Unsaturated Clay Research Articles

Geometrical effective medium model of electric conduction of partially saturated clays

The current existing researches were discussed to assess the advantages and limitations of existing empirical and theoretical models for direct conductivity (DC) prediction. Prior research has demonstrated that an effective medium model may not accurately reflect the actual situation due to the presence of two types of water (bound water and bulk water) in clay-rich materials. Furthermore, the existing models can not satisfy the prediction of electrical conductivity of metal ions adsorbed clay or unsaturated clay. To address this issue, a new Effective Medium Double Water (EMDW) model was proposed based on multiple scattering techniques, which encompasses soil particles, surface-bound water layers, and bulk water and was established by controlled soil types and degrees of saturation. The novel EMDW model includes the Coherent Potential Approximation (CPA), which has consistently demonstrated superior agreement with experimental data when compared to other approximation models. Moreover, the binomial expansion approximation was utilized to simplify the formula and facilitate its use. The developed conductivity model was validated with data from other researchers. In comparison to other well-established conductivity models, the proposed EMDW model has clear physical meaning and can accurately compute matrix conductivity utilizing modified coated particle conductivity and saturation conductivity. The findings suggest that matrix conductivity in clay materials is significantly correlated with electrical-physical parameters, such as porosity, degree of saturation, shape of each discontinuous phase, and conductivity of surface-bound water and bulk water. Consequently, the new EMDW model is a theoretically grounded, physically meaningful, and easy-to-use model for conductivity prediction in clay materials.

Parameters affecting the development of capillary barriers due to conventional geotextiles

The objective of this study is to assess the formation of capillary barriers on unsaturated soils in contact with geotextiles that have been placed with the primary function of acting as a separator between soil or drainage layers, including the geotextile component of a geocomposite. A comprehensive experimental program involving soil column infiltration tests was conducted using a variety of material combinations to assess the hydraulic performance of interfaces between unsaturated clay and geotextiles. Soil moisture was recorded using time domain reflectometers. Custom made acrylic soil columns were comparatively small (20 cm diameter and 33 cm tall) in order to facilitate test setup and reduce total testing time. Multiple column tests were conducted, which included woven and nonwoven geotextiles selected to identify which geotextile characteristics may affect the formation of a capillary barrier. Other tests were conducted to assess the effect of varying relative compaction and inflow rate on the hydraulic performance. Based on data from the moisture sensors, all tests were found to clearly show the formation of a capillary barrier, which resulted in additional moisture storage in the overlying finegrained soil layer. The test results show that currently available conventional geotextiles create a capillary barrier and restrict moisture flow into the underlying soil or drainage layer until the overlying fine-grained soil has become essentially saturated. This includes nonwoven geotextiles that are commonly used as the top layer in geocomposite drainage products or as separators between a soil layer and an underlying gravel drainage layer. The strength of the capillary barrier was found to be similar for the multiple conventional polypropylene geotextiles investigated in this study.

Pore‐Scale Modeling of Water and Ion Diffusion in Partially Saturated Clays

AbstractAn accurate mechanistic understanding of solute diffusion in partially saturated clays is critical for assessing the safety of deep geological repositories for radioactive waste. In this study, a pore‐scale numerical framework is developed to simulate water and ion diffusion in partially saturated clays. First, the two‐phase Shan‐Chen Lattice Boltzmann method is employed to establish the liquid‐gas distribution in a reconstructed three‐dimensional pore geometry of a clay. An equivalent solute method is also developed and validated to improve the numerical stability of the solution at the liquid/gas interface corresponding to steep variations of the concentration and diffusion coefficient of the water tracer. By using a mobility‐distance relationship from molecular simulations, Fick's law is numerically solved to simulate water diffusion in nanopores, while the coupled Poisson‐Boltzmann‐Nernst‐Planck equations are solved to simulate ion diffusion under the influence of the electrical double layer (EDL). Our model reveals that the decrease of relative effective diffusion coefficients during the desaturation is more pronounced for ions than for water, due to the additional transport pathway of water tracers in the gas phase. The obtained effective diffusion coefficients of tritiated water and ions agree well with reported data from compacted sedimentary rocks. By comparing the local electric potential and the distribution of ion concentrations in single pores, the simulation results suggest that the EDL in unsaturated clays has a more complex influence on ion distribution than under fully water‐saturated conditions. This study provides critical insights into the coupled transport processes of solutes in partially saturated clays.

