Compressibility and shear strength of municipal solid waste under short-term leachate recirculation operations

Abstract

This paper describes a comprehensive laboratory study performed to investigate the compressibility and shear strength properties of 1.5-year-old municipal solid waste (MSW) exhumed from a landfill cell where low amounts of leachate were recirculated. The study results are compared with results from a previous study on fresh MSW collected from the same landfill and data from previous studies with known MSW age to assess the variation in properties due to degradation. Laboratory testing was conducted on shredded landfilled and fresh MSW that consisted of similar particle-size distribution, with maximum particle size less than 40 mm and approximately 80% of the waste consisting of particles ranging from 10 to 20 mm. Standard Proctor, compressibility, direct shear, and triaxial consolidated undrained (CU) shear tests were conducted in general accordance with the American Society of Testing and Materials Standard Procedures. These tests were conducted with samples at an in-situ moisture content of 44% (dry weight basis) as well as elevated moisture contents of 60, 80 and 100% (dry weight basis). Standard Proctor compaction tests yielded a maximum dry density of 600 kg/m³ at 77% optimum moisture content for landfilled MSW compared to the 420 kg/m³ maximum dry density at 70% optimum moisture content for fresh MSW. Compression ratio values for landfilled MSW varied in a close range of 0.19—0.24 with a slight increasing trend with increase in moisture content; however, for fresh waste they were in the close range of 0.24—0.33 with no definitive correlation with moisture content. Based on direct shear tests, drained cohesion and friction angle were varied in the range of 12—64 kPa and 31—35° for landfilled MSW and 31—64 kPa and 26—30° for fresh MSW. Neither cohesion nor friction angle demonstrated any correlation with the moisture content. Based on triaxial CU tests, the average total strength parameters (TSP) were found to be 39 kPa and 12° for landfilled MSW and 32 kPa and 12° for fresh MSW, while effective strength parameters (ESP) were 34 kPa and 23° for landfilled MSW and 32 kPa and 16° for fresh MSW. This study was limited to small-scale laboratory testing using MSW samples with the specimen size relative to the maximum particle size in the range of 1.6 to 2.6; therefore, large-scale laboratory and field studies are recommended to systematically assess the influence of composition, particle size distribution and specimen size on the geotechnical properties of MSW.

Keywords

Municipal solid waste (MSW)leachate recirculation bioreactor landfills moisture content biodegradation compressibility compaction characteristics shear strength friction angle cohesion

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