Recent Findings on the Static and Dynamic Properties of Municipal Solid Waste

The design of Municipal Solid Waste (MSW) landfills requires the selection of representative MSW material properties. The profession’s understanding of the mechanical response of MSW has evolved significantly in recent years, but many aspects of the response of MSW remain uncertain. A research project involving five organizations was conducted to systematically evaluate the factors that affect the static and dynamic properties of MSW. A summary of major findings from the work performed to date by this team of investigators is briefly described in this paper. INTRODUCTION The selection of representative MSW properties is important for the reliable performance of landfill analyses such as the static and seismic slope stability, the seismic response of landfills, the design of the containment system, and the design of facilities for post-closure development. The profession’s understanding of the response of MSW has evolved significantly since the early work performed by Sowers (1973) and Landva and Clark (1986). However, despite many advances in our understanding of MSW properties, many of the factors affecting the response of MSW remain largely uncertain. In 2002, the US National Science Foundation funded a collaborative research project that involved the University of California at Berkeley (UCB), the University of Texas at Austin (UT), and Geosyntec Consultants to evaluate systematically factors that affect the static and dynamic properties of MSW by means of in situ investigations and an extensive large-scale laboratory testing program. Subsequently, Arizona State University (ASU) and the University of Patras, Greece GEOCONGRESS 2008: GEOTECHNICS OF WASTE MANAGEMENT AND REMEDIATION 177 (UP), joined the project team. The fieldwork and laboratory testing performed by this team of investigators are briefly described in this paper. Relevant findings are presented that provide insight regarding the mechanical response of MSW. FIELD INVESTIGATION AND WASTE CHARACTERIZATION Waste sampling was coordinated by Geosyntec Consultants. Two large-diameter (760 mm) borings were augered to depths of 10 m and 32 m using a bucket auger at the Tri-Cities landfill, located in the San Francisco Bay Area. Shallow and deep bulk samples of waste were retrieved and stored separately in 39 sealed 55-gallon drums. Bulk samples of similar visual composition from the same depth interval within a borehole were designated as sample groups. Sample groups included relatively young waste (waste placed within the past 2 years) and relatively old waste (waste placed for approximately 15 years) as well as waste of varying composition. In situ unit weight tests were performed using a gravel replacement procedure developed by Geosyntec Consultants (Matasovic and Kavazanjian, 1998) and described in Zekkos et al. (2006a). The MSW unit weight was found to range from 10 kN/m near the surface to 16 kN/m at depth. Shear wave velocity soundings were performed by UT at the boring locations using the Spectral Analyses of Surface Waves (SASW) method. The shear wave velocity was found to vary from 75 to 210 m/sec at the surface, reaching 250 m/sec at a depth of 25 m (Lin et al., 2004). Waste material was characterized using a procedure that was developed to efficiently collect relevant information about the waste material. The procedure is described in Zekkos (2005) and included segregating the waste into material larger and smaller than 20 mm (0.75 in). This segregation is useful because: a) material <20 mm is composed predominantly of equidimensional particles, including soil from daily cover, organic materials, and some fine waste inclusions, whereas material >20 mm consists mostly of fibrous constituents; b) material <20 mm can be characterized using conventional soil mechanics tests and can be tested using conventional size geotechnical testing equipment. Evaluation of the influence of the fibrous >20 mm material on MSW was an important part of this investigation. About 50-75% by weight of each waste sample was <20 mm material. This material contains a significant amount of soil (and soil-like) particles, but also contains organic material so that it is lighter, and softer than many inorganic soils. The remaining coarser material consisted primarily of paper, plastic, wood, and gravel. Constituents such as metals, glass, stiff plastics, and textiles, comprised a significantly lower percentage of the material by weight and by volume. Laboratory tests were performed on the three sample groups summarized in Table 1. Group A3 is older material sampled from a relatively large depth. Group C6 is younger material sampled from a relatively shallow depth. Group C3 was selected for testing as the most different sample group from the previously tested sample groups A3 and C6. An extensive large-scale and small-scale laboratory testing program on material from these groups was performed at the various institutions through a coordinated testing program. A brief summary of the laboratory testing performed on the waste and some of the primary findings are presented in this paper. 