Hydraulic Conductivity of Bentonite Slurry Mixed Sands

The hydraulic conductivity (k) of specimens from columns containing initially dry sands mixed with bentonite slurries was measured. The mixed specimens represented a range in void ratios (0.672 ≤ e ≤ 3.94) and bentonite contents (0.61% ≤ BC ≤ 7.65%, by dry weight). The measured k values, which ranged from 2.4×10?7?cm/s to 6.8×10?4?cm/s, correlated poorly with the total void ratio (e) of the specimens, due to the complicating effect of the bentonite in the sand-bentonite slurry mixtures. However, the measured k values correlated better with the void ratio of the bentonite (eb), which is consistent with the results of previous studies involving permeation of compacted bentonite and sand-bentonite specimens, even though the range in values of eb in this study (42.5 ≤ eb ≤ 127) was much higher than that previously reported. The relatively large range in eb values for the sand-bentonite slurry mixtures was also consistent with the relatively large range in measured k values, which are about one to seven orders of magnitude higher than values of k commonly reported for compacted sand-bentonite mixtures, despite similar bentonite contents. In terms of bentonite content, addition of more than 3% bentonite via slurry injection and mixing with the sands was successful in reducing the k of the unmixed sands (9.4×10?3?cm/s ≤ k ≤ 5.4×10?2?cm/s) by as much as four orders of magnitude to values less than 1.0×10?6?cm/s.     

Hydraulic Conductivity and Compressibility of Soil-Bentonite Backfill Amended with Activated Carbon

Flexible-wall permeability tests and rigid-wall consolidation/permeability tests were performed to evaluate the hydraulic conductivity and compressibility of a model soil-bentonite (SB) backfill amended with granular activated carbon (GAC) or powdered activated carbon (PAC). The tests were performed as part of an assessment of enhanced SB backfill with improved attenuation capacity for greater longevity of barrier containment performance. Backfill specimens containing fine sand, 5.8% sodium bentonite, and GAC or PAC (0, 2, 5, and 10% by dry weight) were prepared to target slumps of 125±12.5?mm. Hydraulic conductivity (k) and compressibility of backfill test specimens were measured in consolidometers as a function of effective stress, σ′ (24 ? σ′ ? 1,532?kPa), whereas flexible-wall k was measured for backfill specimens consolidated to σ′ = 34.5?kPa. The results indicate that addition of GAC has little impact on the hydraulic and consolidation properties of the backfill, whereas addition of PAC causes a decrease in k and consolidation coefficient (cv) and a slight increase in compression index (Cc). Differences in behavior between GAC-amended backfills and PAC-amended backfills are attributed primarily to differences in GAC and PAC particle size.     

Predicting Solute Flux through a Clay Membrane Barrier

Measured solute flux breakthrough curves (FBCs) from column tests performed on a semipermeable clay membrane subjected to KCl solutions are compared with predicted FBCs using independently measured flow and transport properties. The predicted FBCs are based on three scenarios: (1) Advective–dispersive transport that neglects membrane behavior; (2) advective–dispersive transport that accounts for the concentration dependency of the effective salt-diffusion coefficient (Ds?) resulting from membrane behavior, referred to as partially coupled transport; and (3) fully coupled transport that includes both the explicit coupling terms (e.g., hyperfiltration, chemico-osmosis) associated with clay membrane behavior and the concentration dependency of Ds?. The FBCs predicted by fully coupled transport agree best with the measured FBCs. However, for the diffusion-controlled conditions of the column tests, the steady-state solute fluxes predicted by partially coupled transport are only 23–69% higher than the measured steady-state fluxes. The results imply that the advective–dispersive transport theory can be used to provide reasonably accurate, albeit somewhat conservative, estimates of steady-state solute flux through clays that behave as semipermeable membranes, provided diffusion is a significant, if not dominant, solute transport process and the concentration dependency of Ds? are taken into account.     

