SCS Advice from the Field: Long Term Performance of Landfill Final Covers

November 16, 2020

About the Author: Ali Khatami, Ph.D., P.E.

There are several hundreds of Municipal Solid Waste (MSW) landfills in the United States. Many of these landfills are anticipated to remain active for decades to come, and Federal and state rules require slopes reaching permitted final elevations to be closed within 180 days. This means partial closure of slopes is part of the operational requirements of MSW landfills.

Federal and State Rules

Subtitle D of the Resource Conservation and Recovery Act (RCRA), enacted on October 21, 1976, requires the final cover of MSW landfills to include a barrier layer with hydraulic conductivity that is substantially equivalent to or less than the hydraulic conductivity of the bottom liner. State-level regulations developed following the enactment of the federal law also required similar standards for MSW landfills. Many states, pursuing the federal guidelines, require at a minimum, the bottom lining system of MSW landfills include at least one primary barrier layer consisting of Polyvinyl chloride (PVC), high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE). Naturally, the final cover barrier layer should also be PVC, HDPE, LLDPE as well.

According to the Federal and state regulations, following the completion and closure of a MSW landfill, the facility owner maintains the landfill for a minimum of 30 years beyond the final closing date. Extension of the long-term care period beyond the 30-year post-closure period is a hot subject among solid waste professionals. Some states have already implemented matrices for such time extensions; it is anticipated that the remaining states will require similar extensions for MSW landfills over the next several years. Even if regulatory agencies approve completion of the post-closure period for a specific landfill, the landfill’s final cover system is expected to perform for many more years to come. Otherwise, environmental issues associated with a lack of performance may force the regulatory agency to spend money for repairs no longer available through a financial instrument.

Long-Term Performance Designs

For the past few decades, SCS has specifically designed and permitted final cover systems with special features to prolong the final cover system’s performance beyond the post-closure period of the landfill. The final cover system designs:

• Maximize available airspace in the landfill,
• Simplify waste placement in the vicinity of the exterior landfill slopes,
• Simplify stormwater management components over landfill slopes,
• Effectively collect and remove rainwater percolating through the final cover soils,
• Collect lateral leachate seeps below the final cover barrier layer, and
• Effectively encapsulate landfill gas at the landfill perimeter.

Less Maintenance

The first partial final cover with these features was constructed in 1998, and since then, many more partial closures with these types of features have been constructed. All partial closures are performing satisfactorily without failure. Regular maintenance of the final cover vegetation and occasional cleaning of drainage swales, which are common maintenance activities, have been the only measures taken by the operators of the facilities with these final cover systems.

The features incorporated into the final cover systems were:

• Straight 3H:1V slopes to the top of the landfill with no benches or terraces, providing benefits such as maximizing airspace; eliminating complications during filling of the landfill near exterior slopes; allowing final surface water drainage swales to be constructed during the construction of the final cover which provides flexibility for the swale locations, swale slopes, drainage points of swales on the slopes; and downchute pipes that do not require complicated geometric features at the point of connection to drainage swales on the slope;
• A leachate toe drain system (LTDS) collecting and disposing of leachate seeps below the final cover geomembrane reaching the bottom of the landfill slope; and
• A rainwater toe drain system (RTDS) collecting and draining out of the final cover the rainwater that percolates through the final cover reaching the cover system geocomposite drainage layer.

The features above have financial, performance, and stability benefits for the facility for many years to come. So far, such final covers have been constructed on 3H:1V slopes as long as 550 ft. in length with no terraces. Several of the completed final covers were partial closures on a 3H:1V slope, where the next phase was constructed directly above a previous phase with the two phases tied together at the phase boundary.

Proper design and planning for the construction of partial final covers are significantly important for the long-term performance of landfills during the active life, post-closure period, and beyond.

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