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.