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An evaluation is presented on the effects of scale, stress, waste segregation, and waste decomposition on mechanical creep and biocompression of municipal solid waste. Laboratory experiments were conducted in 64-, 100-, and 305-mm-diameter compression cells. A field-scale experiment (Deer Track Bioreactor Experiment) was conducted on fresh waste of the same composition and material properties. The mechanical creep compression ratio () and biocompression ratio () were not affected by scale (i.e., specimen size). The mechanical creep ratio for fresh and degraded wastes and for fresh wastes were not affected by stress (i.e., similar and were obtained for a given waste at two creep stresses). Variation in can be related to the waste compressibility index, which is a function of waste composition, dry unit weight, and dry weight water content, or to the ratio of cellulose plus hemicellulose to lignin (). Larger is coincident with larger waste compressibility index and higher . The elapsed time for onset of biocompression () and the first-order decay rate () are shown to be scale dependent ( increases and decreases as experiment size increases). A dual-model approach is presented for predicting field-scale compression based on laboratory- and empirically-derived compression model parameters. [ABSTRACT FROM AUTHOR]

Abstract: A forensic investigation was conducted to determine the mechanism causing the slide of a final cover on a 4:1 (horizontal: vertical) slope at a municipal solid waste (MSW) landfill where leachate had been recirculated. The slide occurred as a veneer displacement along the interface between the geomembrane (GM) and geosynthetic clay liner (GCL). Site observations suggested that elevated gas pressures were a significant contributor to the failure. Laboratory tests were conducted to determine the shear strength parameters of the GM–GCL interface and the reduction in normal stress required to cause displacement of the GM–GCL interface. Hydraulic conductivity and gas permeability of the GCL were also determined. Slope stability analyses were conducted to determine the gas pressure required to cause a slide and the factor of safety that would have existed if the gas pressures was at zero gage. Good agreement was obtained between gas pressures measured in the field, measurements of the reduction in normal stress required to cause sliding on the GM–GCL interface in a large-scale direct shear test, and the gas pressures corresponding to FS = 1 (imminent sliding) from the slope stability analysis. The findings from this study, and a similar case history, illustrate the importance of managing gas at an acceptable level beneath the cover at MSW landfills. [Copyright &y& Elsevier]

The objective of this study was to evaluate the effects of waste composition and decomposition on the shear strength of municipal solid waste. Waste was collected from two sources (an operating landfill and a transfer station) and degraded in laboratory anaerobic reactors to prepare wastes with different degrees of decomposition. Shear strength was measured in a 280-mm-diameter direct shear ring on nine wastes with normal stress ranging between 12 and 90 kPa. The Mohr-Coulomb failure criterion was used to determine shear strength parameters ( = friction angle and = cohesion intercept) of the wastes, and shear strength was selected at a horizontal displacement of 56 mm (i.e., 20% of the specimen diameter). A composite failure envelope regressed through shear strength versus normal stress data from all wastes was statistically significant, with and . A comparison between tests conducted in this study and in the literature indicates that larger are obtained for waste with a greater fraction of soil-like, gravel, and inert constituents, whereas lower are coincident with higher fractions of paper and cardboard or plastic. This effect of waste composition on is applicable when fibrous particles are primarily parallel with the shear plane, which is the common particle orientation in direct shear. Tests conducted in this study also indicate increases with decreasing volatile solids or the ratio of cellulose + hemicellulose to lignin (i.e., increasing decomposition). Contrasting correlations have been reported in the literature, attributed to the initial waste composition, which influences the effect of decomposition on . No correspondence was found between and waste composition or the degree of waste decomposition. [ABSTRACT FROM AUTHOR]

An evaluation of scale effects, stress, waste segregation, and waste decomposition on the immediate compression behavior of municipal solid waste is presented. Laboratory experiments were conducted in 64-, 100-, and 305-mm-diameter compression cells. A field-scale experiment [Deer Track Bioreactor Experiment (DTBE)] was conducted on waste of the same composition and material properties. A methodology is presented for determining the end-of-immediate compression strain ( ɛEOI) that is applicable to both laboratory- and field-scale data. The compression ratio () was comparable between tests conducted in 100- and 305-mm compression cells. Compression tests in 305-mm cells conducted on six wastes (three size-differentiated fresh wastes and three decomposed wastes) yielded ranging from 0.22 to 0.28 in the stress range of 25-100 kPa. A similar (0.23) was determined for the DTBE (20-67 kPa). The variation in is related to the waste compressibility index (WCI), which is a function of waste dry weight water content, dry unit weight, and the percent contribution of biodegradable organic waste (paper/cardboard, food waste, yard waste). A compilation of laboratory data from this study and the literature yielded a predictive relationship for the and WCI. The can be estimated within ±0.087 for a given WCI using this relationship. [ABSTRACT FROM AUTHOR]

