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01 <br />C <br />C <br />1 In V 0,14191 o.s <br />Leachate containment and removal systems are presently being constructed on txist <br />landfills, which were developed before low -permeability liners and leachate collection syste <br />became a regulatory and industry standard of practice. These systems am being construc <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 potentia <br />subject to large 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 liner 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, tensi <br />cracks will reduce the effectiveness of the liner as a hydraulic barrier by providing a direct fl. <br />path through the liner 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 at the surface of a liner and leachate collection system, leachate w-'' <br />on the liner, 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 a <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 collection system above it. <br />Geogrid t-einforcement is one of the techniques most frequently applied in these cast <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 tl <br />loading from the overlying waste. Structural geogrids are placed to support and protect t) <br />integrity of the liner system (12, 13). The design methodology used in geogrid reinfor- <br />is based on the tensioned membrane theory. When differential settlement takes p1a.... . <br />1498 - Vancouver, Canada - Geosynthetics'93 <br />