Static and dynamic waste characterization

Mechanical properties of the Municipal solid waste ( MSW) materials control many aspe cts of landfill lining system design and performance, incl uding stability issues and integrity of the geosynt hetic components. Stability analyses require extensive kn owledge on the mechanical behavior of waste which c ould be provided by laboratory or in situ testing. Never th less, the physical and mechanical characterizati on of waste is a very complex task, given the testing dif ficulties and uncertainties adapting the theory and techniques used in soil mechanics. Particular attention must be devoted to MSW landfills in seismic areas, where the liners can be damaged by seismic action, as doc umented by recent earthquakes. In the paper the results of a geotechnical investig ation carried on MSW materials retrieved from the “ Cozzo Vuturo” landfill in the Enna area (Italy) are reported an d analyzed. Static and dynamic properties were dete rmined by in situ and laboratory tests. In particula r, Seismic Marchetti Dilatomer Tests ( SMDT) have been carried out for the seismic characterization and the t ests results are used to derive shear wave velocity profiles and the dynamic properties of the deposited waste m at rials. A comprehensive large-scale laboratory pr ogram was also performed to develop insights about the in terpretation of the MSW compressibility. the mechanical behavior of waste materials and factors influencing the deformation processes. Figure 1. “Cozzo Vuturo ” (Enna, Italy) landfill location. 2 GEOTECHNICAL WASTE PARAMETERS The physical and mechanical characterization of waste is a very complex task given the testing diff iculties and uncertainties adapting the theory and techniques used in soil mechanics (Grisolia et al., 1995; Jessberger and Kockel, 1993a). In fact, it is difficult to extrapolate the results obtained from ne situation to another, because of the differences in initial composition, in the type of pre-treatment c arried out and in the way in which the waste was dumped, etc. Waste materials differ widely in type, composition, consistency and state of biochemical decay. Typical examples for different waste naturals are e xcavated soil, construction material debris, sludges or municipal waste showing large variations in type an d properties of its constituents. Generally waste ty pes are divided into two groups: soil-like waste, defin d as “granular waste”, for which conventional soil me chanics principles are applicable, and “other waste ”. For fine-grained, soil-like waste mechanical properties compressibility, shrinkage and swellin g behavior, shear strengthare determined using conventional geotechnical test methods. For mixed and coarse-grained soil-like waste some modifications o f the testing equipment are necessary, i.e. compressi on tests on large scale samples. For other wastes, it is necessary to undertake in situ tests on trial areas . Recent studies (e.g. Kavazanjian et al., 1999; Zekkos et al., 2006; Zekkos et al., 2008; Bray et a l., 2009) have identified a variety of factors that can significantly affect the mechanical properties of MSW. These properties (strength and compressibility) are dependent on the individual composition of the waste material and on the mechanical properties of its constituents. In addition the mechanical parameter are time-dependent related to the state of d composition. Taking account of this effect, parameters for fresh and for decomposed waste must be distinguished. In some cases to provide applicable parameters for stability or deformation analysis, i t might be useful to conduct appropriate tests. To characterize the deformation or compression behavior of waste, parameters like stiffness modulu s or compression index can be used (Dixon et al., 2004). Comparable to the strength parameters, for mixed waste materials, compression parameters from literature also show large variations. The amount of settlement of waste materials is dependen t on the properties of the individual constituents an d on their composition. So the evaluation of compression parameters requires an extensive description and classification of the waste materials. An appropriate method to classify “other wastes” may be the conduction of tests like grain-size anal ysis, determination of the main waste constituents, moisture content, consistency limits, organic content, etc,. Generally the tendency of increasing fi egrained material with increasing age of waste can b e observed. This tendency probably can be explained by different states of decomposition. 3 DYNAMIC MUNICIPAL SOLID WASTE PROPERTIES It is important to stress that the dynamic characte istics of solid waste materials play an important rol e on the seismic response of landfill (Maugeri and Sêco e Pinto, 2005). Some properties are measured directly, such as dry density and water contents, whereas other properties, due the difficulties related with sampling, are obtained from indirect methods combining with the existent knowledge of waste properties. Both in situ (Matasovic et al., 2011) and laboratory tests (Athanasopoulos, 2011) can be performed to evaluate these properties. MSW properties that a re used as input in the seismic analyses of landfills include: − the unit weight of MSW; − the (dynamic) shear strength of MSW; − the shear wave velocity VS and small-strain shear modulus G0 profile; − the shear strain-dependent shear modulus reduction G/G0 and material damping ( λ) curves; and − the dynamic Poisson’s ratio ν. The unit weight of MSW can vary significantly at a landfill site and its variation with depth affect s significantly the seismic response of landfills. Zekko s et al. (2006), based on large-scale laboratory and in situ field data developed a hyperbolic model for th e unit weight of MSW. The unit weight was found to be affected by compaction effort and composition as well as confining stress and is described by the fo llowing equation: where γi is the near-surface in-place unit weight (kN/m), z is the depth (m) at which the MSW unit weight γ is to be estimated, and α and β are modeling parameters with units of m /kN and m/kN, respectively. Calibration of the model using laboratory and field test data, yielded values for γi, α and β that are a function of compaction effort and amount of soil an d are shown in Table 1. Table 1. Unit weight parameters for different compa ction effort and amount of soil cover (Zekkos et al., 2006). Compaction effort and soil amount i γ kN/m β