Enhanced bentonites (EBs) developed for improved hydraulic compatibility (i.e., low hydraulic conductivity, k, to chemical solutions) relative to unamended sodium bentonite (NaB) are increasingly being used in geosynthetic clay liners (GCLs). Both NaB and EBs have been shown to exhibit semipermeable membrane behavior. Thus, predictions of barrier performance for different bentonites that focus only on advection and ignore the influence of membrane behavior and diffusion on solute flux are inaccurate. In this paper, data from solute transport studies conducted over the last 20 years were used to compare expected solute flux through NaB and EB barriers with and without accounting for membrane behavior. Coupled effective diffusion coefficients (D*), membrane efficiency coefficients (w), and k values for NaBs and EBs from the literature were used with an analytical solution for coupled solute transport to predict dimensionless transient and steady-state solute fluxes exiting the barrier (J*j and J*ss) under typical GCL boundary conditions. Values of J*j and J*ss for a range of salt solutions were compared for NaB GCLs and five different EBs. For a given source concentration (Co) of a given salt, EBs generally exhibit lower k, higher w, and similar D* relative to NaB GCLs. As a result, J*ss values for EBs were 10–60% lower relative to NaB for monovalent (KCl, NaCl) solutions and up to 95% lower relative to NaB for divalent (CaCl2) solutions. The bentonites with the highest w, but not necessarily the lowest k, correlated to the lowest J*ss for all solutions.

This paper elucidates the influence of alkaline solutions on shear characteristics of bentonite and bentonite–sand mixtures. The compacted Ca–bentonite and bentonite–sand mixtures were immersed in 0.1 mol/L–KOH and KOH–NaOH solutions at 40 ℃. Triaxial compression tests were executed using the immersed specimens. The mineral composition, montmorillonite content, expansivity of montmorillonite, mean layer charge, and leachable cations of the samples were investigated. The shear characteristics of the compacted bentonite changed after the immersion in alkaline solutions. The reduction of dry density, likely induced by the dissolution of minerals, and the K-exchange of montmorillonite are related to the decrease in shear strength. K-exchanged montmorillonite behaved as a pseudo-non-swelling layer. It is suggested that the precipitation of secondary products, composed of mainly calcium, contributed to the increase in shear strength. The change in the shear characteristics of the compacted bentonite was interpreted as primarily resulting from a combination of dissolution and precipitation.

Cracking in clay is frequently encountered in geoenvironmental and geotechnical engineering, sometimes causing failures in foundations, landfill liners and road and railway embankments. At the same time, desiccated clay materials are known to spontaneously heal their surface cracks when rehydrated. The autogenous healing capacity of clay has been typically inferred from changes in its permeability and mechanical strength. However, very little direct observation of crack sealing in real time has been reported in the literature. In addition, knowledge of the mechanisms driving self-healing in clay, and the effects of different variables on these mechanisms remain poor. In this paper, micro X-ray computerized tomography (µ-XCT scanning) is used to observe and quantify changes of crack volume in a bentonite clay as a result of swelling driven by hydration. A vertical crack is artificially introduced into an intact cylindrical sample of consolidated bentonite and the crack closure observed. The cracked zone is segmented and quantified by image processing. The effects on the pace and extent of sealing of sealing time, consolidation pressure and boundary constraints, are considered. Results from these experiments, bearing in mind the specific limitations of X-ray imaging, can contribute to a possible coupled chemo-hydro-mechanical interpretation of autogenous sealing of clay.

Compacted bentonite was selected as buffer material in deep geological repository to deal with high level nuclear waste (HLW). During the long-term operations of repositories, volume change characteristics of compacted bentonite are affected by several factors, for example: (1) Change of the concentration in infiltration liquid caused by the variation of groundwater level; (2) Vary of additional stress owing to the unpredictable geological tectonic activity and the stress generated by the swelling of buffer material in confined condition. Hence, it is very crucial to study the effects of chemo-mechanical couplings on volume change characteristics of compacted bentonite. To simulate different chemo-mechanical coupled conditions, serial swell-compression-rebound tests were performed infiltrated by distilled water (DIW) and NaCl solutions with different concentrations. Multi-step salinization-desalination tests were carried out as well. Volume change behaviors of compacted bentonite were studied from microstructure to macrostructure. Experimental results showed that: (1) Final swelling strain and compression index of compacted bentonite decreased with increasing NaCl solution concentration; (2) Comparing the volume change behaviors of compacted bentonite underwent desalination-salinization cycles to those did not undergo these cycles, it was found that the plastic property at the macro level exhibits irreversible property to a certain extent. While, microstructural characteristics are somewhat reversible and elastic.

Bentonite is often considered as one of the key components of the Engineered Barrier System (EBS) for the radioactive waste disposal; mechanical, thermal, and hydraulic behaviors of bentonite should be thoroughly studied to ensure the emplaced the quality and to control the performance of EBS. This study examined the effects of dry density and relative humidity (RH) on microstructure and mass transport characteristics of Granulated Bentonite Mixtures (GBM). The tested GBM samples were prepared by sieving and grading Japanese bentonite (trade name: OK Bentonite, Kunimine Industries, Japan). For dry density-controlled conditions, air-dried GBM samples (RH 60%, water content of 6.38% by weight) were packed at six different dry densities ranging from 1.25 to 1.75 g/cm3. The samples for humidity-controlled conditions were prepared at the dry density of 1.45 g/cm3 and stored at RH 90% (0 to 12 weeks). A microfocus X-ray computerized tomography (MFXCT) scanning apparatus was used to visualize the microstructures of packed tested samples. For each tested sample, the mass transport parameters of gas diffusivity (Dp/Do), air permeability (ka), thermal conductivity (λ), and volumetric heat capacity (C) were measured. The results indicated that dry density greatly affected the CT brightness values of GBM; the histogram of CT brightness well captured the internal distribution of density and the CT brightness homogenized more with increasing in dry density. The measured Dp/Do and ka. were mainly controlled by the dry density. On the other hand, the thermal properties such as λ and C were governed both dry density and RH.

