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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]

The matric suction in unsaturated soil depends on the type of soil and various flux conditions i.e., infiltration, evaporation and no flow. The shear strength of unsaturated soil changes with change in matric suction leading to varying resistance of soil under various flow conditions. In this study, the bearing capacity of strip foundations resting over unsaturated clay considering variation of matric suction with depth for different surface flux conditions and position of water table, has been obtained using finite element lower bound limit analysis. Modified Mohr-Coulomb yield criteria based on unified effective stress approach has been employed to incorporate the contribution of matric suction stress to the failure condition. A non-dimensional bearing capacity factor was introduced to estimate the bearing capacity of strip foundations and presented as a function of different influencing parameters such as foundation width, water table depth, soil properties, various flow conditions and their flow rate, and surcharge pressure. The influence of variation of unit weight of soil in the unsaturated zone, air entry and pore size distribution parameters, residual degree of saturation of soil, specific gravity of soil solids, and foundation-soil interface roughness on the bearing capacity has also been examined and found to be insignificant. [ABSTRACT FROM AUTHOR]

Vertical plate anchors are placed in the soil to improve the lateral resistance of various geotechnical structures. Extensive research has been conducted to find out the pullout capacity of vertical anchors in saturated or dry soil. Despite all these attempts, no research is available to study the behavior of anchors in unsaturated soil where the shearing resistance of soil is governed by the matric suction depending on the surface flow conditions and water table positions. Since the soil in field is mostly in unsaturated state; thus, the pullout resistance of vertical plate anchor has been aimed to be evaluated in this study. Finite element limit analysis solutions in terms of a non-dimensional pullout factor have been presented to find the pullout capacity of vertical anchors. The solutions are inclusive of the effect of soil properties, embedment depth, height of anchor plate, depth of water table from the ground surface, flow conditions and flow rate under an immediate breakaway condition. The effect of variation of other factors like the degree of saturation, unit weight of soil, van-Genuchten SWRC parameters, specific gravity of soil solids, and anchor-soil interface conditions has also been evaluated. [ABSTRACT FROM AUTHOR]

Numerous studies have been reported in literature for plate anchors in saturated or dry clays. However, in practical scenario the soil overlying the anchor plate in most of the cases is in unsaturated state, and even then, no study seems to be available for anchors in unsaturated soil. The uplift capacity depends on the resistance of soil overlying the anchor plate, which is in turn affected by the shear strength of soil and varies with variation in the matric suction owing to changes in surface flux conditions and position of the water table. Hence, it has been planned to study the uplift capacity of horizontal strip plate anchors buried in unsaturated clay in this paper. The stress-based limit analysis with the help of finite elements and second-order cone programming has been used for performing the analysis considering the variation of matric suction with depth above the water table under various surface flow conditions (evaporation, no flow and infiltration). Unified effective stress approach-based modified Mohr–Coulomb yield criterion was used to include the influence of stress due to matric suction to the ultimate failure state. The uplift capacity was expressed in terms of a non-dimensional uplift capacity factor, which has been found to be dependent on the embedment depth-to-width ratio of anchor, shear strength parameters of soil, water table position, various flow conditions and their flow rate. The effect of other parameters like air-entry pressure and pore-size distribution parameters, degree of saturation, specific gravity of soil solids, and soil–anchor interface roughness has also been examined and was found to be insignificant. The solutions were presented for an immediate breakaway condition, i.e., considering that the contact between anchor and underlying soil mass is separated at ultimate failure. [ABSTRACT FROM AUTHOR]

The water vapor migration of unsaturated clay affects the safe operation of the subgrade during service and the guarantee of long-term performance. It is of great engineering application value and significance to study the influence of water vapor migration law on unsaturated soil subgrade moisture on subgrade subgrade humidity. Through the indoor test model, the high and low temperature alternating incubator was used to change the ambient temperature to simulate the effect of ambient temperature on water migration in four seasons. The migration process of gaseous water in unsaturated cohesive soil was simulated, and the sensitivity analysis of influencing factors was carried out. The water vapor migration law of unsaturated cohesive soil was obtained and the influencing factors were evaluated. The experimental results show that in the process of water vapor migration in unsaturated soil, the water content gradient has a great influence on the gaseous water migration. The migration time has a great influence on the liquid water migration. Under low temperature environment, the migration of liquid water is dominant, and the proportion of gaseous water migration gradually increases with the increase of ambient temperature. Water content gradient, water content level, degree of compaction, migration time, and different ambient temperatures all contribute to the migration of gaseous water, of which the degree of compaction has the least effect. In practical engineering, more attention should be paid to the influence of water-gas two-phase migration on the performance of subgrade soil. [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]

