Strength and Deformation Behavior of Municipal Solid Waste (MSW) Based on Constitutive Modeling Approach

Abstract

The geotechnical properties of municipal solid waste (MSW) such as compressibility, shear strength and stiffness are of prime importance in design and construction of landfills. However, it is not well clear how the stress-strain and strength characteristics vary with time as the biodegradation of waste continues in the landfill. There is also a need to address the variability of MSW properties and their role in landfill design. The present thesis proposes models for the analyses of stress-strain response of MSW in undrained and drained conditions. The proposed models are based on critical state soil mechanics concept and the modified cam clay model is extended to consider the effects of creep and biodegradation. The models are examined with reference to experimental data and published results of MSW in the form of stress strain response. In addition, the experimental results and the data from published literature are also compared with predictions from hyperbolic model. The proposed models are able to capture the stress strain response of MSW in undrained and drained condition adequately. The applicability of proposed model is presented in terms of shear strength ratio, stiffness ratio and settlement for typical landfill cases. In order to examine the influence of model parameters on shear strength, stiffness ratio and settlement, multilinear regression equations are developed based on response surface method (RSM) for different coefficients of variation (COVs). The effect of variability associated with model parameters is examined using reliability analysis. For better understanding, the present thesis is divided into following seven chapters. Chapter 1 is an introductory chapter, in which the need for use of the constitutive models and its use in engineering response analysis of MSW is presented. Further, the organization of thesis is also presented. Chapter 2 presents various studies with regard to the engineering properties of MSW available in the literature. Different models and approaches proposed by various researchers for the prediction of stress-strain response, time dependent behavior and settlement analysis of the MSW are presented. The uncertainty associated with engineering properties and available methods for reliability analysis and the use of response surface method are presented. Finally, based on the literature review, the scope of the thesis and summary of chapter are presented at the end. Chapter 3 presents composition of MSW, detailed description of the sample preparation, methods adopted in the experimental program and test results of one dimensional compression and consolidated undrained tests. Based on the experimental observations, a constitutive model for municipal solid waste for undrained condition in the framework of modified cam clay model considering mechanical creep and biodegradation mechanisms is proposed. It also provides detailed description of the selection of the input parameters required for the proposed model. Further, the detailed derivation of proposed model and the discussion on evaluation of the input model parameters from triaxial and consolidation tests are presented. The model is examined with reference to the experimental data and published results. The stress strain behavior of MSW is compared with the prediction of stress strain response from hyperbolic model. The comparison of stress strain response is well captured using proposed model for all levels of strain. The major conclusions from the study are presented at the end. Chapter 4 presents experimental results of consolidated drained tests. A constitutive model for MSW for drained condition in the framework of modified cam clay model considering mechanical creep and biodegradation mechanisms is proposed. The model is examined with reference to the experimental data and data from published literature. In addition, stress strain behavior of MSW is compared with the predictions from hyperbolic model. The comparison of stress strain response is well captured using proposed model for all levels of strain. The major conclusions from the study are presented at the end. Chapter 5 presents the applicability of proposed models in terms of shear strength ratio and stiffness ratio for a typical landfill condition. Based on response surface method (RSM), multilinear response surface equations are developed for different variables ( M, λ.b.c.d.Edg ) for different percentages of strain for 10 and 20% COVs of the model parameters. The effect of variability of model parameters is presented in terms of results of reliability analysis for specified performance functions. The major conclusions from the study are presented at the end. Chapter 6 presents an approach for the settlement evaluation of MSW for a typical landfill case of 30 m high. Based on RSM, multilinear response surface equations are developed for the calculation of MSW settlement for 30 years for 10 and 20% COVs. The effect of variability of model parameters is evaluated in term of reliability index for performance function specified in terms of landfill capacity. The major conclusions from the study are presented at the end. Chapter 7 presents a brief summary and conclusions from the various studies reported in the present thesis.