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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 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 liner system. Since 1487 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 landfall, (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 STRAW <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 S percent. <br />(2) <br />