Man-made Caverns for Energy, Why and How?

We are mainly interested in caverns used for energy purposes. Two uses are classic in their antiquity:

• just over a century for underground hydroelectric plants;

• half a century for cavities storing liquid and gaseous hydrocarbons.

Other uses are mentioned by way of comparison, but they always have something to do with energy. Indeed, we know that the primary energy quality of the subsoil is its thermal inertia. Saving energy on air-conditioning therefore favours a wide variety of uses. We also know that the temperature of the ground increases with depth, which is the basis of geothermal applications. In most of the cases considered here, these caves are specially dug by man for a well-defined use, but prehistoric man, rightly called "cave man", knew how to use natural caves, to which he sometimes made a few modifications.

Man-made caverns are used for purposes requiring a high degree of stability over a long period of time, whereas the models provided by Nature, caves and then mining cavities, only offer stability for a limited period of time. Underground mines and quarries, which date back to ancient times, represent a wealth of experience. The transition from underground quarrying to troglodytism is a seamless one, even though the dimensions of the living spaces are much smaller than those of industrial caves. As a result, the voids left by underground mining operations often have a second life (theme of the 2013 Mineral Industry Congress). Caverns modelled on mining caverns are also being evoked for countless purposes, many of them related to underground urban planning.

The characteristics of caverns in hydroelectric power stations and storage caverns are examined in detail, as their wide variety is justified not only by the materials and products they house, but also by local conditions (nature and structure of the terrain, hydrogeology, initial state of stress) and constantly evolving construction methods. A similar approach applies to underground road and rail stations, including subway railways and the switch caves of the Channel Tunnel. Unconventional caverns, including those desired by astrophysicists (neutrino studies), multidisciplinary projects (DUSEL and its successors, e.g. Sadoulet [2005]) and those required for underground nuclear power plants, are the only ones capable, thanks to their exceptional containment, of guaranteeing the safety of the surrounding area in the event of very serious accidents such as those at Chernobyl and Fukushima.

This attempt at synthesis is an opportunity to briefly mention a few theoretical and technological aspects of underground structure projects:

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