GCL – Geosynthetic Clay Liners are bentonite clay, rolled into thin sheets, sandwiched between two nylon carpets, needle punched together to avoid shearing.
GCL is relatively cheap per m2, and is a little more “natural” than plastic liners. GCLs are very tough, being used in landfills and other high risk applications.
It is however very heavy and needs industrial earth moving equipment to be installed. It also needs a 300mm confining layer of soil on top of it to work properly. This means a large amount of material that can cause excessive turbidity and nutrient pollution. The confining layer can be stabilised with cement, or be replaced with cement plaster, but this adds significantly to the overall installation price.
GCL’s are also very difficult to install on vertical surfaces and are most effective when installed on slopes of about 30 degrees from the horizontal. All this means that GCLs are more suited to large earthen dams than domestic natural pools.