Variations of waste unit weight during mechanical and degradation processes at landfills

This manuscript develops a model for assessing the time and space variations in unit weight of traditional municipal solid waste (MSW) landfills. The model considers the variations of unit weight caused by deformation of the waste matrix and degradation of the organic portion. Deformation of the waste matrix includes both short-term effects, resulting from mechanical strain during the filling period, and long-term effects, resulting from superposition of waste skeleton creep and waste degradation. Mass loss, caused by waste degradation, not only affects the stress level within the waste column, but also induces large and long-term deformation. Degradation-induced deformation is caused by the local collapse of the solid matrix weakened by mass loss. Considering that the correlation between mass loss and waste deformation is locally erratic and hard to define, a smooth time-strain curve (represented by Kelvin viscoelastic model) is used to describe approximately the overall long-term deformation. The analytical formulation for unit weight is obtained in Laplace transform domain and can be used to simulate spatial and temporal variations of waste unit weight. Unit weight profiles obtained at four MSW landfills using the proposed model agree well with measurements from in situ large-scale unit weight tests. Evolution of unit weight profiles indicates that there is no monotonous varying trend for unit weight along the whole depth of the landfill. Density first increases and then decreases to a stable value in the lower portion, whereas the opposite occurs in the upper portion. Whether unit weight increases or decreases depends on the competition between matrix deformation and degradation processes.