Development of a technical management tool for settlement and stability behaviour of municipal solid waste landfills
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Landfilling is a common method of disposal of municipal solid waste (MSW). Landfills are engineered structures consisting of bottom liners, leachate collection and removal systems, and final covers. With an expanding world and the remarkable growth of industry, the demand for larger and higher capacity landfills is rapidly increasing. As available space becomes scarce in urban areas, development on the top of or adjacent to old landfills has become increasingly common. Redeveloping on or adjacent to a closed landfill can be challenging and complex. In this regard, it is necessary to review a wide variety of engineering and geotechnical considerations for landfill redevelopment including settlement, foundation systems, gas and leachate management, and utility considerations, etc. Among various factors, which would be considered in redeveloping landfill sites, settlement is the most important factor from the perspective of structural and geotechnical stability. Prediction of the long-term settlement of MSW for the final cover design as well as end-use facility design such as recreational facilities, industrial/commercial facilities is necessary. In addition, estimation of settlement is needed to assess the stability of leachate and gas collection pipes, drainage systems, landfill storage capacity, and the overall landfill operating costs. Excessive settlement may cause fracture in the cover system and may also cause damage to the leachate and gas collection pipes. Many researchers studied the compression response of MSW and proposed different approaches to predict immediate settlement and time dependent settlement under load. Today, there are many published landfill settlement models. The research has been developed from simple soil mechanics based model to the constitutive model of waste. The stages of waste, which primarily only consider the primary and secondary settlements are improved with the development of constitutive model for long term settlement predictions. In this study, the critical and comprehensive literature review about the landfill settlement and compressibility is provided. It is concluded that current methods of settlement prediction have serious shortcomings in accounting for the organic (biodegradable) portion of waste streams and the many factors that control its decomposition; they are also unable to account for changing landfill conditions such as rates of filling or changes in waste type that have major effects on settlement rates and magnitude. The existing methods are therefore difficult to use in a predictive manner and require recalibration for changing waste streams. In this study, a technical management tool for MSW closed landfills has been developed using MATLAB graphical user interface, which aims to understand the process of long term settlement in landfills considering various related parameters, whilst it calculates different properties of wastes, and determines the landfill slope stability under various conditions. Furthermore, to illustrate the role of these parameters in settlement, a detailed parametric study considering variations of different parameters have been conducted in term of the settlement change over time as affected by influencing parameters. This parametric study showed that the variation of parameters can lead to significantly different settlements in landfills. Therefore, it is necessary to carefully select the parameter values for accurate prediction of landfill settlements. Moreover, in order to increase the understanding of the landfill behaviour and to quantify the significance of different parameters, a sensitivity analysis has also been performed to study the sensitivity of the models to variation of input parameters such as the unit weight, the landfill height, the waste properties, and the factors, affecting the biodegradation process of landfills. The results of this sensitivity analysis indicate that there are two prominent characteristics, having significant impacts on the overall landfill settlement. These characteristics are the landfill height and the compressibility parameters, while two other parameters including the gas diffusion coefficient and the van Genuchten parameter have trivial effects when compared to their relevant normal operating point (NOP). However, some other parameters have different degree of impact on the landfill settlement. In this research, a numerical study has also been carried out by applying PLAXIS 2D for prediction of slope stability of landfills. Additionally, a detailed parametric study is conducted to investigate the influence of the slope geometry on the safety factor (SF) considering the variation of waste geotechnical properties, the landfill height, and the waste compaction conditions, in order to develop a set of design charts in terms of variations of the SF with various heights, the slope inclinations, the effective cohesions of the waste, and the effective friction angles. Finally, the field data related to Tehran landfill has been collected for the validation of this landfill technical management tool (LTMT). It has been found that LTMT and its adopted model can be effectively used for MSW landfill settlement estimation. It should be noted that using this technical management tool, the effect of different parameters and conditions on settlement can be investigated. It is expected that the parametric study, which has been carried out in this research, can be applied in landfill redevelopment projects to predict the long term behaviour accurately, resulting in reduced construction and maintenance costs. This dissertation concludes that the most important and influential parameters in landfill redevelopments and prediction of MSW landfill settlement incorporating gas generation and leachate production are the landfill height, the compression ratio, the lift thickness, the waste composition, and the age of MSW.
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