Chemico-Osmotic Efficiency of a Geosynthetic Clay Liner

Chemico-osmotic efficiency coefficients, ω, are determined from measured differential pressures across specimens of a geosynthetic clay liner (GCL) containing granular bentonite in response to applied differences in potassium chloride (KCl) concentrations under no-flow conditions. The results show that the GCL acts as a semipermeable membrane with ω values at steady state, ωss, ranging from 0.08 to 0.69 for KCl concentration differences ranging from 0.0039 to 0.047 M. The chemico-osmotic efficiency of the GCL decreases with increasing porosity and increasing KCl concentration. The decrease in ωss with increasing porosity is consistent with an increase in pore size reflected by an increase in measured hydraulic conductivity. The decrease in ωss with increasing KCl concentration is consistent with compression of the diffuse double layers surrounding the clay particles, and is reflected by a time-dependent decrease in the induced differential pressure as well as an increase in the hydraulic conductivity of the specimen. The results of this study are potentially significant with respect to the evaluation of the hydraulic and contaminant transport performance of GCLs used in waste containment applications.     

Membrane Behavior of Two Backfills from Field-Constructed Soil-Bentonite Cutoff Walls

Two soil-bentonite cutoff-wall backfills obtained from construction sites, one in New Jersey and one in Delaware, were tested for the existence of membrane behavior. Both backfills were designed as a mixture of dry bentonite (3–4% by dry weight) and the locally excavated soil blended with bentonite water slurry to provide slumps ranging from 100 to 150?mm (from 4 to 6?in.). The results of the membrane tests indicate that both backfills exhibit membrane behavior. Further, the magnitude of the membrane behavior increases with decreasing void ratio. However, the magnitude of the increase in membrane behavior in these construction-site backfills was lower than that previously reported for model backfills prepared in the laboratory. The difference in the membrane behavior is attributed, in part, to a lower percentage of clay in the construction-site backfills relative to the model backfills. Nonetheless, based on the measured membrane efficiencies for the two field-constructed backfills, the total liquid flux (q) through the cutoff walls can be expected to be reduced relative to that in the absence of membrane behavior (qh) by 1–10% for the cutoff wall in Delaware and 7–8% for the cutoff wall in New Jersey, depending on the void ratio. Thus, the results of this study suggest that membrane behavior in field-constructed cutoff walls can be significant, depending on the void ratio and the clay content of the backfill.     

Consolidation of a Geosynthetic Clay Liner under Isotropic States of Stress

The consolidation behavior of a geosynthetic clay liner (GCL) was evaluated by consolidating duplicate specimens of the GCL in a flexible-wall cell to a final effective stress, σ′, of 241 kPa (35.0 psi). The hydraulic conductivity, k, also was measured at the end of each loading increment. The results indicated that the GCL was normally consolidated for values of σ′ greater than 34.5 kPa (5.0 psi), which correlates well with limited consolidation data reported in the literature for GCLs based on confined compression using oedometers. Values of the coefficient of consolidation, cv, for the GCL ranged from 5.2×10?10?m2/s to 2.1×10?9?m2/s, and generally decreased with increasing σ′, albeit only slightly. Values of the measured k, kmeasured, for the GCL were low ( ? 5.0×10?9?cm/s) due to the sodium bentonite content of the GCL, and were within a factor of about two of the values of k calculated on the basis of classic (Terzaghi) small-strain consolidation theory, ktheory (i.e., 0.5 ? ktheory/kmeasured ? 2.0), suggesting that the theory is appropriate for describing the consolidation behavior of the GCL. The results also are consistent with the results of previous studies based on one-dimensional consolidation of sodium montmorillonite, suggesting that there would be little difference in the consolidation behavior of the GCL under confined compression.     

