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2.0 SUBSIDENCE <br /> The single most important and influential parameter in a liner support design is the <br /> selection of the foundation criteria. In cases where the landfills are located over old Hones <br /> or karst topography, predicting settlements is straightforward. Old mine maps and local <br /> sinkhole data can be used to develop a subsidence model. Landrbelow finer 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 <br /> risk to the structural components within the landfill's liner P greatest <br /> support projects system. Since l98?liner <br /> su rt .ro'ects 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 fused 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 l0'/.-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 /> ! <br />