General considerations

Duct tunnels and trenches are constructions containing one or generally more ducts belonging to different networks. Tunnels which can be inspected (inspectable tunnels) include one or more gangways for initial assembly work and for subsequent control, maintenance and repair operations. A tunnel without standing room, but designed for crawling should have a clear internal height of at least 0.8 m. Duct gangways may not be entered.

The above principles apply to inspectable tunnels, and apply by analogy to tunnels with crawling room only.

Tunnels may contain ducts belonging to the following types of networks:

- collective antennas;

- telecommunications;

- electricity;

- gas;

- water;

- district heating;

- ducted transport (e.g. pneumatic tubes);

- drainage water.