178 GEOCONGRESS 2008: GEOTECHNICS OF WASTE MANAGEMENT AND REMEDIATION Table 1. MSW sample groups tested Α3 C6 C3 Borehole BH-2 BH-1 BH-2 Depth, m 25.6-26.2 7.6-9.6 3.5-4.5 % moisture content 12 13 23 % organic 15-30 10-16 20-36 Age (years) 15 <1 2 As measured on the smaller than 20 mm material at 55 C. LABORATORY TESTING AT THE UNIVERSITY OF CALIFORNIA AT BERKELEY AND THE UNIVERSITY OF PATRAS Large-scale cyclic triaxial (CTX) equipment (d=300 mm, h=600-630 mm upon specimen preparation) was used at UCB to perform cyclic triaxial, triaxial compression, extension and lateral-extension tests. Additional conventional-scale cyclic triaxial tests were performed on waste that was processed to make all particles less than 20 mm in size. A direct shear (DS) box, 300 mm by 300 mm by 180 mm, was used at UP to perform shear testing. 1-D compression tests were performed prior to shearing. More information about the devices is presented in Zekkos (2005). The laboratory testing program evaluated the effects of unit weight, compaction effort, confining stress, waste composition, and loading rate on the monotonic stressstrain response and shear strength, and the dynamic properties, i.e. the small-strain shear modulus, strain-dependent shear modulus and material damping, of MSW. Poisson’s ratio was also measured during some of the triaxial tests. A total of 23 large-scale direct shear and 27 large-scale monotonic triaxial tests were performed. As shown in Figure 1, direct shear test specimens prepared with the same compaction effort and with 100%, 62% and 12% <20 mm material, were found to have a conventional concave (e.g. hyperbolic) stress-displacement response and similar shear resistances. The similarity in shear resistance is attributed to the fact that shearing occurred parallel to the sub-horizontal orientation of fibrous materials within the waste, and thus, the contribution of the fibrous materials is minimal. The tendency of the fibrous materials to be oriented horizontal was confirmed by sample inspection after testing. This tendency was also observed in triaxial testing of this investigation, and has also been observed in the field by Matasovic and Kavazanjian (1998). Subsequent direct shear tests performed with waste fibers oriented perpendicular to the shear failure surface yielded a convex, upward curvature in the stress-displacement response, which is attributed to the progressive mobilization of the fibrous materials within the waste matrix. Figure 2 illustrates this anisotropic aspect of MSW behavior, comparing two specimens with the same composition, prepared with the same compaction effort, and tested at the same normal stress. The only difference between the two specimens is the orientation of the >20 mm, fibrous materials. The stressdisplacement response is different. Details are provided in Zekkos et al. (2007a) Consistently with the direct shear tests, the stress-strain response in triaxial compression was found to be strongly dependent on fibrous waste content. An upward curvature of the stress-strain curve was observed for all the triaxial compression tests, with the exception being specimens with 100% <20 mm material. This observed behavior may also be attributed to the progressive mobilization of fibrous materials during shearing, as shearing in triaxial compression occurs at an angle from the horizontal orientation of the fibers. This explanation is supported by the observation GEOCONGRESS 2008: GEOTECHNICS OF WASTE MANAGEMENT AND REMEDIATION 179 that the shear resistance in triaxial compression was higher than that observed in direct shear. The secant friction angle was found to decrease with confining stress both in direct shear and triaxial testing, indicating that a nonlinear strength envelope may be most appropriate for MSW. Shear resistance was found to increase with unit weight, compaction effort, and strain rate (for axial strain rates between 0.5%/min and 50%/min). Test results are presented in Zekkos et al. (2007a, 2007b) in greater detail. 0 200 400 600 80