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1.0 INTRODUCTION Today all operating municipal solid waste landfills are required to have a composite liner and leachate collection system at their base. Regulations require that these landfill components be constructed over a stable foundation. In many instances however, the underlying subgrade has the potential for differential settlement and collapse. The site selection process for a landfill is complex. Local economies and tax structures, host fees, infrastructure,geology,hydrogeology, nimby(not in my backyard groups)and a variety of other forces and factors influence the process. Often the most challenging site from an engineering perspective turns out to be the selection that best serves the common good. In 1987, a barge was loaded with bailed waste from Islip New York_ Many months and many miles later, the barge having found no landfill that would accept the waste,.arrived back in Islip. The town council, the consulting engineer and the state regulatory agency all agreed that the best solution.was to construct a new landfill above the old capped landfill. This resulted in the first landfill liner support installation. In the late 1980's and eady 1990's all the states in the country upgraded their landfill liner system1 ions. This left many essentiallandfills f complianee. upgraded cells could be constructedon adjoining property, the airspace abovet landfill sideslopes to be closed was a valuable asset no one wanted to abandon. In many instances, it was in the best interest of all involved to design a"modern"landfill on the sideslope of the landfill to be capped. To date dozens of landfills have been constructed over existing landfills, karst topography and old mine tunnels. The leachate collection systems are monitored to insure that the landfills are operating as designed. A few landfills have installed strain monitoring systems below the new liner system. At the Babylon Landfill in New York and the Sawyer Landfill in Maine, strain readings have indicated that one layer of uniaxial geogrid has reduced liner strains to well below the design limit. This paper reviews the design methodology utilized when landfills are sited above subgrades with the potential for differential settlement and collapse. Subsidence issues and engineering solutions will be discussed. A design example is also presented for review.