Waste Containment Systems Research Articles

Lateritic Soil and Bentonite Mixtures Assessment for Liner Usage Purpose

Among all the constructive elements in waste containment systems, base liner may be the one of greatest concern. In Brazil, the use of compacted lateritic soils for this purpose is widely accepted due to their wide occurrence. While they usually achieve compaction and strength requirements, sometimes they may need some improvement in terms of hydraulic conductivity and reactivity to contaminants. This paper describes a laboratory assessment of two mixtures of a lateritic soil and bentonite for liner usage purpose. Laboratory tests included geotechnical, mineralogical and physicochemical characterization, hydraulic conductivity determination in compacted samples and modified Atterberg Limits and grain-size distribution analysis. These two last tests were intended to evaluate the compatibility between the soil and different chemical solutions. The characterization tests showed that the clay content of the mixtures decreased and the plasticity and activity increased relative to the natural soil sample. The hydraulic conductivity of the compacted mixtures decreased two to four times when compared to the compacted natural soil. The results also have demonstrated that the addition of bentonite in the proportions used in the tests increased the compatibility of the natural soil.

The log-normal distribution for hydraulic conductivity of compacted clays: Two or three parameters?

Risk and uncertainty assessments for waste containment systems employing clay barriers often include spatial variability in the hydraulic conductivity as part of the analysis. The two-parameter log-normal distribution is often used to describe the spatial variability, but for compacted clays the three-parameter form can be more appropriate. A statistical hypothesis test was developed that can be used to determine if the two- or three-parameter form of the log-normal distribution is more appropriate. The test is based on a likelihood ratio, comparing likelihood functions for the two- and three-parameter forms at their maximums. Likelihood functions are used assuming that a data set can be segregated into a set of distinct groups of hydraulic conductivities. A step-by-step calculation procedure is described and the test is applied to data collected from 45 sites.

Influence of the unsaturated zone of soil layer on the solute migration

To design waste containment barriers, low permeability soils or manufactured liner products are required. For hydraulic barriers in waste containment system, it is recommended to use soil layer of 5 m thickness when the coefficient of the permeability is higher than 10 − 6 m/s or a layer of one meter thickness, when the coefficient of permeability is lower than 10 − 9 m/s. So some authors tried to prove analytically, that the barrier thickness can be reduced when the permeability of the soil is less than 10 − 9 m/s (e.g. 10 − 10 or 10 − 11 m/s). This suggestion does not take into account the ion sorption and the solute migration delay which can occur in an unsaturated zone and reduce the solute transfer. Infiltration tests on clayey sand and clay of Gault columns are carried out in our laboratory. TDR probes at different depth allow soil moisture and soil impedance measurements. The solute concentration in the pore water is monitored by determining the electrical conductivity from the soil impedance. A lead slag leachate with an electrical conductivity of 5.3 S/m and a pH 6.3 is used for the infiltration tests. The clayey sand column is percolated during the infiltration test. But the moisture takes one year to reach the clay of Gault column bottom. The electrical conductivity and the moisture become uniform in clayey sand column after 15.5 h. The maximum value of the electrical conductivity is not yet reached in the clay column after 500 days. The forms of the electrical conductivity profiles prove that the leachate solute migration is dispersive–convective along the clayed sand column and only dispersive along the clay column. The soil impedances measured with TDR probes are used to assess the solute retained in the soil layer. These measurement results show that 300 days are needed to reach the solute retention capacity of the 25 mm thickness of the clay. The thickness of 475 mm of the clayey sand reaches its solute retention capacity after 15.5 h. These results prove that the efficiency of liner materials does not depend only on hydraulic behaviour. The use of low permeability materials reduces the water flow but a sufficient thickness is required to reduce the solute migration. The delay in the solute transfer is increased by the thickness of the unsaturated zone.

