Air/water drainage  (under the liner)

In the presence of projects of large lakes and where it is known the proximity of the groundwater from the ground level, it is necessary to install a gas drainage sistem, in order to avoid the formation of air bubbles under the waterproofing liner. 
Laying waterproofing membranes changes the transpiration of the soil and the air or water may be trapped under the liner due to the movements of the water table in the soil.
There is a potential risk that this air or water then find a way to move vertically and this could lead to the formation of air pockets under the waterproof panel. 
It is possible to avoid this by providing a network of drainage slabs, which have vent-chimneys at the ends to allow the escape of air from the bottom of the lake under the liner. 
These slabs are long about 100 mt, about half a meter wide, with a thickness of 6 mm. 
They have an expected life guaranteed for a minimum of 100 years in natural soils.