Foundry Green Sands as Hydraulic Barriers: Laboratory Study

A laboratory testing program was conducted to assess the use of foundry sands from gray iron foundries, typically called green sands, as hydraulic barrier materials. Foundry green sands are mixtures of fine uniform sand, bentonite, and other additives. Specimens of foundry sand were compacted in the laboratory at a variety of water contents and compactive efforts and then permeated in rigid-wall and flexible-wall permeameters to define relationships between hydraulic conductivity, compaction water content, and dry unit weight. Additional tests were conducted to assess how hydraulic conductivity of compacted foundry sand is affected by environmental stresses such as desiccation, freeze-thaw, and chemical permeation. Results of the tests show that the hydraulic conductivity of foundry sand is sensitive to the same variables that affect hydraulic conductivity of compacted clays (i.e., compaction water content, and compactive effort). However, hydraulic conductivities <10?7 cm∕s can be obtained for many foundry sands using a broad range of water contents and compactive efforts, including water contents dry of optimum and at lower compactive effort. The hydraulic conductivity of foundry sand was generally unaffected by freeze-thaw, desiccation, or permeation with 0.1 N salt solution or municipal solid waste leachate, but was incompatible with acetic acid (pH = 3.5). Hydraulic conductivity of foundry sands correlates well with bentonite content and liquid limit, with hydraulic conductivity less than 10?7 cm∕s being achieved for bentonite content ≥6% and∕or liquid limit >20.     

Swell Potential of Pulverized Coal Combustion Bottom Ash Amended with Sodium Bentonite

Within the last decade several studies have been conducted to evaluate the geotechnical properties of bottom ash obtained from electric utilities burning pulverized coal amended with admixtures such as clay, bentonite, and lime. Most of these studies concentrated on evaluating strength and stiffness characteristics of the mixtures. Because of the high volume change characteristics of bentonite and clay, improper and/or excessive use of these admixtures can impart significant swelling characteristics to the mixtures. This study was conducted to evaluate the swelling properties of two bottom ash-bentonite mixtures compacted at various initial moisture contents. Results from this detailed investigation show that the swelling potential increased with the increase in bentonite content and decreased with the increase in initial moisture content. Mixtures with 20% bentonite were observed to have volume change characteristics that may not be suitable for some lightly loaded structures. Mixtures with 15% bentonite; compacted at initial moisture contents of 18% or higher; were observed to have less than 4% free swell, whereas for mixtures compacted at an initial moisture content of 16% or lower, the percent increase in free swell was observed to be greater than 7%.     

Organoclay with Soil-Bentonite Admixture as Waste Containment Barriers

Organoclays, clays modified by cationic surfactants, for engineering applications have recently drawn great attention because of their high organic removal capacity. In this study, the potential use of organoclays with soil-bentonite admixtures as waste containment barriers is investigated by experimental tests such as batch equilibrium sorption studies, compaction tests, and hydraulic conductivity tests. Sorption isotherms of total organic carbon (TOC), a gross organic term, by five different types of soil admixtures are nonlinear. The soil specimen with more organoclays exhibits higher organic sorption capacity and a larger retardation factor. The specimens with 20% by dry weight of bentonite have higher optimum water content and plasticity. With the addition of bentonite in the soil material consisting of completely decomposed volcanic rock (CDV) (natural soils) and organoclays, the hydraulic conductivity to leachate decreases from about 10?7 to 10?8 cm∕s. This indicates that the presence of bentonite in the admixtures is important in reducing hydraulic conductivity.     

Hydraulic Conductivity and Swell of Nonprehydrated Geosynthetic Clay Liners Permeated with Multispecies Inorganic Solutions

