Large-scale Simple Shear Testing Of Municipal Solid Waste

With the development of social economy and the rapid urbanization in China, total municipal solid waste generation has increased greatly. As a result, the safe, environmental-friendly and efficient treatment and disposal of municipal solid waste (MSW) is a huge challenge for China government in the future. Landfills are the primary method of municipal solid waste disposal in many parts of the world, including United States and Canada, especially for China. For the next 20 years or more, there will be more and more landfills in China and old landfill will be expanded for meeting future needs. There are many landfill slope failures and function failures in the world in recent years because of a poor understanding of static and dynamic properties of municipal solid waste. In order to provide reasonable design parameters and evaluate of the stability and integrity of landfill and post-closure use, it's important and essential to research the static and dynamic properties of municipal solid. As part of a collaborative NSF-funded research program, a comprehensive laboratory investigation of the static and dynamic properties of municipal solid waste (MSW) were conducted on reconstituted specimens of MSW collected from the Tri-Cities landfill in Fremont, California, USA using a large-scale simple shear device of Arizona State University (ASU). Based on the information of in-situ drilling and investigation, MSW are first screened to separate the<20 mm soil-like material from the>20 mm fibrous material. Specimens at ASU were then compacted using 100%,65% and 35%<20 mm material, by weight and four different compaction styles in order to investigate the effects of composition and compaction on the static and dynamic properties of MSW.The compression tests results indicated that the initial unit weight is increasing with the increasing of the percentage of the<20 mm soil-like material and compaction energy. In contrast, the compressibility of MSW is decreasing with the increasing of the percentage of the<20 mm soil-like material and compaction energy. The coefficient of earth pressure at rest of MSW was measured using Tekscan pressure sensor. The results showed that the coefficient of earth pressure at rest of MSW is decreasing with the percentage of>20 mm fibrous material as a function of composition and stress history. The values of the coefficient of earth pressure at rest for 100%. 65%,35% three different solid waste groups are 0.48,0.35, and 0.23, respectively. Modified Mayne-Kulhawy empirical model could explain the testing results well.The shear wave velocity of MSW specimens were measured under different composition and compaction styles using self-designed Geophone system. The small strain shear modulus was calculated using shear wave velocity testing results. The research results indicated that both shear wave velocity and small strain shear modulus are increasing with the increasing of unit weight. The cyclic simple shear tests results showed that as the relative amount of>20mm fibrous material increases in the MSW, the absolute value of small strain shear modulus reduces, the normalized shear modulus curve shift to the right and the material damping ratio at large shear strains reduces significantly. With the increasing of cyclic shear strain, the material damping ratio also is increasing.The stress-strain response of monotonic shear showed that the peak shear stress conditions were not reached. Lower unit weights (lower compaction energy) result in softer response and lower shear strength of MSW for the same composition condition. The shear strength is increasing with the increasing of the percentage of the<20 mm soil-like material for the same compaction condition. The shear strength also is increasing with the increasing of vertical stress and rate of shearing. The tests of orientation effects of>20mm fibrous material indicated that the shear strength of MSW is significantly affected by the relative orientation of the fibers to the horizontal shear surface. The strongest response is observed for the specimen that includes fibers in angle of approximately 60 degrees to horizontal surface. Based on the results from this research and back analysis from numerical analysis using modified Cam-Clay model and FIAC6.0 program, the shear strength parameters is characterized by cohesion=9-14kPa. friction angle=36-54°.