Keynote Lecture No. 19 SECOND INTERNATIONAL CONFERENCE ON PERFORMANCE-BASED DESIGN IN EARTHQUAKE GEOTECHNICAL ENGINEERING May 28-30, 2012 - TAORMINA (ITALY)
PERFORMANCE BASED DESIGN FOR SEISMIC DESIGN OF GEOSYNTHETICS-LINED WASTE CONTAINMNT SYSTEMS Edward KAVAZANJIAN, Jr.1, Mohamed ARAB2, and Neven MATASOVIC3 ABSTRACT A performance-based methodology for seismic analysis and design of the geosynthetic elements of waste containment systems, including landfills and heap leach pads, has been developed. The methodology offers a rational alternative to the current state of practice for seismic design of geosynthetic containment system elements in which a decoupled Newmark-type displacement analysis is used to calculate a permanent seismic displacement. This calculated displacement is generally considered to be an index of the performance of the containment system in an earthquake. In this Newmark-type design methodology, no explicit evaluation is made of the stresses and strains in the geosynthetic elements of the containment system. In order to explicitly assess the ability of the geosynthetic elements of a containment system to maintain their integrity in a design earthquake, a finite difference model of waste-liner system interaction has been developed using the computer code FLACTM. A beam element with zero moment of inertia and with interface elements on both sides is employed in the model to represent a geosynthetic element in the liner system. This enables explicit calculation of the axial forces and strains within the liner system element. The beam element model was calibrated using available experimental data from shaking table tests of rigid and compliant blocks sliding on geomembranes. The model was then used to analyze the behavior of the Chiquita Canyon landfill in the Northridge earthquake. Results of the analysis provide insight into the reasons for the tears in the liner system at Chiquita Canyon observed after the Northridge event. This model provides a basis for direct performance based seismic design of geosynthetic elements not only in waste containment systems but in a variety of other civil structures that employ geosynthetic elements wherein earthquake ground motions cause relative displacement between the geosynthetic element and the surrounding soil. Keywords: geosynthetic, geomembrane, seismic design, liner system, waste containment
INTRODUCTION The state of practice for seismic design of geosynthetics-lined waste containment facilities has changed little in the last two decades. While great strides have been made in the understanding of the dynamic properties of solid waste, seismic analyses of the performance geosynthetic liner system elements still generally follows the methodology reported on by Seed and Bonaparte (1992). In this type of analysis, the ability of the geosynthetic elements of the liner system to resist the earthquake strong ground motions is based upon the displacement calculated in a decoupled Newmark-type analysis (Newmark, 1965). The seismic response of the waste mass is calculated without consideration of the influence of the relative displacement (slip) at liner system interfaces on the response. The response is then used to calculate the 1
Professor of Civil Engineering, Arizona State University, Tempe, Arizona, USA, 85287-5306, email:
[email protected] 2 Assitant Professor, Mansoura University, Mansoura, Egypt 3 Associate, Geosyntec Consultants, Huntington Beach, California, USA
II International Conference on Performance Based Design in Earthquake Geotechnical Engineering May 2012, 28-30 - Taormina, Italy relative displacement (slip) at the liner interface (hence the calculation of seismic response is decoupled from the calculation of relative displacement). This calculated relative displacement is then used as an index of seismic performance of the liner system, with calculated values of less than 150 mm generally accepted as being indicative of satisfactory performance, i.e. of the liner system maintaining its integrity in the earthquake. In this type of analysis, the loads on the liner system elements, e.g. tensile strains and axial forces, are never explicitly evaluated. Furthermore, no consideration is given to transient strains and forces induced in the liner system by seismic loading and the method fails to predict liner system performance in the one case history of damage to a geosynthetic liner system in an earthquake, the tearing of the geomembrane at the Chiquita Canyon landfill in the 1994 Northridge, California (USA) earthquake (EMCON, 1994). This paper presents a rational methodology for performance-based analysis and design of the geosynthetic elements of waste containment systems at landfills and mine sites. In this methodology, the strains and axial forces induced in the geosynthetic elements of the containment system in a design earthquake can be explicitly evaluated. A finite difference model of waste-liner system interaction is employed in which a beam element with zero moment of inertia and with interface elements on both sides is used to represent a geosynthetic element in the liner system. The interface model was calibrated using available experimental data from shaking table tests of rigid and compliant blocks sliding on geomembranes and large scale cyclic direct shear testing of high density polyethylene (HDPE) geomembrane / geosynthetic clay liner (GCLs) combinations. The beam model was based upon available information on the stress-strain behavior of HDPE geomembranes subject to tensile loading and the internal shear behavior of geosynthetic clay liners (GCLs). Analysis of the behavior of the Chiquita Canyon landfill in the Northridge earthquake demonstrates the capabilities of the model and provides insight into the reasons for the tears in the liner system at the landfill in the Northridge event. This model provides a basis for direct performance based seismic design not only of the geosynthetic elements of waste containment systems but can also be used in a variety of other situations wherein earthquake ground motions cause relative displacement between soil or waste and other elements of the geotechnical system.