In the garbage disposal site, a liner is installed to prevent contaminant migration and reducing the groundwater and surrounding environment getting polluted. Due to chemical stability, high swelling capacity, high specific surface area, low hydraulic conductivity, and high cation exchange capacity, bentonite clay is utilized as a liner material in the disposal site to stop the contamination of the surrounding environment. However, chemicals present inside the landfills may affect the liner properties. This investigation was carried out to determine the influence of heavy metal solution on the change in properties of compacted bentonite. In this study, the sorption and hydraulic conductivity of lead (Pb2+) and copper (Cu2+) were studied on the bentonite clay at an initial concentration of 100 and 1000 mg/L. Test parameter like pH, initial metal concentration, contact time, and agitation speed was examined. The results showed that the sorption capacity of the bentonite increased with a rise in the initial concentration of heavy metals. For an initial concentration of 1000 mg/L, the removal percentage for Pb2+ and Cu2+ was found to be 90.68 and 72.178, respectively. The outcome also reveals that the hydraulic conductivity value of the bentonite raised with the rise in Pb2+ and Cu2+ concentration.

The ability of bentonite-based barriers to act as semipermeable membranes that inhibit the passage of solutes (ions) is well documented. This behavior induces chemico-osmotic liquid flux that can improve the performance of such barriers by reducing solute mass flux. This paper explores the potential significance of chemico-osmosis on solute transport through bentonite membranes using a phenomenological transport framework combined with a physical model relating the macroscale transport properties (membrane efficiency coefficient, w, and hydraulic conductivity, kh) to the microscale physicochemical and fabric properties of the bentonite. The model was used to simulate the coupled transport of monovalent salt (KCl) through a geosynthetic clay liner. The results indicate that the influence of chemico-osmosis is dependent upon the void ratio of the bentonite and the extent to which clay platelets are aggregated to form tactoids. Chemico-osmosis is predicted to have an increasingly more significant impact on solute transport with increasing source concentration (Cs0), despite decreasing w with increasing Cs0.

Interface shear strength between bentonite-polymer composite geosynthetic clay liners (BPC-GCLs) and a textured geomembrane (GM) was investigated and compared to the interface shear strength between conventional Na-bentonite GCLs (NaB-GCLs) and the GM. Lower peak shear strength was observed in BPC-GCL/GM interfaces compared to NaB-GCL/GM interfaces under the same normal stress. Elution of hydrated polymer gel with low shear strength into the BPC-GCL/GM interface was identified as the cause of lower interface shear strength.. Polymer elution was greatest during hydration, but also occurred during shearing.

Membrane behavior of a geosynthetic clay liner (GCL) exhumed from a bottom liner system at a municipal solid waste landfill in California, USA was investigated in this study. The GCL was installed as part of a geomembrane-GCL bottom liner system at a newly constructed cell at the landfill. The cell was not filled subsequent to the construction of the liner system and the geosynthetics were exposed to the atmosphere for 12 years. Preliminary data on the membrane behavior of GCL samples exhumed from the liner system are provided herein. Laboratory multi-stage membrane behavior tests were used to determine membrane efficiency coefficients (ω) of the GCL for potassium chloride source solutions. Measurable membrane behavior was confirmed in the laboratory tests for the exposed GCLs. Even though membrane behavior was shown to still exist in the GCL after 12 years of exposure, values of ω were very low (0.1 – 5.4 %). Bentonite migration and cation exchange likely contributed to the significantly lower ω for the exposed GCLs relative to values reported in the literature for virgin/unexposed GCLs.

High concentration of ammonium was detected in groundwater in southern Hanoi, Vietnam while municipal solid waste (MSW) landfills were known to generate large amounts of NH4+. Thus, bottom barrier with well performance should be required in Hanoi MSW landfills to minimize NH4+ migration. Hanoi is expected to experience temperature increase, which enables to reduce hydraulic-barrier performance. Hence, study on temperature effect on NH4+ adsorption and diffusion through landfill barriers plays a key role in prevention of NH4+ contaminated aquifers. The objective is to evaluate effect of three kinds of temperature (20oC, 35oC and 50oC) on NH4+ adsorption and diffusion through GCL and in-situ clayey soil sampled in Hanoi (HN clay). The results show that GCL possesses higher non-linear partitioning coefficient (Kd*) and lower diffusion coefficient (De) than HN clay in all cases of temperature. Both GCL and HN clay experienced an increase of Kd* and De in the temperature range of 20oC and 35oC. When temperature rises to 50oC, Kd* tends to decrease but De keep increasing. Hence, the study indicates a more dominant effect of diffusion coefficient than non-linear partitioning coefficient in promotion of ammonium mass transport through HN clay.

Geosynthetics Clay Liners (GCLs) are widely used in geoenvironmental engineering as part of groundwater protection barrier systems. The dehydration and desiccation of bentonite in the GCLs under high temperatures, together with its suppressed self-healing when exposed to aggressive chemicals, has led to concerns about its performance and the durability of engineering barriers in these environments. In this paper, key parameters influencing chemo-thermo-hydro-mechanical behavior of GCLs under high temperature and salinity are reviewed, based on experimental evidence generated by the authors over the last four years. Next, a thermo-hydro-mechanical model is used to analyze the effects of subsoil characteristics on GCL dehydration. Finally, a new concept (I-GCLS), is proposed and experimentally investigated with the aim of preventing dehydration and desiccation cracking.