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° The maxim m tensile strain'generated by the differential settlements w as evaluate the int.--gr'ty of,a composite Finer, which included a exami The geomembrane can sstain tensile strains higher than those th t the clay co membrane and a clay .. composite Liner ca sustain before tensile failum or tension cracks develop within the cl mponcnt of component. The fore, the tensile strain limit of the clay component can be used as acceptable design criterion to evaluate the integrity of composite liners. From the maximum tensile strains versus soil/waste thickness (T) characteristic cure a soil layer thieknes (1) equals to 2.7 m (9 feet), the maximum tensile strain caused by the 0 m (3 foot) wide by 0.9 in (3 foot) deep by I-9 1n (6 foot) long void is.0.2percent "liar . This s is within acceptable limits for a clay layer as illustrated on Figure 5 from Gilbert and Murpl (6)• On the basis f the above c,. lcm.(9 > xamp analysis, a 2.7 foot) thick.soil/waite layer c� serve as a strain Wa " 'tion zone to prevent grade reversal, excessive tensile strains and stresst developed in a liner stem. Therefore, a 2.7 m (9 foot) thick layer of soilor "select" wast can be placed, in thi example, before constructing the vertical landfill containment liner. Summ--a'y- For slope angles other than the 7 percent used in the example analysis, the tensil ? strains and potenxial or grade reversal on a liner surface can be evaluated according to th thickness versus maxim.um 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 liner grades and liner systems can be determined and desig to prevent grade reversal and excessive tensile strains on a vertical expansion liner and leach collection system. CONCLUSIONS The El-astic Met lod-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 BritisE NCB's field mining su sidence surveys. The method provides a conservative estimate of the potential differential se ttement, and therefore can be used to design a vertical expansion liner system. Characteristic cu: es 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 S1ze, 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 h-aracteristic 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 vo d and the surface of the liner, to reduce the differential settlement to Gcosynthebcs'93-vawouver.Canada- Iso7