The influence of multispecies inorganic solutions on swelling and hydraulic conductivity of non-prehydrated geosynthetic clay liners (GCLs) containing sodium bentonite was examined. Ionic strength and the relative abundance of monovalent and divalent cations (RMD) in the permeant solution were found to influence swell of the bentonite, and the hydraulic conductivity of GCLs. Swell is directly related to RMD and inversely related to ionic strength, whereas hydraulic conductivity is directly related to ionic strength and inversely related to RMD. RMD has a greater influence for solutions with low ionic strength (e.g., 0.05?M), whereas concentration effects dominate at high ionic strength (e.g., 0.5?M). No discernable effect of cation species of similar valence was observed in the swell or hydraulic conductivity data for test solutions with similar ionic strength and RMD. A strong relationship between hydraulic conductivity and free swell was found, but the relationship must be defined empirically for a particular bentonite. A regression model relating hydraulic conductivity of the GCL to ionic strength and RMD of the permeant solution was developed. Predictions made with the model indicate that high hydraulic conductivities (i.e., >10?7?cm/s) are not likely for GCLs in base liners in many solid waste containment facilities. However, for wastes with stronger leachates or leachates dominated by polyvalent cations, high hydraulic conductivities may occur.     

Field Performance of a Compacted Clay Landfill Final Cover at a Humid Site

A study was conducted in southern Georgia, USA, to evaluate how the hydraulic properties of the compacted clay barrier layer in a final landfill cover changed over a 4-year service life. The cover was part of a test section constructed in a large drainage lysimeter that allowed continuous monitoring of the water balance. Patterns in the drainage (i.e., flow from the bottom of the cover) record suggest that preferential flow paths developed in the clay barrier soon after construction, apparently in response to desiccation cracking. After four years, the clay barrier was excavated and examined for changes in soil structure and hydraulic conductivity. Tests were conducted in situ with a sealed double-ring infiltrometer and two-stage borehole permeameters and in the laboratory on hand-carved blocks taken during construction and after four years of service. The in situ and laboratory tests indicated that the hydraulic conductivity increased approximately three orders of magnitude (from ≈ 10?7?to? ≈ 10?4?cm?s?1) during the service life. A dye tracer test and soil structure analysis showed that extensive cracking and root development occurred throughout the entire depth of the barrier layer. Laboratory tests on undisturbed specimens of the clay barrier indicated that the hydraulic conductivity of damaged clay barriers can be underestimated significantly if small specimens (e.g., tube samples) are used for hydraulic conductivity assessment. The findings also indicate that clay barriers must be protected from desiccation and root intrusion if they are expected to function as intended, even at sites in warm, humid locations.     

Role of Matric Suction in Collapse of Compacted Clay Soil

This paper examines the influence of variations in matric suction on the collapse behavior of compacted Bangalore clay soil. The ASTM filter paper method measured the matric suctions of the compacted soil specimens. The matric suction of the compacted clay soil specimens ranged between 50 and 8,000 kPa at the as-compacted Sr values of 90 and 35%. Comparison of the matric suction-gravimetric water content relations of various compacted soils showed that the soil with a higher liquid limit has a higher matric suction at a given gravimetric water content. Variations in as-compacted degrees of saturation at a constant relative compaction or variations in relative compaction at a constant as-compacted degree of saturation notably affected the matric suction of the Bangalore clay soil. Experimental results also showed that the influence of matric suction on the collapse behavior of this compacted clay soil greatly depended on the relative compaction of the specimens and the pressure at which they were inundated.     

Volatile Organic Compound (VOC) Transport through Compacted Clay

Movement of volatile organic compounds (VOCs) through compacted clay liners was investigated using laboratory-scale column and tank tests. Hydraulic conductivity of the compacted clay was not significantly impacted by the introduction of VOCs in concentrations up to 20 mg∕L. Soil-water partition coefficients of the seven VOCs tested had a strong logarithmic relationship with the octanol-water partition coefficient. Partition coefficients from batch tests were in good agreement with those measured directly on soil samples at the termination of the column∕tank tests. The VOCs were degraded in the clay, with estimated half-lives ranging from 2 to 116 days. Mechanical dispersion was not significant in the range of the hydraulic conductivities of the test specimens (i.e., <10?7 cm∕s). Effective molecular diffusion coefficients were mostly in 10?6 cm2∕s and generally decreased with increasing aqueous solubility. Mass transport parameters of VOCs in clay liners can be estimated from laboratory batch tests and properly prepared small-scale column tests. However, accounting for degradation of VOCs and minimizing the number of transport parameters that are simultaneously estimated from a single response-time record are important considerations for accurate determination of transport parameters.     

