It’s often a misconception that landfill cell designers choose between soil or impermeable covers when creating a cap for a closed unit. However, almost all modern designs include layers of both materials to take advantage of their different benefits. When you see debates on capping landfill cells, it’s usually a reference to the practice of using soil alone versus combining it with geomembranes. Older cells were often capped with soil alone, as were open dumps when they were first beginning to undergo covering. However, now it’s understood that most sites call for a more intensive capping process. Soil covers alone are rarely enough to completely control leachate production and runoff issues at a landfill cell site. Find out why almost all landfill cells today call for the combined approach and why using a geomembrane cover alone, without soil, isn’t recommended either.
Amount of Rainfall
Soil caps are allowed by EPA and state regulations only in areas where there is very little annual rainfall. This is because the main benefit of capping a cell is keeping water out of the trash, so it produces a minimal amount of leachate. Soil alone, even when it’s piled five feet thick or more, allows water to slowly trickle and seep down to the compacted trash. If there’s more than just a few dozen inches of rain per year, the trickle-down effect will add up to flood the pit over the course of a few years. Impermeable covers shed that water so it can’t penetrate the trash and create leachate issues; reducing the pumping needed and eliminating the chances of an overflow event.
Leachate Risks
The high risks of landfill leachate make it essential to control. The volume of leachate produced must be limited as well. If it was a simple case of harmless runoff, a soil cover would be more than enough to encapsulate most types of solid waste. However, even well-regulated municipal waste supplies tend to create leachate high in heavy metals, organic waste, and more. Geomembranes are essential for limiting leachate creation by redirecting surface water out of the landfill cell pit. By working with the liner below the surface that prevents seepage and other forms of loss, truly impermeable covers are essential to managing liquid levels in each cell for the entire lifespan of the landfill.
Age of the Waste
Very old and stabilized solid waste from former landfill sites may be able to be covered with soil alone. If a compacted mound of municipal waste has already been covered or exposed to the air for 20 years or more, there’s little likelihood of more leachate issues occurring in the future. These stabilized and partially decomposed deposits still need liners to contain what liquids are produced, but they’re not likely to contribute much water from the breakdown of waste anymore. As long as the climate is not wet, a soil cap is likely enough to contain it. On the opposite end of the spectrum, fresh waste can be so high in leachate production that it needs a temporary geomembrane cover to control water gain even during active filling.
Soil for Runoff Control
Even though geomembranes are a better choice than soil layers alone for capping and covering these landfill cells, the impermeable barriers aren’t enough to use on their own. Since they won’t let water through, it’ll accumulate on the surface instead. This leads to runoff that can damage the area surrounding the cell, destabilize the berm, and eventually uncover the waste itself. Erosion issues are common on landfill cell projects that don’t feature adequate soil cover over the geomembrane. A thick layer of soil, ideally covered in healthy vegetation like turf grass, is essential to absorbing and disbursing surface water without causing runoff.
UV Protection
A thick cap of soil over the surface does more than just protect the site from runoff and erosion. It also serves an important purpose as a physical barrier against UV damage. Even the best RPE liners will have a much shorter lifespan when left exposed after installation. While short-term exposure during installation and filling is just fine, the cover material eventually should be covered with something that will block out light. Sand and gravel can work, but they’re also rough on the material. Soil is better at blocking UV rays and extending the lifespan of a covered cap material.
Make the choice to include geomembranes in your landfill capping or covering project. You can find durable reinforced polyethylene covers from us here at BTL Liners that work seamlessly with the liner to completely control leachate loss. Don’t let leachates become an issue when we have multiple products to manage them.
