the concept of soil-water potential for the problem of macroscopic description of physicochemical forces in unsaturated saline clay. Subsequently, the suction stress formula for unsaturated saline clay is presented with the help of the mechanical analysis. The suction stress of unsaturated saline clay is divided into matric suction stress and osmotic suction stress, and its suction stress characteristic curve (SSCC) is analyzed. Finally, the extended suction stress theory is verified by utilizing the test results of the soil-water characteristic curve (SWCC) and shear tests for the four types of clays. The results indicate that the osmotic pressure of clay is affected by the clay's type, osmotic suction, and stress level, so it is key to accurately consider the relationship between osmotic pressure and osmotic suction to describe the osmotic effect of saline clay. The presence of osmotic effects not only generates osmotic suction stress in clay but may also alter its matric suction stress, and the sum of these two components yields the total suction stress in unsaturated saline clays. From the verification, it is noted that the extended suction stress theory can effectively characterize the SSCC of unsaturated saline clay by solely increasing the surface fractal dimension. Additionally, this work provides a theoretical basis for explaining the mechanical behavior of unsaturated clay in response to hydrochemical environments. [ABSTRACT FROM AUTHOR]

Matric suction has an important effect on the behavior of unsaturated soils, and polypropylene fibers are often used to improve soil. In order to probe into the mechanism of matric suction and fiber reinforcement, triaxial shear tests are carried out with changing matric suction and net confining pressure. Unsaturated clay in the Shaoxing section of East Zhejiang Grand Canal is selected with polypropylene fiber as a reinforcement material in the tests. The results show that the total cohesion intercept and effective internal friction angle of soil increase with the increase in matric suction, while the adsorption internal friction angle decreases gradually. Similarly, the contribution of matric suction to shear strength decreases. The total cohesion intercept is more sensitive to matric suction. As the length of fiber is 12 mm, the shear strength parameters of soil will be improved accordingly, which makes the fiber reinforcement achieve the best. The stress–strain relationship is approximately hyperbolic and strain hardening. The characteristics of strain hardening are more obvious with the increase in matric suction, and the soil specimens present plastic failure. The volumetric strain of specimen is more sensitive to the changing net confining pressure. It increases with the increase in net confining pressure, and increases linearly as the matric suction is zero. The failure modes of triaxial tests are divided into tensile failure and friction failure. [ABSTRACT FROM AUTHOR]

In order to clarify the suction characteristics of unsaturated clay under different conditions of water and salt, taking soil in western Jilin province, China as the research object, the suctions (matrix suction and osmotic suction) of samples with different water content, salt type, and salt content were determined by filter paper method. The results indicate that salt type and content have insignificant influence on the matrix suction. With the increase of water content, the matrix suction decreases at different rates in different suction regimes. When water content is constant, the osmotic suction increases with the increase of salt content, and under the condition of a constant salt content, the osmotic suction first increases and then decreases with the increase of ion concentration of the pore water, and can reach tens of MPa. The concept of suction equilibrium water content is proposed. When water content of sample is higher than this value, the magnitude of osmotic suction will always be greater than that of matrix suction. This indicates the necessity to consider osmotic suction in the presence or change of salt in soil. The applicability of the traditional models of matrix suction and osmotic suction in the full suction range of clay is discussed, and it is found that the traditional models cannot make a full explanation to the variation in suction. Based on the experimental data, an improved van Genuchten matrix suction model and an osmotic suction model, transformed by the Planck blackbody radiation formula, are proposed. [ABSTRACT FROM AUTHOR]

The complicated composition of unsaturated clay, e.g., solid mineral particles, water, and air, makes it difficult to get its precise equation of state (EOS) over a wide pressure range. In this paper, the high-pressure EOS of unsaturated clay was discussed at the mesoscale. With the original clay extracted from the southern suburbs of Luoyang city, China, three unsaturated clays with moisture contents of 0%, 8%, and 15%, respectively, were remolded. Their Hugoniot parameters in the pressure range of 0–30 GPa were measured using a one-stage or two-stage light gas gun. With the measured Hugoniot parameters, a high-pressure EOS of the unsaturated clay up to 30 GPa was developed and it is in good agreement with the experimental data.

In this article, we investigate the nanoscale soil-water retention mechanism of unsaturated clay through molecular dynamics and machine learning. Pyrophyllite was chosen due to its stable structure and as the precursor of other 2:1 clay minerals. A series of molecular dynamics simulations of clay at low degrees of saturation were conducted. Soil water was represented by a point cloud through the center-of-mass method. Water-air interface area was measured numerically by the alpha-shape method. The soil-water retention mechanism at the nanoscale was analyzed by distinguishing adsorptive pressure and capillary pressure at different mass water contents and considering the apparent capillary interface area (i.e., water-air interface area per unit water volume). The water number density profile was used to quantify the adsorption effect. A neural-network based machine learning technique was utilized to construct functional relationships among matric suction, the mass water content, and the apparent water-air interface area. Our numerical results have demonstrated from a nanoscale perspective that the adsorption effect is dominated by the van der Waals force and hydroxyl hydration between the clay surface and water. As the mass water content increases, the adsorption pressure decreases, and capillarity plays a prominent role in the soil-water retention mechanism at the nanoscale. [ABSTRACT FROM AUTHOR]

The complicated composition of unsaturated clay, e.g., solid mineral particles, water, and air, makes it difficult to get its precise equation of state (EOS) over a wide pressure range. In this paper, the high-pressure EOS of unsaturated clay was discussed at the mesoscale. With the original clay extracted from the southern suburbs of Luoyang city, China, three unsaturated clays with moisture contents of 0%, 8%, and 15%, respectively, were remolded. Their Hugoniot parameters in the pressure range of 0–30 GPa were measured using a one-stage or two-stage light gas gun. With the measured Hugoniot parameters, a high-pressure EOS of the unsaturated clay up to 30 GPa was developed and it is in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]