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.r UDUCT ON Leachate containment and removal systems are presently being constructed on't-x1,, landfills, which were developed before low-permeability liners and leachate collection systf became a regulatory and industry standard of practice. These systems arse being construe below proposed vdIrtical expansions to reduce the infiI:tration of leachate generated within newly placed waste, into the underlying old landfills. A liner and leachate collection system constructed on an existing landfill is potentit subject to large di.#erential settlements. -The long-term performance of these systems is thu major design consideration. Large differential settlements within existing refuse may occur c to'the collapse or degradation of large objects, which have been disposed of in the old land-1 These settlements Would result in tensile strains at the surface of a liner system. If the ten, •strains within the liner exceed the tensile capacity of the material, whether it is a soil o geomembrane, tension cracks or tensile failure will develop. Under extreme conditions, tenni cracks will reduce the effectiveness of the liner as a hydraulic barrier by providing a direct fl. path through the ling system. Differential settlements occurring below a liner and leachate collection system could a affect the slope at which the liner and leachate collection systems have been constructed. Aga under extreme conditions, this could result. in the reversal of leachate flow grades. If gra reversal takes place 4t the surface of a liner and leachate collection system, leachate w-" - on the liner, and the potential fok infiltration of the leachate into the old landfill will i'......a This paper presents an approach to analyze the differential settlement at the surface a Liner system caused by the collapse of a void within an existing landfill. On the basis of tl analysis, a new design method is presented and described. This paper summarizes metho currently available to estimate "void-induced" differential settlements. One of these methods further discussed and an example using- this design method is provided. Conclusions a recommendations based on the proposed analysis and results are also presented. Current Design Practice Presently, two methods are being used to minimize the potent damage of a liner and leachate collection system constructed between an existing landfill and vertical expansion. 711' ese methods consist of either reinforcing the liner system with synthei geogrids or improving the subgrade by stabilizing the in-situ refuse before constructing a lin and leacha-te collectioin system above it. Geogrid tei.nforcement is one of the techniques most frequently applied in these case The design of the reinforcement is based on the assumption that a void is located immediate underneath the liner. 'ile liner is then treated as a plate bridging over the void and carrying d loading from the ovetlying waste. Structural geogrids are placed to support and protect ti integrity of the liner sstem (12, 13). The design methodology used in geogrid reinfor- --,P is based on the tensioned membrane theory- When differential. settlement takes pi&. ... . 1498- Vancouver.Canada-Geosynthedcs'93