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F <br /> ,.. <br /> ODUMO <br /> Leachate containment and removal systems are presently being constructed on <br /> landfills, which were developed before low-permeability liners and leachate collection syste <br /> became a regulatory and industry standard of practice. These systems are being construe <br /> below proposed vertical expansions to reduce the infiltration of leachate generated within <br /> newly placed waste, into the underlying old landfills. <br /> A liner and ;leachate collection system constructed on an existing landfill is potentit <br /> subject to Iarge differential settlements.:-The long-term performance of these systems is thu <br /> major design consideration. Large differential settlements within existing refuse may occur c <br /> to*the collapse or degradation of large objects, which have been disposed of in the old lands <br /> These settlements could result in tensile strains at the surface of a liner system. If the ten., <br /> strains within the Finer exceed the tensile capacity of the material, whether it is a soil o. <br /> geomembrane, tension cracks or tensile failure will develop. Under extreme conditions, tenni <br /> cracks will reduce the eff-ectiveness of the liner as a hydraulic barrier by providing a direct fl, <br /> path through the ling system. <br /> Differential settlements occurring below a liner and leachate collection system could a <br /> affect the slope at which the liner and leachate collection systems have been constructed. Aga <br /> under extreme conditions, this could result in the reversal of leachate flow grades. If gra <br /> reversal takes place 4t the surface of a liner and leachate collection system, leachate <br /> on the 11-ner, and the potential for infiltration of the leachate into the old landfill will i,... <br /> This paper presents an approach to analyze the differential settlement at the surface o- <br /> liner system caused by the collapse of a void within an existing landfill. On the basis of tl <br /> analysis, a new design method is presented and described. This paper summarizes metho <br /> currently available to estimate "void'-induced' differential settlements. One of these methods <br /> further discussed and an example using- this design method is provided. Conclusions a <br /> recommendations based on the proposed analysis and results are also presented. <br /> current Design Practice Presently, two methods are being used to minimize the potent <br /> damage of a liner and leachate collection system constructed between an existing landfill and <br /> vertical expansion. these methods consist of either reinforcing the liner system with synthei <br /> geogrids or improving the subgrade by stabilizing the in-situ refuse before constructing a lin <br /> and leachate collectioln system above it. <br /> Geogrid reinforcement is one of the techniques most frequently applied in these case <br /> The design of the reinforcement is based on the assumption that a void is located immediate <br /> underneath the liner. 'the liner is then treated as a plate bridging over the void and carrying 0 <br /> loading from the overlying waste. Structural geogrids are placed to support and protect 0 <br /> integrity of the liner system (12, 13). The design methodology used in geogrid reinfor- -7-. <br /> is based on the tensioned membrane theory. When differential settlement tapes pla. ... <br /> • <br /> 1498- vancouvci,Canada-Geosynthetics'93 <br />