Landfill cap systems prevent water from entering a landfill site; water which could contribute to the formation of leachate. The cap system uses a combination of drainage and low permeability layers to accomplish this task. Traditional soil liners or granular materials are commonly being replaced with geosynthetic materials. Geosynthetics offer a significant advantage in landfill cap systems because they take up little landfill space (compared to soil layers) and allow the operator to accept additional waste.
A number of different geosynthetic lining materials can be applied for landfill caps. For example, Edmonton, Alberta-based Layfield Plastics supplies several types of liners: polyvinyl chloride (PVC), polypropylene (PP), and Enviro Liner liners. The advantage of these materials is that they are more flexible (highly extensible) than the high density polyethylene (HDPE) that is commonly installed in base liner applications. Efficient landfill cap designers fabricate these flexible materials into large panel sizes so that construction time is significantly reduced.
Depending on the landfill site, the level of precipitation may require a complex drainage system and sophisticated liner design or a more simplified design. Geosynthetic drainage materials such as geo-nets and geo-composites come in a variety of styles. These materials allow the passage of water (or gas) in the plane of the material. A 6 mm (0.25-inch) geo-net can drain approximately the same amount of water as 300 mm (one foot) of select drainage sand while maximizing landfill volume. A drainage geo-composite is often formed by bonding a non-woven geotextile to one or both sides of a geo-net. Bonding the geotextile helps to improve the frictional stability of the composite so that steeper slopes can be used in the cap.
Although it is possible to use an exposed geomembrane liner material as a landfill cap (i.e., for industrial sites), it is more common to back-fill and vegetate the finished landfill cap system (i.e., for municipal sites). A back-filled vegetated cap reduces maintenance and allows for other uses (i.e., parkland).
In an effort to maximize landfill volumes, finished slopes may often be steep. Ensuring stability of the backfill can be difficult. High friction angle PVC and PP geomembranes can help hold a soil veneer on a steep landfill slope. Erosion control products placed over finished seeded slopes prevent erosion until permanent vegetation can become established.
Final landfill cap systems provide a permanent entombment of the waste mass, but much of the liquid content in the landfill is added during operations. Operators use a number of techniques to prevent the accumulation of liquid in the operating area, such as rejection of liquid waste, working face area reduction, and placement of a daily cover. However, daily soil covers consume valuable landfill volumes. Geosynthetic daily covers can be made from geotextiles (woven, non-woven or Typar style) or lining materials such as reinforced polyethylene (RPE). These daily covers can be used to cover the working face at the end of each day and do not take up any landfill volume.
In larger landfills, a number of cells must be completed before a final landfill cap is practical. An interim landfill cap can prevent water accumulation in a completed cell for three to five years until a final cap can be constructed. Interim landfill caps are made from inexpensive RPE materials in sizes up to 10,000 m2 (100,000 ft2) in a single piece. An interim landfill cap keeps the water out of the waste mass until a final cover can be constructed.
Written by Andrew Mills, C.E.T., product manager (geosynthetics) at Layfield Plastics Ltd. in Edmonton, Alberta.