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The maximum tensile strain generated by the differential settlements was examined evaluate the integrity of a composite liner, which included a geomembrane and a clay Ia The geomembrane can sustain tensile strains higher than those that the clay component olk composite liner can sustain before tensile failure or tension cracks develop within the c component. Therefore, the tensile strain limit of the clay component can be used as : acceptable design criterion to evaluate the integrity of composite Finers. From the maximum tensile strains versus soil/waste thickness ('1) characteristic cure a soil layer thickness M equals to 2.7 m (9 feet), the maximum tensile strain caused by the 0 m (3 foot) wide by 0.9 m (3 foot) deep by 1.8 m (6 foot) long void is .0.2 percent. This stra is within acceptable limits for a clay layer as illustrated on Figure 5 from Gilbert and Murpl (6)• On the basis of the above example analysis, a 2.7 m.(9 foot) thick.soil/waste layer CE serve as a strain transition zone to prevent grade reversal, excessive tensile strains and stressc developed in a Iiner system. "Therefore, a 2.7 m (9 foot) thick layer of soil or "select" wast can be placed, in this example, before constructing the vertical landfill containment liner. Summary__ For slope angles other than the 7 percent used in the example analysis, the tensil strains and potential for grade reversal on a liner surface can be evaluated according to th thickness versus maximum tensile strain and thickness versus liner grade characteristic curve! settlement contours and containment liner design criteria. From this evaluation, the require backfill thicknesses in different finer grades and liner systems can be determined and designe, to prevent grade reversal and excessive tensile strains on a vertical expansion liner and leach collection system. CONCLUSIONS The Elastic Method provides a conservative and efficient method to evaluate the potentia differential settlement caused by a void within an existing landfill when a vertical expansion o the landfill is planned. The results from this method closely agree with the results from Britist NCB's field mining subsidence surveys. The method provides a conservative estimate of th( potential differential settlement, and therefore can be used to design a vertical expansion linea system. Characteristic curves of thickness over the void (T) versus maximum tensile strain, and of thickness (T) versus maximum differential settlement slope can be developed for specific void size, as shown in the example presented in this paper. These characteristic curves can then be used for the design of containment liner systems in a landfill vertical expansion. As shown in the characteristic curves provided, the differential settlement diminishes as the soil/waste thickness over the void increases. Thus, it is possible to increase the distance between the .potential void and the surface of the liner, to reduce the differential settlement to Geosynd)c ies '93 - Vancouver, Canada - 1507