Dynamic characteristics of municipal solid waste (MSW) in the linear and nonlinear strain ranges

Abstract

A series of resonant column and torsional shear (RCTS) and large scale resonant column (LSRC) tests were performed to investigate the dynamic properties (shear modulus and material damping ratio) of municipal solid waste (MSW). the MSW materials were recovered from the Tri-Cities landfill adjacent to the San Francisco Bay in California. A total of 30 specimens 2.8-in. (71.1-mm) and 6.0-in. (152.4-mm) of old, fresh, and mixed MSW were reconstituted in accordance with established sample preparation procedures. Ten of specimens were small-diameter (2.8-in. (71.1-mm)) RCTS specimen and 20 specimens were larger (6.0-in. (152.4-mm)) LSRC specimens. Dynamic laboratory measurements were performed in the linear and nonlinear strain ranges. Test parameters affecting the dynamic properties in the linear range included: (1) duration of confinement, (2) isotropic total confining pressure, [sigma]o, (3) excitation frequency, f, and (4) specimen size. Other test parameters affecting dynamic properties in the nonlinear strain range were: (1) shearing strain amplitude, [gamma], (2) isotropic total confining pressure, (3) overconsolidation ratio, (4) number of loading cycles, and (5) excitation frequency. In addition, the effects on dynamic properties of MSW specimens of material parameters such as (1) waste composition, (2) water content, (3) unit weight of waste, and (4) particle size were evaluated. The total unit weights of old, fresh, and mixed MSW specimens were estimated during testing in the RCTS and LSRC devices. These estimated total unit weights in the laboratory were compared with those measured at other MSW landfills and were found to generally be less than the field measurements. At a given [sigma]o, Gmax decreases with decreasing weight percentage of soil-size (passing the 3/4-in. (19.1-mm) sieve) material. However, Dmin increases slightly with decreasing weight percentage of soil-size material. Another relationship was developed between estimated total unit weight, [gamma]t, and confining pressure, including weigh percentage of soil-size material. The Vs profiles of old, fresh, and mixed MSW specimens obtained in the laboratory tests were compared with those measured at other MSW landfills in situ. The 62 to 76% soil-size material groups are in good agreement with in-situ Vs profiles. The variation in normalized shear modulus and material damping ratio curves were patterned after the Darendeli model (2001) for different weight percentages of soilsize material. An empirical relationship between normalized shear modulus (G/Gmax) and modified material damping ratio (D-Dmin) was developed in the nonlinear strain range. As part of collaborative research project, nonlinear shear modulus reduction and material damping curves generated by The University of Texas at Austin (UT) and The University of California at Berkeley (UCB) were compared according to different weight percentages of soil-size material. Furthermore, nonlinear shear modulus reduction and material damping ratio curves generated by UT were also compared with ones previously proposed by other researchers.