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MMODUCTION Leachate containment and removal systems are presently being constructed on "st landfills, which were developed before low-permeability liners and leachate collection syste became a regulatory and industry standard of practice. These systems are being construe below proposed vertical expansions to reduce the infiltration of leachate generated within newly placed waste, into the underlying old landfills. A liner and leachate collection system constructed on an existing landfill is potentid, subject to large differential settlements. The long-term performance of these systems is thu major design consideration. Large differential settlements within existing refuse may occur c t6the collapse or degradation of large objects, which have been disposed of in the,old landf These settlements could 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 liner 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 grs reversal takes place at the surface of a Iiner and leachate collection system, leachate w-"* on the liner, and the potential for infiltration of the leachate into the old landfill will ii.- This paper presents an approach to analyze the differential settlement at the surface o liner system caused by the collapse of a void within an existing landfill. On the basis of U analysis, a new design method is presented and described. This paper summarizes methe 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 ane vertical expansion. These methods consist of either reinforcing the liner system with synthei geognds or improving the subgrade by stabilizing the in-situ refuse before constructing a lin and leachate collection system above it. Geogrid teinforcement 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. The liner is then treated as a plate bridging over the void and carrying C loading from the overlying waste. Structural geogrids are placed to support and protect C integrity of the liner system (12, 13). The design methodology used in geogrid reinfor- is based on the tensioned membrane theory. When differential settlement takes pla. 1498- Vancouver-,Canada-Geosynthebcs'93