CURRENT STATE OF PRACTICE The current state of the practice for seismic analysis and design of geomembranes in waste containment systems is described by Bray and Rathje (1998), Bray et al. (1998), Kavazanjian (1999), and Matasovic and Kavazanjian (2006). In current practice, the adequacy of a geomembrane to withstand seismic loading is based upon the permanent seismic displacement calculated in a decoupled Newmark-type analysis. In this type of analysis seismic response is calculated assuming there is no relative displacement at the liner system interfaces and then the calculated response is used as input to calculate permanent seismic displacement in a Newmark analysis (i.e. the seismic displacement is decoupled from the seismic response). As reported by Seed and Bonaparte (1992), a calculated permanent seismic displacement no greater than 150 to 300 mm is generally used as the seismic performance criterion in this type of analysis (Kavazanjian 1999, Kavazanjian et al. 1998), with the more stringent criterion of 150 mm typically employed in California. However, as noted by Kavazanjian (1999, 1998), this calculated displacement should not be considered the actual seismic displacement, but rather serves as an index of seismic performance. Bray and Rathje (1998) showed that for typical landfill problems, the decoupled approach significantly overestimates the actual seismic displacement at the liner interface. Matasovic et al. (1998) showed that if the large displacement shear strength is used to calculate the yield acceleration in the Newmark analysis, the actual permanent seismic displacement may be negligibly small for calculated Newmark displacement of 300 mm or more (depending on the ratio of the peak to large displacement shear strength and the displacement necessary to mobilize the large displacement strength). Furthermore, regardless of the magnitude of the permanent seismic displacement, this type of analysis does not reflect
II International Conference on Performance Based Design in Earthquake Geotechnical Engineering May 2012, 28-30 - Taormina, Italy the actual strains and displacements induced in the liner system and thus can never be more than an index of seismic performance. Augello et al (1995) and Matasovic et al. (1995) summarize the performance of landfills in the 1994 Northridge earthquake. This event remains the only major earthquake in which landfills with modern geomembrane liner systems were subjected to high levels of seismic shaking. Most notable of the case histories summarized by these investigators is the performance of the Chiquita Canyon landfill, where tears were observed in the side slope geomembrane liner system at or near the top of the slope in after the earthquake at two different locations, Canyon C and Canyon D. Figure 1 shows the tear observed in the Canyon D liner system after the earthquake.
Figure 1. Tear Observed in the Canyon D side slope geomembrane at Chiquita Canyon Following the Northridge Earthquake
Augello (1997) reported that the displacement predicted in a conventional seismic performance assessment (i.e. a decoupled Newmark-type analysis) at both of these locations was less than 150 mm, bringing into question the adequacy of the conventional seismic performance criteria for geomembrane liner systems. EMCON (1994) noted that the tears in the Chiquita Canyon liner systems at both locations appear to have emanated from locations where a sample had been cut from the geomembrane for construction quality assurance (CQA) testing and a patch had been extrusion welded over the area where the sample had been taken. While not explicitly stated, this observation suggests that stress and/or strain concentrations associated with removal of samples and placement of patches for CQA provide a possible explanation as to why conventional seismic performance analyses fail to predict the tears in the
II International Conference on Performance Based Design in Earthquake Geotechnical Engineering May 2012, 28-30 - Taormina, Italy geomembrane at the Chiquita Canyon landfill in the Northridge earthquake. Giroud (2005) presents strain concentration factors that account for the presence of geomembrane seams perpendicular to the direction of loading of a geomembrane (i.e. due to the presence of a patch) as well as for the presence of scratches and defects in the geomembrane. These factors indicate that the strain in a geomembrane can more than double in the vicinity of an extrusion-welded patch and that additional strain amplification can occur around scratches or gouges in the geomembrane. However, as geomembrane strains are not explicitly evaluated, the Giroud strain concentration factors do not influence current practice for the seismic performance of the geosynthetic elements of waste containment systems. PERFORMANCE-BASED ANALYSIS OF GEOSYNTHETIC LINER SYSTEM ELEMENTS Methodology Fowmes et al. (2006) and Fowmes (2007) developed a methodology for calculating actual axial forces and strains in waste containment geomembranes subject to relative displacement at the interface between the geomembrane and an adjacent material due to waste settlement. In their analyses, conducted using the large displacement finite difference formulation in the computer program FLACTM (Itasca, 2008), these investigators modeled the geomembrane as a linear elastic beam with zero moment of inertia (to allow for buckling) and modeled the interface using a linear elastic-perfectly plastic stress-stain model with a high shear stiffness and a yield strength described by the Mohr-Coulomb criterion (with parameters based upon direct shear interface shear strength test results). The tensile stiffness of the geomembrane was based upon manufacturer-cited stiffness for high density polyethylene (HDPE), the typical material used for waste containment geomembrane liner elements. The compressive stiffness of the geomembrane was by assumed equal to zero in the analyses conducted by these investigators. Arab and Kavazanjian (2010) and Arab et al. (2010) describe extension of the elastic-perfectly plastic interface model of Fowmes et al. (2006) and Fowmes (2007) to include cyclic loading. Arab and Kavazanjian (2010) and Arab et al. (2010) demonstrated the ability of this interface model to describe relative displacement across an interface subject to cyclic loading by comparison of numerical analysis to the results of shaking table model tests on a sliding block on a plane. Figure 2 illustrates the numerical models used to analyze a block on a horizontal plane and a block on an inclined plane.
