Date of Award

9-2009

Degree Name

MS in Civil and Environmental Engineering

Department

Civil and Environmental Engineering

Advisor

Jim Hanson

Abstract

An investigation of the variability of engineering properties of municipal solid waste as a function of placement conditions was conducted. Limited data have been reported for the engineering properties of municipal solid waste (MSW) as a function of placement conditions. Wastes have high variability of engineering properties due to heterogeneity in composition and component size; influence from time based effects; and presence of compressible solids. Control of moisture content of MSW at the time of waste placement provides opportunity for increased capacity at a given landfill site due to higher compacted unit weight as well as for control of other geotechnical properties. A laboratory experimental test program was conducted on manufactured municipal solid waste (MMSW) that was representative of waste stream in the United States. Large scale test equipment was used to minimize the effects of scaling on results. The experimental program included compaction, compressibility, hydraulic conductivity, and shear strength testing over moisture contents ranging from 11% to 110%. Baseline compaction curves were developed for different compactive efforts. Similar to soils, the MMSW had bell shaped compaction curves that peaked at a maximum dry unit weight and associated optimum moisture content. The compaction curve generated at modified compactive effort had a maximum dry unit weight of 5.1 kN/m3 and optimum moisture content of 66%. Four times modified compactive effort testing resulted in a maximum dry unit weight of 5.9 kN/m3 and corresponding optimum moisture content of 56%. The compaction curve generated for four times modified compactive effort was used as a baseline for subsequent testing. Compression index was calculated from the strain-log stress curves for total stress conditions and is referred to as apparent compression index. Apparent compression index decreased from 1.1 to 0.34 with increasing moisture content. Secant modulus of elasticity was calculated between 1% and 25% strain and ranged from approximately 200 kPa to 4,800 kPa over the range of tested moisture contents. Tangent modulus ranged from 400 kPa to 6,200 kPa between 1% and 25% strain. Both the secant and tangent modulus peaked between 30% and 56% moisture content. Wet of optimum, the moduli of elasticity decreased with increasing moisture content. The hydraulic conductivity was measured under constant head at a hydraulic gradient of 1 and decreased asymptotically from approximately 1.3x10-2 cm/s to 8x10-5 cm/s as the moisture content was increased to optimum. The hydraulic conductivity of the MMSW increased slightly wet of optimum. The internal angle of friction of the MMSW was measured at 15% shear strain and decreased from approximately 40° to 30° with increasing moisture content. Test results demonstrated that both the molding moisture content and dry unit weight have significant impact on the MMSW geotechnical properties, although it appears that molding moisture content ultimately controls the behavior. Based on the results of the tests it was speculated that, similar to clay soils, increases in moisture content allowed for breakdown of the fabric and physical rearrangement of waste components which in turn controlled geotechnical behavior. Overall trends were comparable for MMSW and soil and included: increased dry density and increased stiffness to optimum moisture content; decreased hydraulic conductivity with increased compaction moisture content; and decreased shear strength with increased compaction moisture content. The results of the test program have environmental and economic implications for design and operation of landfills as well as post closure use.