Influence of Osmotic Suction on the Soil-Water Characteristic Curves of Compacted Expansive Clay

Unsaturated clays are subject to osmotic suction gradients in geoenvironmental engineering applications and it therefore becomes important to understand the effect of these chemical concentration gradients on soil-water characteristic curves (SWCCs). This paper brings out the influence of induced osmotic suction gradient on the wetting SWCCs of compacted clay specimens inundated with sodium chloride solutions/distilled water at vertical stress of 6.25 kPa in oedometer cells. The experimental results illustrate that variations in initial osmotic suction difference induce different magnitudes of osmotic induced consolidation and osmotic consolidation strains thereby impacting the wetting SWCCs and equilibrium water contents of identically compacted clay specimens. Osmotic suction induced by chemical concentration gradients between reservoir salt solution and soil-water can be treated as an equivalent net stress component, (pπ) that decreases the swelling strains of unsaturated specimens from reduction in microstructural and macrostructural swelling components. The direction of osmotic flow affects the matric SWCCs. Unsaturated specimens experiencing osmotic induced consolidation and osmotic consolidation develop lower equilibrium water content than specimens experiencing osmotic swelling during the wetting path. The findings of the study illustrate the need to incorporate the influence of osmotic suction in determination of the matric SWCCs.     

Cytotoxicity of aluminum silicates in primary neuronal cultures

To study their cytotoxicity, clays containing aluminum silicates were added to cultures of primary murine spinal cord neurons and differentiated N1E-115 neuroblastoma cells. Bentonite (0.1 mg/ml) and montmorillonite (0.1 mg/ml) rapidly associated with the outer membrane of both N1E-115 and neuronal cells. Erionite (0.1 mg/ml) was randomly distributed throughout the culture. Both bentonite and montmorillonite caused complete cell lysis in the neuronal cultures within 60 min following addition. Erionite had no effect. None of the clays appeared to be cytotoxic to the differentiated N1E-115 cells even though bentonite and montmorillonite were closely associated with the cell membrane. N1E-115 cell lysis did not occur up to 18 h after addition of the clay. Aluminum silicate-containing clays caused a rapid lysis of primary neuronal cells. Differentiated N1E-115 neuroblastoma cells were not susceptible to clay-induced lysis, suggesting that the lytic mechanism is not a general phenomenon that affects all cell types equally.     

Theoretical Equations on Hydraulic Conductivities of Bentonite-Based Buffer and Backfill for Underground Disposal of Radioactive Wastes

Compacted bentonite and sand-bentonite mixtures are sought as buffer and backfill materials for high-level radioactive waste disposal facilities because they have very low permeability. To establish specifications such as the dry density and sand-bentonite mass ratio for buffer and backfill materials, we must quantitatively evaluate a material’s hydraulic conductivities. This study presents theoretical new equations for evaluating the hydraulic conductivity of compacted bentonites and sand-bentonite mixtures. New equations are proposed for evaluating the flow velocity of interlayer water between two montmorillonite parallel-plate layers considering the swelling behaviors of montmorillonite. Furthermore, a prediction method for hydraulic conductivity of compacted bentonite and sand-bentonite mixtures is presented by combining new equations with previous equations for evaluating swelling behavior of montmorillonite in bentonite. The applicability of this method is clarified by comparing predicted results with experimental data reported by previous research on hydraulic conductivities of compacted bentonites and sand-bentonite mixtures.     

