Effects of Waste Composition and Decomposition on the Shear Strength of Municipal Solid Waste

Abstract

The objective of this study was to evaluate the effects of waste composition and decomposition on the shear strength of municipal solid waste. Waste was collected from two sources (an operating landfill and a transfer station) and degraded in laboratory anaerobic reactors to prepare wastes with different degrees of decomposition. Shear strength was measured in a 280-mm-diameter direct shear ring on nine wastes with normal stress ranging between 12 and 90 kPa. The Mohr-Coulomb failure criterion was used to determine shear strength parameters (ϕ = friction angle and c = cohesion intercept) of the wastes, and shear strength was selected at a horizontal displacement of 56 mm (i.e., 20% of the specimen diameter). A composite failure envelope regressed through shear strength versus normal stress data from all wastes was statistically significant, with ϕ=37° and c=20 kPa. A comparison between tests conducted in this study and in the literature indicates that larger ϕ are obtained for waste with a greater fraction of soil-like, gravel, and inert constituents, whereas lower ϕ are coincident with higher fractions of paper and cardboard or plastic. This effect of waste composition on ϕ is applicable when fibrous particles are primarily parallel with the shear plane, which is the common particle orientation in direct shear. Tests conducted in this study also indicate ϕ increases with decreasing volatile solids or the ratio of cellulose + hemicellulose to lignin (i.e., increasing decomposition). Contrasting correlations have been reported in the literature, attributed to the initial waste composition, which influences the effect of decomposition on ϕ. No correspondence was found between c and waste composition or the degree of waste decomposition.