Once a landfill cell has been filled to capacity it must be capped. Due to the settlement of the waste, the lining material must be able to stretch without being breached. The most common materials used are 40 mil low density polyethylene or 30 mil polyvinyl chloride due to their elongation properties. Geocomposites are also used in capping applications directly above or below the liner. On top of the liner they provide drainage of rainwater to prevent saturation of the cover soil. Below the liner, they are used as a gas collection layer. The methane gas is then either burned with a flare or used to power an engine to generate electricity.

Various different geosynthetics are currently incorporated into the design of a state of the art landfill. The most common lining material used in municipal landfills is 60 mil high density polyethylene. This material is unrolled and seamed together in the bottom of the landfill cell to contain the leachate generated in the landfill and prevent contamination of the groundwater. The material is unrolled and then seamed together using heat and pressure. For steep slopes, the surface of the material can be textured to increase the stability of the design. 100% of all the seams are tested to ensure both the strength and integrity of the seam.

Geocomposite is another commonly used product in landfill cells. It is placed directly on top of the liner to be used for leachate collection or directly below the liner for leak detection. The geonet core has a nonwoven geotextile bonded to one or both sides to prevent clogging. In either application, the Geocomposite allows the leachate to flow to a sump where it is removed and treated.