Federal mandates to close unlined landfills have inspired renewed interest in one innovative concept - the piggyback landfill.
To site today's landfills, solid waste managers must balance regulatory requirements, public opposition and landfill capacity. The piggyback approach, which constructs a new landfill on top of one that is either closed or scheduled to be closed, can help to meet those needs. Another configuration is to construct the new landfill adjacent to the closed landfill, using the side slope as part of the new landfill. With both methods, the piggyback landfill allows for vertical and/or horizontal expansion at an already-approved site.
The piggyback landfill's primary advantage is that municipalities can create more landfill space without a rigorous siting process. Also, the expenditure to cap a landfill can be incorporated into the liner system of the piggyback landfill and offset some closure costs. Communities also can avoid hauling expenses for moving MSW to a new, more remote site.
Piggyback landfills may sound ideal in theory, but there are still concerns about building a new landfill on top of existing solid waste. Since the MSW settles as it biodegrades and shifts under the weight of the new waste, the former landfill's instability can threaten the integrity of a new liner system.
Landfill engineers have developed techniques to curtail settlement. Deep dynamic compaction, or dropping a weight to compact the existing waste prior to new construction, reduces the magnitude of future settlement. An earth preload works in a similar manner but relies on the weight of a soil fill. However, both of these alternatives are only effective on compacting materials to a depth of approximately 40 feet, and a deeper, more active landfill may not be adequately compacted.
Injecting slurry grout, a costly but effective technique, fills the voids within a landfill and hardens to strengthen the foundation of new landfills. To improve the subgrade and to protect the new liner system, the liner can be reinforced with geogrids, which are layers made of various plastics. This support method is helpful to prevent damage from decaying appliances and vehicles. A rusted refrigerator, for example, can cause the liner to dip and put stress on the liner containment system.
A piggyback landfill is not recommended for existing sites with leachate and groundwater contamination problems since it would be difficult to determine if the source of the problem was the old or the new landfill.
Even as engineers develop technology (see chart on page 12) to support piggyback landfills, regulators remain cautious about approving piggyback landfills due to long-term performance uncertainties. As landfill engineers and regulators learn more about landfill settlement and the related issues, piggy back landfills are expected to gain more support.
Long-term monitoring of existing piggyback landfills such as the Blydenburgh landfill, Islip, N.Y., the Brookhaven landfill, Long Island, N.Y., the Oaks sanitary landfill, Montgomery County, Md., and a landfill in St. Lucie County, Fla., that is situated adjacent to a closed landfill and uses its side slope, will determine if piggyback landfills will be viable for the future.