Analyses of Seismic Damage to Interfaces in Waste Containment Systems: A Review

Interfaces are generally the weak links in most structural systems because they are locations of stress concentration and physicochemical/biological attack from both external and internal causes. For waste containment systems in seismically active zones, stress concentration and possible damage initiation at interfaces among different layers of materials depend on input seismic parameters, site characteristics, containment system design configuration, and both material and interface characteristics of components of the system. The approaches and quantitative methods that have been used to analyze the strength and stability of waste containment structural components and systems, including adaptations from earthdam design techniques, are discussed. Using U.S. regionalization data from HAZUS (Natural Hazard Loss Estimation Methodology), containment system liner displacements in lateral and vertical directions are predicted for a liner base at 5 m depth shaken by waves with periods 0.1–0.5 at amplified ground...

Improvement of Approaches to Waste Containment System Development, Management, and Evaluation

Waste-containment sites number in the hundreds of thousands throughout the world as a result of past and continuing industrial, municipal, and military activities. Within the United States alone, recent records of the United States Environmental Protection Agency USEPA indicate the existence of more than 200,000 contaminated sites, a large fraction of which involve or need waste containment. In addition to contaminated sites and facilities, a large waste-generation inventory comprises billions of metric tons of metal mining and processing wastes, oil and gas processing wastes, industrial wastes, municipal wastes, and solid wastes from electric power generation stations. At the Hanford site, which had been a plutonium production site for 50 years, approximately 7,600 cubic meters of actinides, as well as 1.2 million cubic meters of low-level radioactive wastes, are buried. It is not feasible to chemically or biologically clean up all contaminated sites because of excessive costs including opportunity costs . Furthermore, the complexities of both the subsurface geohydrology of contaminated sites and the lack of field-scale cleanup technologies that can apply mechanisms proved at the bench scale for some persistent contaminants to field scale make it infeasible to implement cleanup programs as the sole means of managing contaminated sites. With respect to solid waste management, recycling has not kept pace with generation rates. Consequently, waste storage in containment systems will remain as a waste-management option for the foreseeable future. Among these systems are landfills, waste piles, covers and liners, grout curtains, slurry walls, reactive walls and beds, concrete tombs, and radioactive waste repositories. For these systems, the primary design function is minimizing the rates of contaminant release into the environment for service lives that range from about 10 years to 1,000 years. The performance of this function requires that the structural integrity of these systems be maintained for very long time periods. Considering the high uncertainties associated with system performance over very long time spans within which transient stressors such as floods and earthquakes may affect some facilities, the challenge is to develop effective ap-

Design Principles and Concepts for Enhancing Long-Term Cap Performance and Confidence

The siting of new waste containment systems is becoming increasing difficult as the public and stakeholders want more confidence that contaminant barrier systems will perform effectively for very long durations and owners want to store more wastes in the least area while knowing and limiting their long-term liabilities. These developments motivate reexamination of long-term performance issues and their implications for barrier designs. Accordingly, design principles are herein considered from the standpoint of long-term performance and management, including the ability to monitor and repair barriers. Then, some design concepts that may be implemented on the basis of these principles, especially evapotranspiration (ET) caps are discussed. Five design principles are recommended on the basis of considerations of infrastructure implementation experience and facility management experiences in other fields. The principles are: establishment of a clear and defendable design basis; design for ease of monitoring and repair; analysis of barriers as dynamic systems; working with nature and not against it; and recognition that increased complexity can reduce, not enhance, net performance. The ET caps are an excellent embodiment of these design principles. The principles are applied to ET caps, as well as design variants such as erosion armor, capillary breaks, biointrusion layers, and low permeability material layers.

Effect of Acidic Mine Drainage on the Mechanical and Hydraulic Properties of Three Geosynthetics

This paper describes a study of the physical effects of exposing three geosynthetics commonly used in waste containment systems (a geomembrane, geotextile, and drainage geocomposite) to a synthetic acidic mine drainage (AMD) from metallic mine wastes in an environment with limited access to oxygen and ultraviolet (UV) radiation. The geosynthetic materials were immersed in tanks containing synthetic AMD, acidic water, and de-ionized water at 20, 40, or 60°C. Specimens were periodically removed from the tanks over a 22 month period and tested to determine their mechanical (tensile strength, puncture strength, and trapezoidal tear strength) and hydraulic properties (transmissivity and permittivity). Results of the tests were evaluated using analysis of variance (ANOVA) and linear regression to determine if statistically significant changes in the mechanical or hydraulic properties occurred during exposure. Both ANOVA and linear regression showed that none of the mechanical or hydraulic properties exhibited s...