Investigation of desiccation cracking in unsaturated clays improved by recycled micro-concrete particles

Investigation of desiccation cracking in unsaturated clays improved by recycled micro-concrete particles

Bearing capacity factor for strip foundations on unsaturated clay

ABSTRACT 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.

Time-step constraints in coupled hydro-mechanical finite element analysis of unsaturated soils

In the finite element (FE) modelling of transient coupled problems for soils, the associated equations need to be integrated numerically over a time interval during the solution process. A minimum time-step size has been observed to exist, below which spatial oscillations in the obtained solutions can occur. These oscillations might lead to an accumulation of the resultant errors, eventually leading to invalid final predictions. This paper presents an analytical approach for deriving the time-step constraints for FE modelling of the transient behaviour of unsaturated soils with a fully coupled hydro-mechanical (HM) formulation. The effects of both element type and nonlinear soil properties are accounted for in the derivation. The proposed time-step constraints were subsequently verified through a series of coupled HM FE analyses of unsaturated soils, showing an excellent agreement. The present work shows that, when dealing with unsaturated soils, the minimum time-step size depends on the suction range investigated. This dependency derives from the nonlinear variation of water content and permeability with suction, as demonstrated through a set of comparative studies with three different types of unsaturated clays.

Experimental investigation of gas migration behaviour in unsaturated sand–clay mixtures

Understanding gas migration behaviours in unsaturated sand–clay mixtures is of great importance to design and construction involving applications of shield-cutting-tool replacement using pressure. A gas permeability test apparatus for unsaturated soil was upgraded to investigate gas migration behaviours further. A series of gas, liquid and solid three-phase coupling gas injection tests were performed on unsaturated clay under different moisture contents and sand–clay ratios. There are three stages of gas migration in unsaturated sand–clay: steep drop stage, transition stage and stability stage. In unsaturated sand–clay with low moisture contents and gas pressures, gas migration is affected primarily by the slippage effect. When the gas pressure is greater than 300 kPa, the permeability remains nearly unchanged. In this case, the gas permeability of the soil sample is affected primarily by the internal structural characteristics of the unsaturated soil sample and those of the shrinkage film. The ideal gas permeability of the soil used in actual engineering and the empirical formula for calculating the pressure of the shield pressure-holding system are summarised.

An experimental study on the measurement of gas diffusivities in un-saturated clay based materials

Gas migration in porous media in the context of nuclear waste disposal is dominated by diffusion. The availability of abundant data on diffusion coefficients of different gases in many clay based materials such as Boom Clay, Opalinus Clay and Callovo-Oxfordian Clays provides a good overview of the rates of gas transport through these different clay rocks [1]. Such formations are being considered as potential host rocks for the disposal of high and intermediate level radioactive wastes across Europe. Diffusion coefficients have been studied with particular interest due to the unavoidable gas generation phase in the nuclear waste repository [2]. However, all the diffusion measurements performed so far have been under fully saturated conditions, with little experimental data on gas diffusivities in partially saturated conditions, which have been hypothesized to exist during the early part of the life cycle of the repository [3]. The objective of the current study is to provide an overview of the effect of desaturation on the rate of diffusive transport of gases in partially saturated clay based materials.
 The experimental methodology is based on the double through diffusion setup used in previous studies to measure gas diffusivities in saturated clays [1]. However, the setup is modified so as to keep a fixed level of unsaturation in the clay sample. This is done by employing the vapour equilibrium technique, using oversaturated salt solutions in both upstream and downstream reservoirs to maintain a constant relative humidity in the entire setup [4].
 Preliminary pore network modelling has already shown that the gas diffusivity increases with a decrease in water saturation of the clay material. However, the predicted trend also suggests that this increase is only marginal and should not normally exceed an order of magnitude. This is shown in the figure below.
 All the diffusion measurements are carried out on synthetic clay samples of dimensions 3.8 cm (diameter) and 2 cm (height). In Table 1, the Sw stands for water saturation. The compositions of the synthetic clay samples (60% clay, 20% sand, 20% silt) have been adjusted so as to most closely mimic the mineralogical compostion of Boom Clay. Synthetic samples are used instead of natural samples like Boom Clay because of their better physical response to drying than Boom Clay.
 The selected control variable to lower the saturation of the material is suction, which is imposed by changing the relative humidity of the environment using oversaturated salt solutions [4]. Suction can be kept constant given constant ambient temperature and pressure. Thus, the clay sample is conditioned at a certain suction, which in turn leads to desaturation in the range of 70-100% of total saturation depending on the level of suction. For this study, the saturation range has been fixed in between 70-100% of total saturation. This range has been chosen because of what is expected in a real life nuclear waste repository. The first measured diffusion coefficients in two partially saturated samples are shown in Table 1. The initial measurements of gas diffusion coefficients in unsaturated synthetic clays show that the increase in diffusivity of helium is higher than that of argon. Helium is the second lightest known element and has an extremely small molecular size and hence the impact of desaturation on helium diffusivity could be more pronounced than the same for heavier gases.

