80 research outputs found
Leachate Pressure Effect on a System Reliability-Based Design of Reinforced Soil Walls for a Vertical Expansion of MSW Landfills
The issue of vertical capacity expansion of municipal solid waste (MSW) landfills with reinforced soil walls (RSWs) is addressed in the present investigation. The influence of different conditions of leachate levels in MSW landfills is a major cause of translational failures. Poor hydraulic conductivity, clogging of drainage because of fines, and freezing of drainage in landfills are the crucial factors of the buildup of leachate pressure. Heterogeneity and different fill ages in MSW landfills gradually change the inherent properties of landfills. The assumption-independent component failures of sliding, eccentricity, bearing capacity, tension, and pullout modes in predicting the series system reliability index of RSWs against translational failure under six leachate level conditions may produce large errors because component failures are usually dependent on one another. Therefore, the present paper demonstrates the feasibility of considering dependent failure modes to estimate the dimensions of RSWs to maintain the external and internal stability under six different leachate buildup conditions. The variability associated with the cohesion of solid waste, the apparent cohesion between liner components beneath the wedges, the friction angle of MSW, and the interface friction angle beneath the wedges is considered for the estimation of the lower bound of the series system reliability index. The limit equilibrium method is employed to assess the stability of an RSW for an expanded MSW landfill. Further, the design charts for the optimum values of width and height of the RSW are provided for different leachate levels (hw) under six leachate buildup conditions by targeting various lower bounds of a system reliability index ≥3.0. The design values of the number of reinforcement layers (n) are also provided corresponding to the optimum dimensions of the RSW subjected to different leachate levels
Reliability Analysis of Soil Slopes
Slope stability analysis is a classical problem of geotechnical engineering characterized by many sources of uncertainty. Some of these sources are connected to the uncertainties of soil properties involved in the analysis. Current practice of slope stability analysis relies in the deterministic characterization and assessment of performance of embankments, excavations and Municipal Solid Waste (MSW) landfills. These slopes have been evaluated in terms of the factor of safety, where the shear strength mobilized along the failure envelop is compared with the shear stresses generated due to self-weight of the soil mass and surcharge loading on the slope. The significant uncertainties associated with the shear strength and shear stresses render deterministic modeling potentially misleading. For example, two slopes with the same factor of safety can have significantly different probabilities of failure
Experimental and statistical evaluation of compressibility of fresh and landfilled municipal solid waste under elevated moisture contents
An investigation of the variability associated with primary and secondary compression indices of municipal solid waste (MSW) is conducted in the present study. A controlled laboratory experimental program was conducted to quantify the compressibility of fresh MSW and landfilled MSW (subjected to leachate recirculation for a year in the field) under different elevated moisture content conditions. Several series of one-dimensional compressibility experiments were conducted on fresh and landfilled waste samples under the field moisture content of 44% (by dry weight) and three elevated moisture contents of 60, 80 and 100% (by dry weight). The compression of waste samples was measured at different elapsed time periods under incremental normal stresses of 48, 96, 192, 383, and 766 kPa. The modified compression indices (or compression ratios) were calculated based on the measured compression versus stress data. Long term secondary compression behavior was determined by performing long term compression tests on fresh and landfilled waste samples under normal stress of 383 kPa. The steep slope is not evident on the vertical stress - strain plot for the 44% moisture content sample, potentially owing to breakdown of micro-fabric and mini-fabric of fresh waste and rearrangement of the particles. It is observed from the present study that the magnitudes of modified primary compression index (C'(c)) for fresh MSW exhibited no specific correlation with an increase in moisture content from 44 to 100% owing to variations in the initial composition of fresh MSW, small scale laboratory testing, and rate of biodegradation of MSW. For a constant vertical stress, the landfilled waste compressed less than the fresh waste at initial and elevated sample moisture contents owing to reduction of organic content in the degraded waste. Based on the compilation of compression indices from several published studies, the average values of mean, standard deviation and coefficient of variation for modified primary compression and secondary compression indices are computed. Overall, this study demonstrated that the long term compression characteristics can highly vary depending on the waste composition, moisture content, and biodegradation. From the statistical analysis, it is determined that the variability associated with secondary compression index (C'(alpha)) is significantly higher than the primary compression index (C'(c)), which may be attributed to significant differences in the biodegradable content of waste and associated extent and rate of biodegradation of waste
Resistance Factor Calculations for Load Resistance Factor Design (LRFD) of MSW Landfill Slopes
