In certain situations water or gas can become trapped under the liner, for example; if the pond is built on clay, the soil contains organic matter that could generate methane gas, or if there is a high / fluctuating water table. Over a period of time, any of these conditions may cause damage to the pond liner, but they can be prevented by installing a drainage system of porous pipes laid in a sand bed or other permeable layer.
Perforated single wall land drainage. Used as a collector drain for the removal of excess surface water on gardens and fields to a suitable collection point. The minimum depth of cover on any piped drain should be at least 450mm to protect the pipe from damage due to the surface traffic or maintenance operations such as deep spiking.