Slag-Cement-Bentonite Slurry Walls

In both the United States and the United Kingdom, slurry walls are used as vertical barriers to control groundwater flow and to contain contaminants as part of waste containment systems. In the United States, slurry walls are commonly constructed using soil-bentonite ~SB! and the barrier typically consists of a mixture of select soil, bentonite, and bentonite-water slurry. Alternatively, in the United Kingdom, the barrier wall comprises a mixture of cement, blast furnace slag, and bentonite-water slurry. After a comparison of the two techniques, this paper presents the results of permeability and unconfined compressive strength tests on twenty-one different mixtures of slag-cement-bentonite ~slag-CB!. The slurry wall materials tested in this study were prepared using sample formulations originating in the United Kingdom and materials originating in the United States. Unconfined compression tests were performed on samples after one month of curing, while permeability tests were performed after one, two, three, six, and twelve months of curing. For the mixtures tested and cured twelve months, two mixtures ~one having 20% cementitious material with 70% slag replacement and another having 15% cementitious material with 80% slag replacement! were found to have the lowest hydraulic conductivity s2 3 10 ˛8 cm/ sd. The data show that 0 to 60% slag replacement had little effect on hydraulic conductivity of the resulting slag-CB mixtures. However, the hydraulic conductivity drastically decreases as the slag replacement increases from 70 to 80%. As expected, the unconfined compressive strength increased as the cementitious material content increased from 10 to 15 to 20%. The slag-CB consolidates rapidly and has compression characteristics similar to other high moisture materials.

Étude de la migration du soluté d’un lixiviat dans un sol non saturé par la méthode TDR

The protection of the environment requires building waste containment systems made up of liners of compacted soil. In this paper, infiltration tests are carried out on soil columns equipped with TDR probes to investigate how water and pollutants migrate in the liners. The TDR method is a technique to measure simultaneously the volumetric water content and the electrical conductivity lor the impedance) of the soil in a continuous manner. Using six soils and three different leachates, new relationships between the volumetric water content (θ) of the soil and its dielectric constant (e) is established. These relationships appear to be more accurate than the one of Topp et al. (1980). They are used to determine water content profiles in a soil column. The relationship between the leachate solute concentration and the electrical conductivity allows to compute the solute concentration profiles in the soil column and the solute breakthrough curve. These curves are used to determine solute convection rate and solute apparent dispersion coefficient. Infiltration test results show that the soil permeability and the diffusivity increase when the leachate is infiltrated. The infiltration rate and the solute convection rate are higher than the leachate outflow rate and the three rates yield to a same value which depends on the soil permeability. The Green and Ampt (1911) model applied to the infiltration shows that the movement of the wet front is faster than the movement of the pollution front. The apparent dispersion coefficient and the solute convection rate are computed by applying the statistic distribution law to the moisture and the solute concentration profiles. It is observed that the apparent dispersion coefficient increases during the saturation of the sample. The electrical conductivity and the solute concentration of the percolated leachate are scaled with the initial values. The evolution curves of these scaled values with elapsed time are used to compute the apparent dispersion coefficient. There is little difference between the apparent dispersion values obtained with the scaled electrical conductivity evolution curve and the scaled solute concentration evolution curve. The use of the TDR measurements allows comparison between solute and water migration parameters and the obtained results can be applied to in situ permeability measurements.