It is obvious to note that there is a significant amount of variability connected with shear parameters of municipal solid waste (MSW) landfills. To ensure uniform safety and reliability, the design approaches in the US have progressively transformed to the load and resistance factor design (LRFD) format. It may be desirable to the successful development and adoption of reliability-based resistance factors for the design of landfill slopes taking into account the significant variability of shear strength parameters. The exhaustive studies reported on shear parameters of MSW are compiled and reviewed. The mean, standard deviation, and coefficient of variation (COV) associated with shear parameters are obtained using statistical analysis. The probability density functions (PDFs) are plotted for unit weight, cohesion, and friction angle. The PDFs show that high range of variability associated with shear parameters and should be given due consideration in the optimum designs. Therefore, the present work reports a procedure for determining the resistance factors for stability number (in terms of unit weight, cohesion) and friction angle of MSW in accordance with LRFD of MSW landfill slopes that target a specific reliability index. A simple first-order reliability method (FORM) is reported to compute the ranges for the resistance factors. Perhaps, this is the first study to propose resistance factors for the design of MSW slopes. The stability number (in terms of unit weight, cohesion) and friction angle of the MSW are treated as random variables. The Spencer method of slices has been employed to formulate the performance function against the sliding failure of finite slopes. It is illustrated that the uniform safety levels can be obtained by using the proposed resistance factors
Effect of Fiber Reinforcement on CBR Behavior of Lime Blended Expansive Soils: Reliability Approach
Use of synthetic fibers as reinforcement to stabilize expansive soils is gaining momentum. As a contribution towards this growing field of research two different types of synthetic fibers, Fiber Mesh® and Fiber Cast®, were evaluated as a stabilization alternative for expansive soils in the presence of lime. California Bearing Ratio (CBR) is chosen as a performance indicator as it is a good pointer towards pavement effectiveness. Variables such as length and amount of the fibers as well as curing period were studied. Both deterministic and probabilistic (or reliability) analysis is presented in this paper. While the deterministic analysis helps in understanding the measured experimental data, the probabilistic approach accounts for the stochastic nature of the experimental data and provides a better rationale for the design methods. The deterministic approach showed that the improvement in CBR increased with higher fiber contents and longer lengths and the effect was prominent when lime was used as a stabilizer. There were some exceptions to this behavior, which were noted in the paper. The probabilistic analysis showed that the amount and lengths of fibers were important factors in CBR strength. It was also determined that the variation in the target CBR value had considerable effect on optimizing the length and amount of the fibers
Computation of the Probabilistic Critical Centers and Reliability Indices of MSW Landfill Slopes Using the Spencer Method of Slices
The shear strength properties of Municipal Solid Waste (MSW) are of special importance when evaluating the stability of landfill slopes. Geoenvironmental engineers are well aware of the existence of many sources of uncertainties associated with shear strength parameters of MSW due to various reasons. The significant uncertainties associated with the shear strength and shear stresses render deterministic modeling potentially misleading. The traditional engineering approaches like method of slices used for evaluating MSW slopes are frequently questionable as they do not adequately account for uncertainties included in analytical modeling and natural variability. In order to quantify the slope stability precisely by taking into account the variability, the Reliability Based Design Optimization (RBDO) framework is presented. The mean and standard deviations associated with unit weight, cohesion and angle of internal friction of the MSW are taken into account in the probabilistic optimization. Reliability analysis is performed using first order reliability method (FORM). A limit state function is formulated against sliding slope failure using Spencer method of slices. The influence of coefficients of variation (COV) of stability number and friction angle on critical center coordinates and reliability index is presented in the form of charts
Coupled Effect of Granite Sand and Calcium Lignosulphonate on the Strength Behavior of Cohesive Soil
This paper assesses the significance of stabilizing clay soil with calcium lignosulphonate (CLS) and granite sand (GS). Unconfined compressive strength ( (Formula presented.) ) and hydraulic conductivity (K) are taken as performance indicators and the effect of varying dosages of GS (30%, 40%, and 50%) and CLS (0.5%, 1%, 1.5%, and 2%) at different curing periods on (Formula presented.) and K are examined. The best fit regression equations have been proposed to relate (Formula presented.) and K of untreated clay soil and stabilized clay using GS and CLS. The proposed nonlinear regression equations provide details of experimental data and aid in estimating (Formula presented.) and K very efficiently and reliably for targeted geotechnical applications from a sustainable perspective. © 2022 by the authors
Target Reliability Approach to Study the Effect of Fiber Reinforcement on UCS Behavior of Lime Treated Semiarid Soil
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