Hydraulic tunnels convey water for power and water supply purposes, both in free-flow and under pressure. These tunnels can be subject to severe service conditions, such as high hydrostatic heads and water velocity, that may cause severe deterioration of the concrete. The resultant cracking and spalling will allow seepage of water through the structure that can undermine its stability. In addition, the increased roughness of the surface will reduce the water flow.

The installation of a very smooth synthetic geomembrane on the tunnel’s inner surface provides the following benefits:

- stops leakage

- provides watertightness even for cracked structures

- protects the tunnel structure from further deterioration

- increases the water flow considerably

- reduces the formation of vegetation and algae

- minimizes future maintenance.

When used for rehabilitation, the geomembrane can be left exposed even in water velocities of several meters/second.

When used in new construction,the geomembrane may be left exposed or can be embedded between the excavation line and the inner layer of concrete. In this configuration, the concrete provides structural stability while the geomembrane provides watertightness.

The CARPI lining system can be very useful to protect the ground water in the case of leaking waste water tunnels.