An assessment of state-of-the-practice at five full-scale North American landfills operating as bioreactors is presented in this two-paper set. This paper focuses on effectiveness of liners and leachate collection systems, leachate generation rates, leachate recirculation practices and rates, effectiveness in moistening the waste, and settlement of the waste over time. Except in one case, the liner and leachate collection systems at the bioreactor landfills were similar to those used for landfills operated conventionally. Leachate generation rates increased approximately linearly with recirculation rate, but in all cases, the leachate generation rate was <300 L/m2 year. Leachate depths generally were maintained within regulatory requirements, even with the highest recirculation rates. Leakage rates from liners at bioreactor landfills, including alternative liner designs employing geosynthetic clay liners, are comparable to leakage rates from conventional landfills. Thus, based on the information gathered in this study, additional requirements or features for liners or leachate collection systems are not warranted for bioreactor landfills. Diminishing capacity of horizontal recirculation trenches is common. Experience at one landfill suggests that small doses at high frequency under substantial injection pressure can deter loss of trench capacity. Only those landfills that were aggressive in recirculation had achieved water contents near the field capacity. Increasing the amount of liquid that is added may be required to achieve field capacity at some landfills, particularly if a final cover is placed soon after waste grades are reached. The rate of time-dependent waste settlement attributed to biodegradation is about 1.6 times larger in bioreactor landfills than in conventional landfills, and increases as the recirculation dosage increases. [ABSTRACT FROM AUTHOR]

Leachates from municipal solid waste (MSW) and hazardous waste landfills contain a wide range of volatile organic compounds (VOCs) in addition to inorganic compounds. VOCs have been shown to migrate and contaminate the surrounding environment and impair the use of groundwater. Therefore, the effectiveness of modern landfill liner systems to minimize migration of VOCs is of concern. Most modern landfills employ a composite liner consisting of a geomembrane overlying a compacted clay liner or a geosynthetic clay liner. The geomembrane is often believed to be the primary barrier to contaminant transport. However, for VOCs, the clay component usually controls the rate of transport since VOCs are shown to diffuse through geomembrane at appreciable rates. Additionally, analyses have shown that transport of volatile organic compounds (VOCs) generally is more critical than transport of inorganic compounds (e.g., toxic heavy metals), even though VOCs are often found at lower concentrations in leachates. Therefore, the effectiveness of modern landfill liner systems to minimize migration of VOCs and transport of VOCs through clay liners and modeling of transport through composite liners merit scrutiny. This paper presents a review of recent research by the author and others on these topics. A systematic and comprehensive approach to determine mass transport parameters for transport of VOCs in liquid phase through compacted clay liners, geosynthetic clay liners (GCLs), and geomembranes has enabled to develop realistic models to predict mass flux of VOCs through modern composite liners and provide a quantitative basis to evaluate potential for transport of dissolved VOCs and the equivalency of different composite liners. [Copyright &y& Elsevier]

Presents information on a study which analyzed leakage through composite landfill liners that have various characteristics. Review of related literature; Methodology; Results and discussion.

Abstract: Partition and diffusion coefficients for volatile organic compounds (VOCs) and geomembranes obtained using batch and double compartment (DC) tests are compared. Tests were conducted with a high-density polyethylene (HDPE) geomembrane and solutions containing five common VOCs ranging in hydrophobicity. Batch tests were conducted in equilibrium and kinetic modes using solutions with single or multiple solutes. The findings indicate that competitive sorption between solutes is negligible for the concentrations examined, except for the most hydrophobic compounds, and that partition coefficients obtained from equilibrium and kinetic batch tests cannot be distinguished statistically. DC tests yield larger partition coefficients, and smaller diffusion coefficients, due to losses in the DC apparatus. Kinetic batch tests are recommended to simultaneously obtain partition and diffusion coefficients for VOCs and geomembranes. The kinetic batch test method is relatively simple and expedient, the partition and diffusion coefficients are reliable, and the data can be analyzed using an analytical solution programmed in a spreadsheet application. Empirical relationships are recommended to estimate the partition coefficient, diffusion coefficient, and permeation coefficient for VOCs and HDPE geomembranes using common chemical properties. [Copyright &y& Elsevier]