Peer Reviewed: Modeling the Long-Term Performance of Waste Containment Systems

An excessively large number of sites in the United States are contaminated with A hazardous substances. The prohibitive costs of cleaning up all sites and the lack of effective technologies for destroying some hazardous materials often leave containment systems as the only viable option to control contaminant migration into sensitive media. Examples of such systems are covers and liners, vertical slurry and geosynthetic walls, concrete containments, and radioactive waste repositories. Designers predict that the lives of these systems range from about 50 to more than 1000 years. How long do these facilities really last? No model can predict that perfectly yet. However, the following discussion illustrates how an integrated approach that includes time-scaled material deterioration models, system reliability frameworks, and adaptive monitoring could improve long-term performance estimates and provide a sound technical basis for facility design and maintenance, environmental risk assessments, and regulatory specifications.

The Application of Innovative Materials in Waste Containment Barriers

Barriers are the critical components of waste containment systems such as landfills, slurry walls, grout curtains, permeable walls, and designed covers ~caps!. Examples of such containment systems are illustrated in Fig. 1. For a given set of conditions, the long-term performance of well-constructed, multicomponent containment systems depends on three principal factors: barrier material properties, component dimensions, and system configurations. Barrier layers of waste containment systems serve one or

Estimation of Polyethylene Oxide Polymer Train and Loop Densities on Contaminant Barrier Materials

Desiccation of exposed clayey materials in cover layers of waste containment systems can ultimately result in the development of cracks that can promote the infiltration of rainwater and/or snowmelt, and escape of gases from buried wastes. Clayey barriers can be stabilized against desiccation by introducing binders such as concentrated polymer liquids or aqueous solutions. In this research, a theoretical analysis of the morphological configuration of molecules of polyethylene oxide (PEO) on montmorillonite (a common barrier mineral) was performed. A methodology for estimating polymer train and loop densities using batch sorption measurements was developed and demonstrated using PEO sorption test data for sodium montmorillonite (specific surface area: 31.82±0.22 m\U2\N/g) at initial aqueous PEO (molecular weight 8,000,000) concentrations of 0, 0.5, 1.0, 2.0, and 4.0 g/L. From test data, the distribution coefficient of PEO was determined to be 0.1599 mL/g. The analyses indicate that the distribution of sorbed PEO molecules into trains and loops is very sensitive to PEO chain flexibility. For example, for a function of partition function (λ) value of 0.1, the numbers of PEO molecular trains on 1 g of sodium montmorillonite are estimated to be 3.410\U20\N and 4.710\U20\N for chain flexibilities of 0.2 and 0.8, respectively. The greater the chain flexibility, the greater the opportunity for polymer molecules to lie flat on clay particles. Within an initial PEO concentration range of 0.4.0 g/L, no significant dependence of sorbed polymer molecular configuration on initial concentration was found. Possibly, extension of sorbed polymer molecular loops into barrier soil pores narrows fluid flow channels and hence the desiccation rate. This research enhances understanding of the physico-chemical processes that underlie clay stabilization by aqueous polymers.

The positive effects of silica fume on the permeability, swelling pressure and compressive strength of natural clay liners

Clays are essential materials to reduce the hydraulic conductivity of natural clay liners in landfill sites. Impermeable compacted clay liners are needed the landfill to be designed. It is known that clays with high plasticity absorb water several times as much as their weights. The clay liners subjected to water pressure in landfills generate high permeability in time and instability problems in their body due to their expansive capacity. Though the compacted clay liners possess many advantages such as low permeability and large capacity of attenuation, they have high shrinkage and high expansive potential causing instability problem. The aim of this study is to examine the effects of silica fume on permeability, swelling pressure and compressive strength of the compacted clay liners as a hydraulic barrier. The test results showed that the compacted clay samples with silica fume exhibit quite low permeability, swelling pressure and significantly high compressive strength as compared to raw clay samples. Thus, silica fume appears to be promising for construction material of liners subjected to leachate in solid waste containment systems.