Nanoscale soil-water retention mechanism of unsaturated clay via MD and machine learning

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.

Advanced numerical modelling of the nonlinear mechanical behaviour of a laterally loaded pile embedded in stiff unsaturated clay

Capturing and understanding the ultimate limit state behaviour of reinforced concrete piles embedded in soil requires the use of advanced tools or the performance of expensive tests. An experiment was performed where reinforced concrete piles embedded in a stiff unsaturated clay profile were load-tested on-site. However, even though in-situ experiments can provide engineers with valuable insight, their cost and time limitations come with restrictions, especially when dealing with a parametric investigation on the soil's material properties, the size of the piles, or the piles' material properties. The objective of this research work was to numerically model the nonlinear mechanical behaviour of laterally loaded full-scale piles through detailed 3D modelling, and perform an in-depth parametric investigation to provide answers to unknown factors that the actual physical experiment could not answer. Furthermore, this work serves as a pilot project that will be used to pave the way in developing multiple soil-structure interaction models that will be used to generate a dataset that helps the creation of predictive models through machine learning algorithms. For the needs of this research work, the reinforced concrete piles were discretised with 8-noded isoparametric hexahedral elements that accounted for cracking through the smeared crack approach. Steel reinforcement bars and stirrups were simulated as embedded rebar elements, while the soil domain was also discretised through 8-noded hexahedral elements. Most of the required material properties assumed during the nonlinear analyses were defined according to relevant laboratory experiments. According to the numerical investigation, it was found that the proposed numerical model has the ability to reproduce the experimental results with high accuracy, while providing in-depth insight on the failure mechanisms for both the soil and reinforced concrete domains.

Upheaval Buckling Resistance of Pipelines Buried in Unsaturated Clay Backfills

Upheaval Buckling Resistance of Pipelines Buried in Unsaturated Clay Backfills

An Experimental Investigation on Shear Strength Behaviour of Unsaturated Clays on Effect of Spent Bleaching Earth and Misspend Cement

Abstract: In any kind of structure, expansive clay is a major cause of undulations. Numerous structures suffer damage and serious problems as a result of expansive soil swelling. The viability and environmental suitability of waste materials are the subject of extensive research by numerous research organizations. Waste products from the cement warehouse and oil industry include wasted bleaching earth and misspend cement. It is the best method to use in expansive soils to avoid issues with dumping and storage. Unconfined compressive strength and tri-axial compressive strength were used to identify a stabilization process. Any structure's stability depends on the strength of the underground soil on which it is built. All of a structure's loads are basically transferred to the ground directly. There are a variety of failures that can occur if the underlying soil is not stable enough to support transferred loads, including structure settlement, cracks, and so on. Soil improvement is necessary to solve this problem because it reduces the risk of structural damage in the future and reduces construction costs. To make ordinary soil stable enough to support the structural loads, a variety of improvements can be made. In this study, both regular and stabilized soil can be used in a number of tests. This paper explains the strength behavior of SBE treated black cotton soil reinforced with MC. The various percentage of SBE as 5%, 10%, 15%, 20% and 25% was used to find out the optimum value of RBI Grade. MC has been randomly included into the SBE treated soil at four different percentages of MC content, i.e. 2%, 4%, 6%, 8% and 10%.

