Toland Road Seismic Analysis - Slope Stability Analysis And Geosynthetic Material Selection
Dynamic Site Response Analysis and Design Methodology of a Geosynthetic Liner System for a Municipal Solid Waste Landfill Located in a High Seismic Risk Zone - A Case Study
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Slope Stability Analysis And Geosynthetic Material Selection Slope stability analyses are typically performed using representative two-dimensional cross sections through the assumed failure mass along the line of failure. There are several computer programs available which simplify this procedure by allowing the user the ability to quickly analyze a variety of failure modes (i.e., circular, random, block, and user defined) and search for the lowest factor of safety. A two-dimensional analysis assumes the failure mass is infinitely wide and does not account for three-dimensional effects, specifically, shear resistance along the edges. Therefore, the factor of safety will be conservative and may increase by 10-25% upon using a three-dimensional analysis.A factor of safety of 1.5 is generally accepted as the minimum for the stability for refuse fills or geosynthetic lined slope. Furthermore, is the minimum requirement under California Code of Regulations, Title 27, Section 21750(f)(5). If the factor of safety calculated using the two-dimensional analysis is less than 1.5 and design modifications do not appear to increase the factor of safety appreciably, a three-dimensional analysis should be performed in order to provide a more realistic factor of safety. The progressive use of more rigorous methodologies (i.e. 2D to 3D), reduces the degree of conservatism with each step, often resulting in higher (and more realistic) factors of safety. For this case study, EBA used the more rigorous three-dimensional analysis due to the complex geometry of the liner system and potential failure mass which incorporated a portion of existing waste. Of particular concern were the effects of strain softening (the progressive increase in strength with increasing strain) caused by the strain incompatibility between the liner which can mobilize peak strength with less than an inch of displacement, and the existing waste which may not mobilize peak strength until after several feet of displacement. MATERIAL SELECTION EBA performed laboratory testing of geosynthetic materials used in the base liner design in order to obtain friction angles and adhesion values for various interfaces. A preferential slip plane, or shear sheet, was incorporated into the liner design above the FML to aid in preventing future landfill settlement and possible seismic displacements from placing damaging stress on the FML primary barrier layer. The composite liner design consisted of (from bottom to top) a GCL, an HDPE FML, and the geotextile shear sheet. The LCRS composite drainage net was placed against the geotextile shear sheet and provided the interface for the preferential slip plane.Material properties of waste were difficult to measure and were estimated from values published for other sanitary landfill sites (Kavazanjian et al., 1996). Fortunately, solid waste properties were not a major factor in the stability analyses because potential sliding was determined to occur along weaker geosynthetic interfaces. YIELD ACCELERATION Once the critical failure mode was determined, the yield acceleration (Ky) of the failure mass was evaluated by varying the horizontal acceleration input parameter in the program until the factor of safety reached unity. Because the yield acceleration was less than the PGA determined for the site, a seismic response analysis was performed to estimate the amount of permanent seismic displacement.References
Toland Road Seismic Analysis - Table of Contents
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