A double liner is a fluid barrier system that incorporates two impermeable layers separated by a permeable drainage layer also called a leak detection layer. Typically the impermeable layers are made from geomembranes with a permeable layer in between. The uppermost layer is called the primary liner while the lower layer is called the secondary liner. This combination of layers is designed to prevent hydraulic head from building on the secondary liner, thereby limiting or preventing any permeation into the secondary liner. Due to the difficulty of constructing a single large scale impermeable layer without any defects, a double liner system is more robust, as it can deal with leakage through the primary liner. A double liner system is required by the United States EPA for landfill, surface impoundments, and waste piles.