Energy piles are a new renewable energy technology that is suitable for use as a heat exchanger in ground source heat pump systems. In this study, hollow steel balls macro-encapsulated phase change materials (PCMs) were used for the development of concrete pile, the resulting pile is referred to as the "PCM energy pile." A laboratory-scale PCM energy pile in unsaturated clay was constructed, and the thermal responses of the pile and surrounding soil subjected to different thermal operation modes were examined and compared with those of a traditional concrete energy pile. The test results showed that the soil zone experiencing temperature changes around the PCM pile is approximately twice the pile diameter. The PCM energy pile showed less temperature change but a much higher heat transfer power than that of the traditional energy pile. However, the heat transfer power was significantly influenced by the operation modes, and a reasonable flow rate was required to optimize the efficiency of the PCM energy pile. Water content in the soil decreased gradually because of water seepage and evaporation. The test results proved that the proposed PCM energy pile was an effective solution for improving the heat exchange capacity and saving underground space resources. • Hollow steel balls encapsulated with paraffin were used for the PCM concrete pile. • The thermal responses under different operation modes and flow rates were examined. • PCM in concrete pile can reduce the temperature variations of the pile and soil. • Heat transfer power of the PCM pile is larger than that of the concrete pile. [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 calculation process of the traditional test method of unsaturated clay permeability coefficient is complicated, so a prediction model of unsaturated clay permeability is established. The e-lnp curve is drawn through K0 consolidation test of unsaturated remolded clay. The linear relationship between state variables and soil saturation was found, and the influence of unsaturated clay on soil properties was obtained. According to the adsorption storage mechanism and the position of soil compactness in the coordinate system, the characteristic curve of soil moisture and the prediction model of unsaturated clay permeability were established. The permeability test of unsaturated remolded clay is carried out by using transient profile method, and the results show that the model can well describe the change of permeability of unsaturated clay. [ABSTRACT FROM AUTHOR]

To explore the feasibility of the application of a compacted red clay as an engineered barrier in landfills, the effect of the loading rate on its unsaturated mechanical behavior has been investigated through a series of triaxial shear tests in this work. The loading rate was changed stepwise numerous times at the designed matric suction and net confining pressure. Based on the deviator stress–axial strain curves, the coefficient of rate-sensitivity (β) was obtained. A model describing the relationships among stress, strain, strain rate, time, and matric suction was also proposed. The experimental results show that the value of β is the largest at the beginning of loading, then decreases sharply. With time and strain increasing, the value of β does not change significantly. At a given value of the net confining pressure, β is larger at larger matric suctions. Moreover, under different matric suctions the respective stress–strain curves are all isotachic. The proposed model was able to describe the effect of the loading rate on the behavior of the red clay almost satisfactorily. These results might be beneficial in the design of compacted clay barriers in landfills under unsaturated conditions. [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.

The thermal properties of soils vary because of the different textures of soils; thus, it is valuable to conduct research on the development of thermal conductivity models for soils with different textures. Clay samples with different dry densities and increasing incremental water contents were prepared, their thermal conductivities were measured by the transient plane source (TPS) method. The experimental results show that the thermal conductivity of clay increases with increasing water content and dry density. The water content has a stronger influence than the dry density on the thermal conductivity. This paper presents an improved model to predict the thermal conductivity of unsaturated clay. The model considers the different contributions of minerals in the soil and the distribution of water and air in pores. The accuracy of the model is verified by soil thermal conductivity data that was previously measured and collected from ten regions. Compared with the three similar models proposed in the literature, the new model proposed in this paper has a higher degree of correlation to describe the thermal conductivity of clay. [ABSTRACT FROM AUTHOR]

Summary: The mechanical and hydraulic properties of unsaturated clay under nonisothermal conditions have practical implications in geotechnical engineering applications such as geothermal energy harvest, landfill cover design, and nuclear waste disposal facilities. The water menisci among clay particles impact the mechanical and hydraulic properties of unsaturated clay. Molecular dynamics (MD) modeling has been proven to be an effective method in investigating clay structures and their hydromechanical behavior at the atomic scale. In this study, we examine the impact of temperature increase on the capillary force and capillary pressure of the partially saturated clay‐water system through high‐performance computing. The water meniscus formed between two parallel clay particles is studied via a full‐scale MD modeling at different elevated temperatures. The numerical results have shown that the temperature increase impacts the capillary force, capillary pressure, and contact angle at the atomic scale. The capillary force on the clay particle obtained from MD simulations is also compared with the results from the macroscopic theory. The full‐scale MD simulation of the partially saturated clay‐water system can not only provide a fundamental understanding of the impact of temperature on the interface physics of such system at the atomic scale, but also has practical implication in formulating physics‐based multiscale models for unsaturated soils by providing interface physical properties of such materials directly through high‐performance computing. [ABSTRACT FROM AUTHOR]

