Landfill cell design requires more than just correct volume calculations and properly sloped berms. Each cell needs both a liner on the bottom and a cap over the top to stay stable and secure for many decades to come. A landfill that is missing either impermeable lining or a durable cap is likely to develop serious issues either while it’s still operating or many years after it is closed. These two parts of a landfill waste management system might be installed at the opposite ends of the lifespan of the cell, but they still work together to completely encapsulate the waste products inside.
Starting at the Bottom
The landfill goes in long before the cap material, but it’s just the first step in the complete system for leachate control. Most landfill cells involve multiple layers of different liners separated by gravel and geotextile mats. The first few layers will be somewhat permeable to allow leachate and other liquids to accumulate above the final impermeable liner. The geomembrane used for the final liner must be as impermeable as possible to control seepage through to the soil below.
Leachate from landfills can cause environmental damage and contaminate groundwater supplies. The leachate is usually pumped or drained out from between these layers of the liner to dispose of it safely elsewhere. In addition to controlling where leachate gathers and goes, the impermeable liner also prevents groundwater from rising up and mixing into the trash layers. Areas where groundwater rises seasonally or is always high can make landfill cell building tricky, but properly anchored liners can prevent water transfer in either direction.
Finishing with a Top Cap
Once the individual landfill cell has been filled and compacted as much as possible, it’s time to add the cap material. Like with the liners, most cell caps are more than just a single layer of material. Caps generally contain a combination of impermeable cover materials, thick layers of soil and vegetation, or an artificial alternative. Without the soil and vegetation at the surface, the wear and tear on the impermeable liner can shorten its lifespan. Yet, top caps and covers also experience more than just the weight of cover soil and plants pressing down on them. The materials are also stressed from the bottom as gas accumulates and pushes up from below. The cover or cap must finally prevent access to the trash layer and keep it from blowing away, which can be a bigger challenge than it sounds.
Controlling Leachate at All Levels
The reason that liners and caps work best together is that they’re both designed to control leachate. Leachate forms within a landfill cell in two distinct ways. First, some water is formed during the breakdown of waste in the storage area, especially deposits of food waste and other high-moisture materials. This is internally produced leachate that is particularly high in contaminants, but usually predictable and limited in volume. Second, leachate can also form as rainwater enters the landfill cell and trickles down through the trash. This external water is greater in volume, but it can flush new, problem contaminants into the general supply as it travels through the layers of compacted waste. Liners keep both types of leachate from escaping the cell, but caps are also essential for controlling the total volume. Without capping at the end of the use of the cell, rainwater continues to flow into it and adds to the total amount of leachate.
Why Skipping One Damages the Other
Landfill cells may seem like they can work well without liners as long as they’re capped as soon as possible. However, a surprising amount of rainfall travels through the compacted waste of a cell while it’s actively being filled. Liners are essential for controlling the leachate that is produced during this stage so the soil and ground water isn’t affected in the process. On the other hand, closing down a cell without capping it will only cause the liner to eventually overflow with leachate. The extra weight of all the liquid accumulating at the bottom against the final liner can tear it, resulting in major leaks that damage the surrounding environment.
What Materials Work Best for Cell Liners and Caps?
The wetter the climate, the more advanced the liners and caps need to be to secure the landfill cell. Increasing volumes of water put strain on the entire system from top to bottom. Durable materials like reinforced polyethylene (RPE) are a good choice for both parts of the system, but other options like reinforced polypropylene (RPP) can also work well. Either way, the materials must be able to withstand a variety of challenges like:
- UV exposure, especially when the cell is active and accepting waste
- Friction and slump, which add wear and tear and increase the likelihood of the material sliding loose
- Crush resistance, since the weight of the landfill cell waste can increase over time due to water infiltration.
If you’re looking for custom recommendations on selecting landfill cap and liner materials that can work well together, contact us here at BTL Liners. We’ll put our industry experience to use helping you choose the right product for each step of your project.