Thermal desorption mechanism of n-dodecane on unsaturated clay: Experimental study and molecular dynamics simulation

Thermal desorption technology can effectively remediate fuels-contaminated clayey soil. However, the microscopic mechanism for the contaminant desorption on clay is still unclear, especially with the existence of water on clay surfaces. In this study, a combination method including TGA experiments, multi-phase and multi-component kinetic models, and MD simulation was proposed to reveal the thermal desorption mechanism of n-dodecane in unsaturated clay. Results showed that the thermal desorption behavior of the free nonaqueous phase liquid (NAPL) and the adsorbed phase of n-dodecane or water could be identified by the multi-phase and multi-component kinetic models based on the desorption process rate obtained by thermogravimetric analysis (TGA) experiments. The activation energy of the NAPL phase (49–69.9 kJ/mol) was lower than the adsorbed phase (90 kJ/mol). The activation energy of the NAPL phase had the same linear relationship with its mass on kaolinite and montmorillonite, while adsorbed phase only existed on the kaolinite surface. MD simulation showed that water demonstrated competitive adsorption with n-dodecane on montmorillonite surfaces and prevented the formation of the adsorbed phase while having little influence on the n-dodecane adsorption on kaolinite surface, which agrees well with the kinetic analysis of the TGA experiments. With the combination of macroscopic experimental analysis and microscopic molecular simulation, it can be concluded that the mass of the NAPL phase limited the desorption behavior, while the interaction between the clay mineral surface and n-dodecane was the key factor that dominated the thermal desorption behavior of the adsorbed phase. The presented results provide new insight into the desorption mechanism of hydrocarbon on clay minerals, which is of significance for the design of thermal desorption remediation.

Uplift capacity of strip plate anchors in unsaturated clay

Uplift capacity of strip plate anchors in unsaturated clay

Behaviour of a building foundation on unsaturated expansive soil

The paper presents a case study of the behaviour of a building foundation on expansive unsaturated clay. The load-bearing masonry building started exhibiting severe cracking in its superstructure, immediately after its completion around 1950. Despite the interventions, the problems continued to exist sixty years later. In the context of identifying the causes of these problems, the paper presents the results of the laboratory tests conducted on the expansive clay in order to estimate its swelling pressure and also understand its behaviour. It is shown that the seasonal variation of the water content of the foundation soil, combined with the intrusion of the root system of the nearby trees at the level of the foundation, subjected the soil to wetting-drying cycles, resulting in its corresponding swelling-shrinking and consequently the settlement of the building. Finally, the proposed countermeasures for the solution of problems are presented, which aimed mainly in minimizing the variation of the moisture of the soil around and at the foundation of the building.

Nonisothermal Failure Envelopes of Strip Shallow Foundations Resting on Partially Saturated Clay Subjected to Combined Inclined and Eccentric Loadings

Shallow foundations are commonly resting on partially saturated soil and are very likely to be exposed to severe variations of temperature. The degree of saturation and matric suction will be changed at the elevated temperatures, thus resulting in different hydromechanical behavior and a considerable variation in the imposed suction stress. In this study, the effect of temperature increase on the ultimate bearing capacity of strip shallow foundations resting on partially saturated clay layer subjected to vertical (V)–horizontal (H)–moment (M) combined loading is examined through a set of finite-element limit analyses (FELA) adopting lower bound theorems and second-order cone programming (SOCP). The unified effective stress for partially saturated soils is first incorporated into the soil yield function, considering the significant effect of temperature variations on suction stress by means of the well-established previously developed nonisothermal soil–water retention curve (SWRC) model. The equilibrium equations corresponding to the combined loading are also incorporated into the FELA formulations to account for the effects of inclined and eccentric loadings. Accordingly, the failure loci of obliquely and eccentrically loaded shallow foundations are presented for a wide range of temperatures. It is shown that with an increase of the temperature in the underlying unsaturated clay, the suction stress within the soil medium significantly rises, and thus, the failure wedges as well as the failure loci for both eccentric and inclined loadings substantially expand at elevated temperatures, revealing the considerably greater bearing capacity of the strip footing. The contact pressure beneath the foundation in all loading scenarios increases with an increase in the temperature of the underlying clay. This trend is much less pronounced at higher angles of load inclination, while it is barely affected by the load eccentricity. In addition, with an increase in the induced temperature and consequently the suction stress within the partially saturated clay beneath the foundation, the failure wedge expands, thereby resulting in an overall increase in the bearing capacity of the shallow foundation against vertical–horizontal–moment combined loading.

