Prediction of Municipal Solid Waste Landfill Settlement with Leachate Recirculation

The degradation of Municipal Solid Waste (MSW) over time, and associated settlement, is of special importance when estimating air space, designing interim and final closure covers of landfills, as well as planning vertical expansion of existing facilities. Most settlement models applied to solid waste were developed for inorganic soils or peat. Furthermore, these models were not developed considering the leachate recirculation effect on settlement. In case of bioreactor landfills, waste settlement will include creep as well as biological components due to the accelerated degradability of waste particles as leachate is recycled. The objective of this paper is to present a settlement prediction model, which accounts for changes in material characteristics as a function of the waste degradation rate. As biodegradation takes place, the organic solid mass is converted to gas and the void ratio increases with a subsequent increase in waste settlement. In general, published settlement models do not capture these phenomena. The model developed herein is based on the results of an experimental program. Settlement components including creep and biodegradation effects are identified as a function of the state of decomposition. Twenty four one-dimensional oedometer tests (63.5 mm cell) were performed on shredded refuse in laboratory-scale reactors for measurement of compression indices representing primary (C_c), creep (C_αi) and biological (C_βi) on samples ranging from fresh to well-decomposed refuse. Particles of length equal to no more than half the diameter of the test cell were included. The time factors, t_i, t₂, t₃ and t₄ for the compressibility were determined from the gas production curve and existing field settlement data, and utilized for model development. The proposed model was verified using the settlement model parameters obtained from laboratory test and comparing predicted settlement with observed field settlement from several landfills.