Modeling deformation behaviour of clay liners in a small centrifuge

In modern solid waste landfills, liner systems are essential structural elements that ensure that waste materials are safely separated from the environment. Many liner systems have been developed and used over the past decade. Among impermeable layers, clay liners are regarded as one of the very significant components of liner system and are being used worldwide as a waste containment system in landfills. One of the failures associated with clay liners is the occurrence of nonuniform settlements, resulting from the sudden collapse of waste, or the decomposition of waste materials, and (or) the subgrade over which the liner is laid. This paper deals with the use of a small centrifuge to model the deformation behaviour of clay liners. Model tests were performed in a small geotechnical centrifuge to investigate the behaviour of clay liners in landfills subjected to nonuniform settlements at sharp curvatures. A parametric study was conducted to analyze the influence of parameters like thickness, consistency, and overburden on the behaviour of clay liners. The clay liner without any overburden is observed to experience severe cracking in the form of deep and wide cracks at the maximum curvature zone. The depth and width of the cracks are found to decrease with an increase in clay liner thickness. The cracking failure pattern is suppressed by shearing for a clay liner with an increase in overburden pressure. Based on model test results, the clay liner with an adequate overburden is found to be free from cracking failure even when subjected to sharp curvatures. The effective usage of a small geotechnical centrifuge to model the deformation behaviour of clay liners is demonstrated adequately.Key words: landfills, municipal solid waste, clay liners, centrifuge modeling, nonuniform settlements, model tests.

Reliability of state of practice for selection of shear strength parameters for waste containment system stability analyses

In this paper, reliability analyses are performed using the results of slope stability analyses on example waste slopes to address two sources of uncertainty involved in waste slope stability assessments. These include the uncertainty associated with using shear strength parameters derived from non-project-specific sources and the uncertainty associated with the conditions required to make the development of a progressive failure mechanism, and subsequent mobilization of large-displacement shear strengths, possible. The results of the slope stability and reliability analyses performed demonstrate that less reliable or overly conservative designs can result from the use of shear strength parameters obtained from non-project-specific sources and that reliability analyses, even simple ones, provide information that can be used to establish confidence levels in factor of safety calculations. Reliability analysis results also show that slope stability calculations based on the full mobilization of large-displacement shear strengths are conservative since such analyses implicitly assume that a progressive failure mechanism develops. A method is presented that considers one particular progressive failure mechanism for the development of large-displacement conditions. The method provides a framework for evaluating the reliability of slopes where the potential for progressive failure exists.

Geomembrane Research Needs

A US National Science Foundation/Auburn University Workshop was held in Summer 2000 to identify geomembrane-related topics that require research. Three publications were produced: a geomembrane state-of-practice paper, a paper on research needs, and a final paper on organizing similar workshops. The present paper focuses on geomembrane research needs, with the intent of moving the profession forward by identifying what new knowledge/research is needed and how to get it. Stimulating discussion on the subject is a secondary purpose of the paper. Ten professionals, selected for their expertise and experience, attended the Workshop. Most facets of the geomembrane/waste disposal industry were represented. The participants almost exclusively represented United States geomembrane practice. Most of the discussions related to high-density polyethylene, linear low-density polyethylene, polyvinyl chloride, and polypropylene geomembranes. The Workshop considered seventeen topics related to geomembranes in waste, water, and product containment systems. The topics were seam tests, seam strength, wrinkles, lifetime predictions, seams types, geomembranes on steep walls, thermal seams in PVC geomembranes, geomembranes in bioreactors, durability, remote leak repair, defect significance, shear displacements, seams in different geomembrane types, leak location and significance, puncture protection, and multiaxial testing.