Nanoscale soil-water retention curve of unsaturated clay via molecular dynamics

This paper characterizes nanoscale soil-water retention mechanism of unsaturated clay through molecular dynamics simulation. Series of molecular dynamics simulations of clay at low degrees of saturation were conducted. Soil water was represented by a point cloud through the centre-of-massmethod. Water-air interface area was measured numerically by the alpha shape method. Spatial variation of water number density is characterized and used to determine the adsorbed water layer. The soil-water retention mechanism at the nanoscale was analysed by distinguishing adsorptive pressure and capillary pressure at different mass water contents and considering apparent interface area (water-air interface area per unit water volume).

Unified state boundary surface model for clay and sand under saturated and unsaturated conditions

A unified state boundary surface that defines the upper limit of the specific volume of saturated clay and sand is first derived. It is extended to unsaturated soils by applying Bishop’s effective stress and using the effective degree of saturation as an independent state variable. Then, a critical state model for unsaturated clay and sand is formulated based on the unified state boundary surface. The model combines a rational soil–water characteristic curve incorporating packing density and hydraulic hysteresis. The model is validated by comparing the calculated results and results of extensive experiments, including one-dimensional and isotropic compression tests, triaxial shearing tests, and soaking-collapse tests under isotropic and anisotropic stress conditions on sand, clay, and mixed soil under saturated and unsaturated conditions. The proposed unified soil model presents a noble and universal framework for the rational description of broad soils’ behavior, in which the state boundary surface plays a central role. The model accurately predicts the compression, shearing, and soaking-induced collapse behaviors of various soil types in saturated and unsaturated conditions.

The Tension-Shear and Compression-Shear Joint Strength Model for Unsaturated Clay and Its Application to Slopes

The capillary component and adsorptive component of matric suction differently impact the soil strength. Due to the cavitation effects of pore water, the adsorption effect dominates the behavior of soil when matric suction exceeds the cavitation tension. Based on the binary medium theory, a compression-shear strength model for unsaturated soils considering both capillary effect and adsorption effect is established. Compared with test data, the proposed compression-shear strength model has better prediction performance on the compression-shear strength of soil over a range of wide suction. The soil failure depends both on tension-shear stress and compression-shear stress. The tension-shear coupling mechanism in the soil is first investigated. A concept of closed stress point is introduced to divide the two zones of tension-shear coupling stress and compression-shear stress. According to the compression-shear strength model and tension-shear failure mechanism, the tension-shear and compression-shear joint strength model applicable to plane stress conditions is then established. Compared with test data, the proposed model in this article can better predict the nonlinear strength characteristics of clays and has better applicability. Finally, using the user material subroutine (UMAT), the secondary development of the joint strength model is conducted in ABAQUS and then applied to the slope stability analysis. The calculation results show that the established strength model presents a reasonable description of the development of the tension-shear coupling plastic zone in slope and gives an accurate safety factor.

Thermo-Hydromechanical Coupling Responses Driven by a Central Heat Source in Unsaturated Soils

Based on the thermodynamic and thermoelastic theory, the coupled governing equations of deformation, heat transfer, and moisture migration in unsaturated soils were given. The coupled calculation process was realized by the finite element method. The hydrothermal coupling characteristics of two cases of unsaturated kaolin clay were studied by using a self-developed test device, and the test results were compared with the numerical results. The results showed that when the initial saturation of the soil is high, the volumetric water content of the measured point increases, and then it is in a stable state during the heating process. When the initial saturation is low, the volumetric water content near the heat source increases firstly and then decreases during the heating process. The rise and fall of temperature make the volumetric water content of the soil irreversible. The volumetric water content of each measured point is lower than the initial state. The closer it is to the internal measured heat source, the more obvious this phenomenon is; the lower the initial saturation is, the more obvious it is.