Compression of Municipal Solid Waste in Bioreactor Landfills: Mechanical Creep and Biocompression

An evaluation is presented on the effects of scale, stress, waste segregation, and waste decomposition on mechanical creep and biocompression of municipal solid waste. Laboratory experiments were conducted in 64-, 100-, and 305-mm-diameter compression cells. A field-scale experiment (Deer Track Bioreactor Experiment) was conducted on fresh waste of the same composition and material properties. The mechanical creep compression ratio ($C_{\alpha M}^{\prime }$) and biocompression ratio ($C_{\alpha B}^{\prime }$) were not affected by scale (i.e., specimen size). The mechanical creep ratio for fresh and degraded wastes and $C_{\alpha B}^{\prime }$ for fresh wastes were not affected by stress (i.e., similar $C_{\alpha M}^{\prime }$ and $C_{\alpha B}^{\prime }$ were obtained for a given waste at two creep stresses). Variation in $C_{\alpha M}^{\prime }$ can be related to the waste compressibility index, which is a function of waste composition, dry unit weight, and dry weight water content, or to the ratio of cellulose plus hemicellulose to lignin ($[\text{C}+\text{H}]/\text{L}$). Larger $C_{\alpha M}^{\prime }$ is coincident with larger waste compressibility index and higher $[\text{C}+\text{H}]/\text{L}$. The elapsed time for onset of biocompression ($t_B$) and the first-order decay rate ($k$) are shown to be scale dependent ($t_B$ increases and $k$ decreases as experiment size increases). A dual-model approach is presented for predicting field-scale compression based on laboratory- and empirically-derived compression model parameters.