Tunnels excavated for construction of dams make up one of the most important underground networks in the country - there are 1,500 km in total in France.
Necessary primarily for drying out of the site, such tunnels may be of very large diameter, more than 10 metres in some cases. The first ones were bored out at the end of the XIXth century, mainly in the Alps, aided by progress in compressed air and explosives technology.
In the case of certain structures, the catchment area may turn out to be inadequate to fill the basin outside periods of seasonal floodwater, during Spring thaws. In such cases, neighbouring mountain streams must be diverted, such as at Roselend where the total length of the galleries reaches 100 km.
It is within the framework of sites of this type that tunnelling techniques were developed, such as on the Isère-Arc diversion.
Large diameter diversion tunnels must be able to take high water pressures, and this assumes a high build quality, meaning that nowadays they are manufactured from concrete.
The other type of tunnels built within the confines of dams are those which, generally in the mountains, enable supply of factories located 500 or 1000 m in altitude below the reservoir. These highly important structures have demanded extensive knowledge of rock strata, which contributes to the development of the science of rock mechanics.