Unsaturated clay is a heterogeneous porous medium consisting of three phases, namely solid soil skeleton, pore water, and pore air. It has been well recognized that the variation of the chemical property of pore fluid in clay can affect the hydromechanical behavior of this material remarkably. In this study, we formulate a non-local chemo-hydromechanical model for unsaturated clay via the constitutive correspondence principle in the state-based peridynamics—a reformulation of classical continuum mechanics using integral equations instead of partial differential equations. We numerically implement this non-local constitutive model through the implicit return mapping algorithm at the material particle level and then integrate the material subroutine into a computational peridynamics code. We conduct a series of numerical simulations of unsaturated clay samples under different chemical loading rates. The numerical results demonstrate that the proposed non-local model can capture the dramatic impact of organic chemicals on the mechanical behavior of unsaturated clay. The numerical results also show that the proposed non-local numerical model can simulate localized deformation in chemically active unsaturated clay because of the intrinsic length scale embedded in the integral equations. [ABSTRACT FROM AUTHOR]

In this study, an attempt is made to investigate the load transfer in pile groups constructed in saturated and unsaturated soil using aluminum model piles of 20 × 20 mm in cross section and 200 mm in length with six group configurations, single, 2 × 1 , 3 × 1 , 2 × 2 , 3 × 2 , and 3 × 3 , groups in addition to pull-out test. The samples are prepared at three different soil properties: two models with the same degree of saturation and two with different dry unit weights and the third is unsaturated soil model with. A relationship between the soil suction and water content is introduced using a digital monitor, accompanied by eight suction probes planted in CBR molds. This relationship is used to predict the value of suction when conducting tests on models of the unsaturated soil. Further, the soil suction is measured using a filter paper method. Results of ultimate load capacity obtained from the load–displacement curves reveal an increase in the ultimate load with increasing the number of piles in the group for the same soil properties, and also an increase in the soil dry unit weight. Further, the results indicate an increase in pile capacity when the soil becomes in unsaturated state compared with saturated soil for the same pile group tested. On the other hand, the pull-out test gives a maximum load carrying capacity when the single pile tested was in saturated soil with high dry density, and minimum results are obtained when tested in unsaturated condition with the percentage of increase being about 463 and 34% for two saturated soils, compared with that for unsaturated soil. [ABSTRACT FROM AUTHOR]

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]

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This paper presents a new method for testing the behaviour of soils placed under tensile load and demonstrates its suitability for testing a number of soil types under various conditions including saturation, compaction and stabilisation. Validation of the results obtained for the soils at relatively low saturation has been conducted using the established Brazilian (indirect) test for measuring the tensile strength of brittle materials. A fair comparison has been found and the results highlight the limited applicability of the Brazilian method to soils at very low water contents at which the tensile failure criterion has been assumed using this methodology. Optical characterisation of the performance of both testing methods has also been conducted using Digital Image Correlation. The consistent, accurate measurement of directly induced tensile strains using the proposed new method has been confirmed, verifying its capability to apply a direct tensile stress in the absence of shearing, a problem commonly associated with other tensile testing methods. The developed technique has then been used to investigate the water content–tensile strength relationship for compacted, unsaturated soils and offers significant advantages in the characterisation of clay soils subjected to variable climatic loading. [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]

Abstract: In a previous paper [Åkesson, M., Hökmark, H., 2007. Mechanical model for unsaturated MX-80. In: Schanz, T. (Ed.), Theoretical and Numerical Unsaturated Soil Mechanics, Springer Proceeding in Physics, vol. 113, pp. 3–10] a new elastoplastic model for unsaturated swelling clay was presented. The model was formulated as two differential equations, for elastic and plastic strains, respectively. Each equation described a relation between the (air-filled) macro void ratio and the two independent variables stress and (water-filled) micro void ratio. The established concept of swelling pressure and its void ratio dependence were incorporated in the model. To do this, the grain-to-grain contact stress and the contact area had to be considered. In its original form the model only handled one-dimensional problems, e.g. compression tests with uniaxial strain and water uptake test at constant volume with assumed isotropic conditions. This paper describes a first attempt to generalize the model to address multidimensional problems with the intention to integrate it into the framework of Code_Bright. This was made through definition of an expression for the retention properties and through variable substitution. In this way, four functions for the kappa moduli were identified: two elastic and two plastic. These functions provide unique values for each given state. In the used framework of the Barcelona Basic Model (BBM), all strains are treated as elastic, and there is thus no explicit treatment of any deviatoric yield condition. The only remaining parameter outside the new model is the Poisson’s ratio. [Copyright &y& Elsevier]