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Figure 2. Finite Difference Models of a Block on a Sliding Plane Slip at the Interface Arab and Kavazanjian (2010) report on comparison of results from numerical analyses compared to shaking table tests of a geomembrane lined block on a horizontal plane lined with a geomembrane, tests
II International Conference on Performance Based Design in Earthquake Geotechnical Engineering
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May 2012, 28-30 - Taormina, Italy conducted to evaluate the potential use of geomembranes for frictional base isolation. Values of bulk and shear modulus representative of structural steel were used to model both the rigid block and shaking table. The interface was assigned an elastic shear and normal stiffness approximately equal to ten times the bulk modulus of the mesh elements and a shear strength based upon tilt-table testing. Figure 3 presents the elasto-plastic interface stress-displacement model along with the results of the comparison for uniform sinusoidal loading from Kavazanjian et al. (1991). Arab and Kavazanjian (2010) also show a favorable comparison between numerical analysis and experimental results for a geomembrane lined (base isolated) block on a horizontal plane subject to earthquake loading from Yegian and Kadakal (2004). 0.8 0.6 0.4 0.2 0 ‐0.2 ‐0.4 ‐0.6
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Figure 3. Measured and Calculated Accelerations of a Geomembrane Lined Black Subject to Uniform Sinusoidal Loading (Arab and Kavazanjian, 2012)
Arab et al. (2012) report on results from numerical analyses for the block on an inclined plane model compared to shaking table test results from Elgamal et al. (1990) and from Wartman (1999) and Wartman et al. (2003). In the Elgamal et al. (1990) shaking table tests, the plane was coated with sandpaper, sand was glued to the base of the rigid block and the plane was inclined at an angle of 10o and subject to horizontal sinusoidal loading. Comparison between the block acceleration from the numerical analysis and the block acceleration measured in the Elgamal et al. (1990) tests for sinusoidal loading using a friction angle based upon the static friction coefficient of 0.36 reported by Elgamal et al. (1990) is presented in Figure 4. Similar good agreement was shown between predicted and observed block displacement by Arab et al. (2010). Wartman (1999) and Wartman et al. (2003) reported the results of shaking table tests of a rigid block on a plane inclined at 13.37o and subject to horizontal shaking. The interface between the block and the plane was a smooth high-density polyethylene (HDPE)/non-woven geotextile interface similar to one that might be found in a side-slope liner system for a landfill. Wartman (1999) and Wartman et al. (2003) used a suite of 22 uniform sinusoidal motions, three sinusoidal frequency sweep motions, and one earthquake-like input motion. For each test, Wartman et al.(2003) varied the interface friction angle in a Newmark-type displacement analysis until a calculated cumulative relative displacement approximately equal to the one observed in the shaking table test was achieved. Results of this analysis indicated that the interface friction angle depended upon the frequency of the input motion. This frequency dependence was interpreted by Wartman et al. (2003) as a dependence of interface friction angle on sliding velocity. Arab et al. (2010) repeated the Wartman et al. (2003) analysis using the numerical model. Figure 5 presents a comparison the results of the Arab et al. (2010) analysis to the Wartman et al. (2003) analysis.
II International Conference on Performance Based Design in Earthquake Geotechnical Engineering
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May 2012, 28-30 - Taormina, Italy 0.8 0.6 0.4 0.2 0 ‐0.2 ‐0.4 ‐0.6 ‐0.8
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Figure 4. Comparison of Numerical Analyses to the Experimental Results of Elgamal (et al. (1990) for a Block on an Inclined Plane (Arab et al. 2010) 20 19
Numerical Analysis Experimental small disp. (
