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EVTRODUCTIEON _- Leachate containment and removals stems are presently being constructed landfills, which were developed before low-permeability liners and leachate collection systt 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 potentit subject to large difierential settlements.:'Thee long-term performance of these systems is thu major designconsideration. Large di tial settlements within existing refuse may occur c to*the collapse or degradation of large objects, which have been disposed of in the old landt 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 davelop. Under extreme conditions, tensi cracks will reduce the effectiveness of the liner as a hydraulic barrier by proving a direct fl. path through the liner system. Differential settlements occurring below a liner and leachate collection system could a affect the slope at w1 ich the liner and leachate collection systems have been constructed. Aga under extreme conditions, this could result in the xrversal of Ieachate 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 for infiltration of the leachate into the old landfill will This paper pr6sents 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 d analysis, a new design method is presented and described. This paper summarizes me-tho ' 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 Prach_ce. Presently, two methods are being used to minimize the potent damage of a liner anJ leachate collection system constructed between an existing landfill and vertical expansion. These methods consist of either reinforcing the liner system with synthee geogrids or improvin# the subgrade by stabilizing the in-situ refuse before constructing a lin and leachate collection system above it. Geogrid reinforcement is one of the techniques most frequently applied in these cast The design of the reinforcement is based on the assumption that a void is located immediate underneath the liner. 1"he liner is then-treated as a plate bridging over the void and carrying tl loading from the overlying waste. Structural geogrids are placed to support and protect 0 integrity of the liner system (12, 13). The design methodology used in geogrid reinfor- -n,° is based on the tensioned membrane theory. When differential settlement takes pla.... 1'498- Vancouver Canada-Ccosynthetics'93