Shear strength parameters of municipal solid waste with leachate recirculation

The objective of this study was to characterize relative changes in waste shear strength parameters during waste decomposition. Twelve direct shear tests (100 mm diameter by 50 mm thickness) were performed on waste specimens ranging from fresh to well-decomposed residential refuse. In addition, nine direct shear tests were performed on selected waste components including fresh paper, partially decomposed refuse, and plastics. Results indicate that the friction angle of refuse decreased with decomposition. As refuse decomposed, the plastic content increased, which contributed to a decrease in friction angle as the friction angle of plastics is 18-19 degrees as compared to 33 degrees for fresh shredded waste. The extent of refuse decomposition was characterized by the cellulose plus hemicellulose to lignin ratio [(C+H)/L]. The measured friction angle decreased from 32 to 24 degrees as (C+H)/L decreased from 1.29 to 0.25. The shearing pattern for decomposed refuse showed a peak, followed by residual, which was then followed by a steady increase in shear stresses with displacement; the final rate of increase was similar to that observed in fresh paper specimens. Results from this work were comparable to data from previous reports, though it is important to characterize the extent of solids decomposition for a valid comparison with published studies.