Pozzolanic fly ash as a hydraulic barrier in land fills

The liner plays an important role in controlling migration of contaminants present in the leachate in waste containment systems such as land fills and impoundments. Although questions have been raised about the performance of clay liners, they are increasingly used singly or as double liners in disposal sites. Though the clay liners possess many advantages such as low permeability and large attenuative capacity, they also possess high shrinkage potential and hence can crack under unsaturated conditions causing instability and increase in leakage rates. Further, the permeability of the clay linear can increase due to clay–pollutant interaction. This study examines the potential of pozzolanic fly ash as a hydraulic barrier in land fill. The behaviour of three different types of fly ashes, showing a range of physical properties and chemical composition from three different sources are reported in the study. Geotechnical properties, needed to evaluate the use of fly ashes as barriers, such as shrinkage, compaction, permeability, consolidation and strength characteristics are reported. The results show that fly ashes possess low shrinkage and hence do not crack. Compacted fly ashes undergo very little volume changes. They also show that pozzolanic fly ashes develop good strength properties with time. Pozzolanic fly ashes containing sufficient lime develop strength even without addition of lime. Non-pozzolanic fly ashes do not develop strength even on addition of lime. Fly ashes generally consist of silt size particles and consequently possess high permeability. However, pozzolanic fly ashes with lime exhibit low permeability on curing because of the formation of gelatinous compounds which block the pores. Thus, pozzolanic fly ashes appear to be promising for construction of liners to contain alkaline leachate.

Analytical Equations for Predicting Concentration and Mass Flux from Composite Liners

Composite liners, consisting of a geomembrane overlying a soil liner, are often required as bottom liners for waste containment systems. In many applications, regulations prescribe the type of liner that must be installed. However, regulations often have provisions that permit an alternative liner design, provided that the alternative liner is equivalent to, or as effective as, the prescriptive liner. The current approach for demonstrating equivalency is based on comparing leakage rates for the alternative and prescriptive liners. This approach is too simplistic because leakage rate is not always indicative of the mass of contaminants discharged from the liner. In this paper, practical equations are presented for predicting the discharge of contaminants from composite liners and for designing alternative liners. The equations can be implemented using hand-held calculators or computer spreadsheet applications. Some of these equations are shown to provide predictions of mass flux and contaminant breakthrough that are similar to predictions made with more complex one- and three-dimensional numerical models for composite liners having perfect contact, circular defects, a constant source concentration, and relatively simple boundary conditions.

A Probabilistic Method for Estimating Monitoring Point Density for Containment System Leak Detection

AbstractThe use of physical and hydraulic containment systems for the isolation of contaminated ground water and aquifer materials associated with hazardous waste sites has increased during the last decade. The existing methodologies for monitoring and evaluating leakage from hazardous waste containment systems rely primarily on limited hydraulic head data. The number of hydraulic head monitoring points available at most sites employing physical containment systems may be insufficient to identify significant leaks from the systems. A probabilistic approach for evaluating the performance of containment systems, based on estimations of apparent leakage rates, is used to introduce a methodology for determining the minimum number of monitoring points necessary to identify the hydraulic signature of leakage from a containment system. The probabilistic method is based on the principles of geometric probability. The method is demonstrated using three‐dimensional ground water flow modeling results of leakage through a vertical barrier. The results indicate that the monitoring point spacing used at many hazardous waste sites likely is inadequate to detect the hydraulic signatures of all but the largest leaks.

Index Properties of Illite-Bentonite Mixtures in Electrolyte Solutions

Abstract Bentonite, commonly used for liner constructions in waste containment systems, possesses many limitations. Illite or illite containing bentonite has been proposed as an alternative material for liner construction. Their properties in different types of pore fluids are important to assess the long-term performance of the liner. Further, the illite-bentonite interaction occurs and changes their properties. The effect of these interactions is known when the pore fluid is only water. How their properties are modified in electrolyte solutions has been brought out in this paper. The index properties have been studied since they give an indication of their engineering properties. Due to reduction in the thickness of the diffused double layer and consequent particle aggregation in bentonite, the effect of clay-clay interaction reduces in electrolyte solutions. In electrolyte solutions, the liquid limit, the plasticity index, and free swell index of bentonite are lower than illite. The plasticity index of bentonite is further reduced in KCl solution. Clays with a higher plasticity index perform better to retain pollutants and reduce permeability. Hence, the presence of both illite and bentonite ensures better performance of the liner in different fluids.