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.;... ..., , .�.•.- x •r•.r.-y r,;`t..:oA :se3i¢ .......y rr.?;i' -�rrClb?5:11.t9'9 <br /> e <br /> e <br /> r! <br /> i. <br /> i ODUCTION <br /> Leachate containment and removal systems are presently being constructed on est <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 potentie <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 land! <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 gn <br /> reversal takes place at the surface of a liner and leachate tion system, leachate •" <br /> on the liner, and the potential for infiltration f the leachate into the old landfill will i' ,a <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 methc <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 teinforcement 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 /> undemeath the liner. The liner is then treated as a plate bridging over the void and carrying C <br /> loading from the overlying waste. Structural geogrids are placed to support and protect C <br /> integrity of the liner system (12, 13). The design methodology used in geogrid reinfor- -nP <br /> is based on the tensioned membrane theory. When differential settlement takes pla <br /> I i <br /> 1498-Vancouver Canada-Geosynthetics'93 <br />