Flow of Gasoline through Composite Liners

A composite liner composed of a soil/clay liner and a flexible membrane is widely used for waste containment facilities. In this research, an organically modified clay (organoclay BB-40) liner and a high-density polyethylene (HDPE) membrane were studied for preventing the leakage and migration of gasoline from underground storage tanks into the surrounding environment. The equivalent hydraulic conductivity of intact HDPE to gasoline was determined using a specially built system, and the conventional hydraulic conductivity testing method was employed to determine the hydraulic conductivity of compacted organoclays and the permeation rate of gasoline through composite liners. The equivalent hydraulic conductivity of intact HDPE to gasoline was about 10–13 cm/s, and the hydraulic conductivity of the organoclay liner was approximately 6.0×10?9?cm/s, which is nearly 4 orders of magnitude lower than that obtained for unmodified clay. These results show that both organoclay and HDPE are effective in reducing the release of gasoline by advective flow, especially the intact HDPE. The flow of gasoline through the composite liners under the worst condition, was of the same magnitude as that through a single organoclay liner, independent of the flow shape. It can be anticipated that under good contact conditions, the defective HDPE would still be beneficial in reducing the permeation of gasoline due to the decrease of the wetted area of the underlying layer exposed to gasoline leakage.     

Determining Intrinsic Compressibility of Fine-Grained Soils

Results of laboratory oedometer tests on reconstituted specimens of four clays prepared at different initial water contents, ranging from the liquid limit to 1.75 times the liquid limit, show that the intrinsic compression line may not be “unique” for a given soil. This suggests that the “intrinsic” parameter Iv, which is based on the constants of intrinsic compressibility, e100?, (void ratio corresponding to σv′ = 100?kPa), and Cc?, (e100??e1000?), may in fact not be a truly intrinsic parameter of the soil, but is dependent on sample preparation. The positioning of the normalized compression curve in e–log–σv′ space is significantly influenced by the initial remolding water content, therefore resulting in differing values of e100? for a given soil depending on the initial water content. The influence of initial water content was greater for kaolinitic and illitic clay than for montmorillonitic clay. It is hypothesized that the difference in behavior may be attributed to differences in mineralogy. The results illustrate that caution should be used when comparing tests results from widespread sources and suggest that a standard level of initial water content be used to evaluate the intrinsic compressibility.     

Shear Behavior of Compacted Rubber Fiber-Clay Composite in Drained and Undrained Loading

The ductility, toughness, and resistance to tensile cracking of clays can be improved with the inclusion of short fibers. Tire buffings are derived from the tire retread process and because of their elongated shape, may be used as fiber inclusions. The objective of this study was to evaluate the drained and undrained shear strength of mixtures of clay and tire buffings. Mixtures of silty low plasticity kaolinitic clay and 10% by dry weight of tire buffings were compacted at both Standard and Modified compaction energy. Consolidated-drained and consolidated-undrained triaxial tests were run at confining stresses ranging from 50?to?300?kPa. Preshear and postshear permeability tests were conducted. Results showed that the peak strength of the composite is comparable to or greater than that of clay alone when tested at confining stresses below 200–300?kPa. Above this threshold, the presence of inclusions tends to degrade the strength of the clay. Changes in permeability were not significant.     

Use of Organoclay as Secondary Containment for Gasoline Storage Tanks

Conventional clay liners, which are widely used as hydraulic barriers to water, have been shown to be adversely affected by organic fluids. However, the addition of quaternary amines into bentonite greatly enhances its compatibility with organic fluids and thus allows the clay barrier technology to be extended to the treatment of organic contaminants. In this study, an organically modified clay was studied for use as a secondary containment for gasoline underground storage tanks. Free swelling and hydraulic conductivity tests were performed on organoclay, pure bentonite, and natural soils. Results show that organoclay has a large swelling capacity in gasoline, whereas bentonite and natural soils shrink when immersed in gasoline. The hydraulic conductivities of bentonite and natural soils to gasoline are 2–5 orders of magnitude higher than that to water. In contrast, the hydraulic conductivity decreases by 2 orders of magnitude for organoclay and this low value can be maintained even under freeze-thaw and dry-wet cycles.