Risk assessment of surface emission of gas from abandoned coal mines in France and techniques of prevention
Résumé
The statutory procedures in France for abandoning mines-and coal mines, in particular-call for the operator to analyse any risks relating to the existence of past underground workings and take measures to reduce them. The risk of gases rising to the surface is perhaps the least easy to assess. The voids left by mining provide a reservoir in which methane and other gases, such as carbon dioxide, can collect. This mine gas can migrate to the ground surface under the effect of several mechanisms-rise in the water-table, variations in barometric pressure, etc.-and build up within enclosed parts of surface structures, presenting considerable hazards to individuals (of explosions, asphyxia, etc.) and to property. The general method used in France to assess this risk, on the basis of the geological context and the specific mining characteristics of the field concerned, is presented and the measures taken to manage such risks are described.