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2.0 SUBSIDENCE <br /> The single most important and influential parameter in a finer support design is the <br /> selection of the foundation criteria. In cases where the landfills are located over old mines <br /> or karst topography, predicting settlements is straightforward. Old mine maps and local <br /> sinkhole data can be used to develop a subsidence model. Landfills below liner systems <br /> are more difficult to characterize. Subsidence associated with decomposition and decay <br /> will depend on many factors and will vary not only between landfills, but within a single <br /> landfill. Recognizing that there is no direct method to predict settlement one must rely <br /> on experience and documented performance to insure that the design is conservative. <br /> From the beginning it was recognized that a localized subsidence would pose the greatest <br /> risk to the structural components within the landfill's finer system. Since 1987 liner <br /> support projects have assumed a complete loss of bearing capacity immediately below the <br /> new liner system. At first, the dimension was based on the potential collapse of a"white <br /> good" near the surface of the landfill, (often referred to as the"refrigerator theory"). As <br /> time went on, the typical void diameter of six feet was adopted as the engineering state of <br /> practice based on the successful performance of previous designs. <br /> 3.0. STRAIN <br /> Liner support applications are based on a strain limited design. The strain at which the <br /> geomembrane ruptures controls the design. An HDPE geomembrane, the predominant <br /> landfill liner because of its resistance to chemical degradation, will elongate significantly <br /> before breaking when stretched in one direction. However, when subjected to multiaxial <br /> elongation, rupture occurs in the 10%-16% strain range. In a strain limited design, the <br /> factor of safety should be applied to the minimum rupture strain. As a result, the current <br /> state of practice for liner support applications is to utilize a safety factor equal to 2 and <br /> design at a strain limit of 5 percent. <br /> (2) <br />